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Journal of Virology, October 2007, p. 11381-11391, Vol. 81, No. 20
0022-538X/07/$08.00+0     doi:10.1128/JVI.00767-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The Cytoplasmic Tails of Uukuniemi Virus (Bunyaviridae) G_N and G_C Glycoproteins Are Important for Intracellular Targeting and the Budding of Virus-Like Particles

Anna K. Överby,¹ Vsevolod L. Popov,² Ralf F. Pettersson,^1* and Etienne P. A. Neve¹

Ludwig Institute for Cancer Research, Stockholm Branch, Karolinska Institute, Box 240, SE-17177 Stockholm, Sweden,¹ University of Texas Medical Branch, Department of Pathology, 301 University Boulevard, Galveston, Texas 77555-0609²

Received 10 April 2007/ Accepted 24 July 2007

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Functional motifs within the cytoplasmic tails of the two glycoproteins G_N and G_C of Uukuniemi virus (UUK) (Bunyaviridae family) were identified with the help of our recently developed virus-like particle (VLP) system for UUK virus (A. K. Overby, V. Popov, E. P. Neve, and R. F. Pettersson, J. Virol. 80:10428-10435, 2006). We previously reported that information necessary for the packaging of ribonucleoproteins into VLPs is located within the G_N cytoplasmic tail (A. K. Overby, R. F. Pettersson, and E. P. Neve, J. Virol. 81:3198-3205, 2007). The G_N glycoprotein cytoplasmic tail specifically interacts with the ribonucleoproteins and is critical for genome packaging. In addition, two other regions in the G_N cytoplasmic tail, encompassing residues 21 to 25 and 46 to 50, were shown to be important for particle generation and release. By the introduction of point mutations within these two regions, we demonstrate that leucines at positions 23 and 24 are crucial for the initiation of VLP budding, while leucine 46, glutamate 47, and leucine 50 are important for efficient exit from the endoplasmic reticulum and subsequent transport to the Golgi complex. We found that budding and particle generation are highly dependent on the intracellular localization of both glycoproteins. The short cytoplasmic tail of UUK G_C contains a lysine at position –3 from the C terminus that is highly conserved among members of the Phlebovirus, Hantavirus, and Orthobunyavirus genera. Mutating this single amino acid residue in G_C resulted in the mislocalization of not only G_C but also G_N to the plasma membrane, and VLP generation was compromised in cells expressing this mutant. Together, these results demonstrate that the cytoplasmic tails of both G_N and G_C contain specific information necessary for efficient virus particle generation.

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Most enveloped viruses acquire their envelope from the plasma membrane; however, members of the bunyavirus (34), flavivirus, and coronavirus families assemble and bud at intracellular membranes (reviewed in reference 33). Not much is known about the budding mechanism of bunyaviruses, and most studies on bunyaviruses have focused on the processing of the two glycoproteins G_N and G_C and their transport to and retention in the Golgi complex, the site of budding. The two glycoproteins are translated as a common precursor encoded by the M segment. The precursor is cotranslationally inserted into the membrane of the endoplasmic reticulum (ER), where it is cleaved into G_N and G_C. After folding and maturation, G_N and G_C form a heterodimer, which is transported to the Golgi region, where it is retained due to a retention signal present in the G_N (1, 3, 15, 24, 25, 39). When expressed alone, the G_C is unable to exit the ER, and it has been proposed that the short cytoplasmic tail, being rich in lysines, contains an ER retention motif (15, 37). The G_N, when expressed alone, can exit the ER and proceed to the Golgi complex, where it is retained due to a retention signal located in the cytoplasmic tail (1), the transmembrane domain (39), or both (15, 24, 25), depending on the specific virus in the Bunyaviridae family. The G_N/G_C heterodimers accumulate in the Golgi complex together with the ribonucleoproteins (RNPs), and virus particles are formed by budding into the Golgi complex (23, 34, 38). The budding probably occurs when a critical concentration of the glycoproteins is reached (30), although the precise mechanism remains obscure. For Uukuniemi (UUK) virus, expression of the two glycoproteins alone is sufficient for initiating the budding and for the generation of infectious virus-like particles (VLPs) (29, 30). Therefore, it is important that the RNPs are in close proximity to the budding site in order to ensure efficient incorporation into progeny particles (23). The interaction between the RNPs and the glycoproteins is mediated by the G_N cytoplasmic tail and the nucleoprotein (22), and we have recently demonstrated that only four amino acids present in the C terminus of the G_N cytoplasmic tail adjacent to the G_C signal sequence are responsible for this interaction (29).

An infectious VLP system has been developed for UUK virus (30), a member of the Phlebovirus genus, one of five genera in the Bunyaviridae family, and a model virus for this family of viruses (7). These VLPs were shown to be able to package an artificial RNA segment, a minigenome containing a reporter gene flanked by the noncoding regions from one of the three viral segments (8, 30). These VLPs can infect new cells and transfer the minigenome that can be replicated and transcribed in these newly infected cells if the required UUK proteins (the polymerase and nucleoprotein, open reading frames of the L and the S segment, respectively) are coexpressed (30). The VLPs display morphology and cellular tropism identical to those of wild-type (wt) UUK virus (30) and are therefore a useful system for studying packaging and budding mechanisms of this family of viruses. This system has been successfully used to map amino acids in the G_N cytoplasmic tail, which is important for the packaging of RNPs into VLPs for UUK virus (29). In addition to the four amino acids located in the end of the G_N cytoplasmic tail demonstrated to be important for packaging, two other regions were shown to be important for the generation and release of VLPs. These two regions were shown to span residues 21 to 25 and 46 to 50 in the G_N cytoplasmic tail (29). In the present study, we have analyzed and identified specific amino acids within these two regions in the G_N cytoplasmic tail that are responsible for the generation and release of VLPs. Moreover, a functional motif within the G_C cytoplasmic tail containing a well-conserved lysine residue was shown to be important for proper targeting and retention of both glycoproteins to the Golgi complex and subsequent particle release.

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Plasmids. pUUK-G_N/G_C, pUUK-L, and pUUK-N are cytomegalovirus-driven plasmids expressing the UUK viral glycoprotein precursor, RNA-dependent RNA polymerase, and nucleoprotein, respectively (9). M-CAT is the Pol I-driven UUK virus minigenome plasmid containing the noncoding regions from the M segment, flanking the reporter gene for chloramphenicol acetyltransferase (CAT) (8). The construction of all alanine mutant plasmids used in the alanine scan was performed by standard PCR cloning methods, such as overlap PCR and two- and three-fragment ligation, as described previously (29). All derivatives of pUUK-G_N/G_C were sequenced to verify the correct introduction of mutations in the absence of undesirable mutations. Primer sequences are available upon request.

Cell culture and transfection. BHK-21 (ATCC) cells were grown in plastic dishes in minimum essential medium with Earle's salt supplemented with 5% fetal calf serum, 5% tryptose phosphate broth, 2 mM L-glutamine, 50 IU penicillin/ml, and 50 µg streptomycin/ml (Invitrogen). For the VLP reporter gene system (30), BHK-21 cells were transfected with pUUK-L, pUUK-N, pUUK-G_N/G_C, and M-CAT using Lipofectamine 2000 reagent (Invitrogen). Transfections were performed as described previously (29, 30). Cells were analyzed for reporter gene expression 24 h posttransfection, and the corresponding supernatants were used for VLP infection. Twenty-four hours prior to the passage of the supernatant, BHK-21 cells were transfected with L and N expression plasmids in order to support minigenome replication, transcription, and detection. After a 3-h incubation period, fresh medium was added to the VLP-infected cells, and cells were incubated for 24 h before reporter gene analysis.

CAT assays. Cells were resuspended in 50 µl of 0.25 M Tris-HCl (pH 7.4) and were lysed by two freeze-thaw cycles. The cell lysates were centrifuged for 10 min at 9,000 x g, and CAT activity was determined using a commercially available Fast Cat kit (Invitrogen) as described previously (9). The reaction products were visualized by UV illumination and were documented by photography.

Harvesting and purification of UUK VLPs. The harvesting and purification of UUK VLPs were done as previously described (30). Briefly, supernatants from VLP-expressing cells were collected and clarified by centrifugation (4,000 x g for 10 min at 4°C). The particles were concentrated through a 20% (wt/vol) sucrose cushion and dissolved in TN buffer (0.05 M Tris-HCl, pH 7.4, and 0.1 M NaCl) by centrifugation at 100,000 x g for 1 h at 4°C. The pellet was dried for 10 min before resuspension in nonreducing Laemmli/sodium dodecyl sulfate-polyacrylamide gel electrophoresis sample buffer for subsequent analysis.

Western blotting. Precast 10% polyacrylamide gels (Invitrogen) were used according to the manufacturer's recommendations and run under nonreducing conditions. Rabbit polyclonal antibodies recognizing both G_N and G_C were used to detect G_N and G_C simultaneously, and polyclonal antibodies recognizing the UUK virus N protein were used to detect the N protein (23).

Immunofluorescence microscopy. BHK-21 cells were grown on coverslips, transfected with mutant or wt glycoprotein, and fixed 24 h posttransfection with 3% paraformaldehyde, and staining was performed as described previously (29). For the cycloheximide treatment, BHK-21 cells were treated 5 h posttransfection with 50 µg/ml cycloheximide for 4 h before fixation with paraformaldehyde. The UUK virus glycoproteins G_N/G_C and Golgi marker protein GM130 were detected using a mix of polyclonal UUK virus G_N/G_C antibodies and monoclonal GM130 antibodies (BD Biosciences), followed by Alexa Fluor 488-conjugated goat anti-rabbit immunoglobulin G and Alexa Fluor 594-conjugated goat anti-mouse immunoglobulin G (Molecular Probes). Polyclonal antibodies recognizing the G_N cytoplasmic tail (K1228) (42) and monoclonal antibodies recognizing the G_N (6G9) or G_C (8B11) protein (31) were used to detect each of the glycoproteins separately. Images were collected using a Zeiss microscope equipped with a charge-coupled-device camera.

Glycosidase treatment. BHK-21 cells were transfected with wt G_N or with the mutant G_N23-24, G_N46-47, or G_N48-50, and 24 h posttransfection the cells were lysed and 30 µg cell lysate was treated with 12.5 mU endoglycosidase H (Endo H) or 2.5 U N-glycosidase F (PNGase F) (Roche), as described by the manufacturer, or was left untreated.

FACS and transmission electron microscopy (TEM). BHK-21 cells were cotransfected with a green fluorescent protein (GFP) expression plasmid (pHL2823) and the wt G_N/G_C or the G_NL23A/wt G_C mutant plasmid at a ratio of 1:2 and trypsinized 24 h posttransfection, and approximately one million GFP-expressing cells were sorted out by fluorescent-activated cell sorting (FACS) analysis. The cells expressing GFP were collected, and an aliquot was plated onto coverslips for immunofluorescence analysis. The remaining cells were fixed and prepared for ultrathin-section electron microscopy, as described previously (30). Cells plated onto coverslips were incubated at 37°C for 16 h and were processed for immunofluorescent staining as described above.

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To evaluate and identify functional motifs in the cytoplasmic tails of G_N and G_C of UUK virus, we used the newly developed VLP system for UUK virus (30). The G_N cytoplasmic tail of 81 residues has previously been shown to contain a Golgi complex targeting and retention signal (amino acids 10 to 40) (3), two palmitylation sites (amino acids 25 and 28) (2), and four amino acids important for packaging (amino acids 76 and 79 to 81) (29) (Fig. 1A). We recently used the VLP system to identify the packaging interactions between the G_N cytoplasmic tail and the RNPs (Fig. 1A, residues indicated by black boxes) (29). In addition, two other regions in the G_N cytoplasmic tail, namely residues 21 to 25 and 46 to 50 (Fig. 1A, underlined), were shown to be important for particle formation and release but not for packaging of RNPs. No UUK virus-specific proteins could be detected in the supernatant of BHK-21 cells transfected with the minigenome system and these glycoprotein mutants, suggesting that no VLPs were generated and released by the cells expressing these mutants (29).

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FIG. 1. Transfer of reporter gene activity and protein analysis of VLPs generated by GN glycoprotein mutants. (A) Schematic representation of the glycoprotein precursor open reading frame. The amino acid sequences of GN and GC cytoplasmic tails are shown, and functional motifs that were previously identified are highlighted. Underlined residues are analyzed in the present study using a mutagenesis screen. TM, transmembrane domain; SS, signal sequence. (B and C) Transfer of CAT activity by VLPs generated with mutated and wt glycoproteins. BHK-21 cells were transfected with the minigenome pUUK-N and pUUK-L were left untransfected (lane 1), were transfected with wt GN/GC (lane 2), or were transfected with mutated pUUK-GN/wt GC (lanes 3 to 5). The amino acid residues in the GN tail that were mutated to alanines are indicated by their numbers according to the GN sequence shown in panel A. Cells were harvested and analyzed for CAT activity 24 h posttransfection (upper panel). The corresponding supernatants, containing VLPs, were used to infect new cells pretransfected with pUUK-N and pUUK-L, and reporter activity was determined 24 h after VLP infection (lower panels). (D) Western blot analysis of the VLPs. Transfected cells (upper panels) and the corresponding supernatants containing VLPs, after concentration through a sucrose cushion (lower panels), were analyzed for glycoprotein (GN/GC) and nucleoprotein (N) expression. BHK-21 cells were transfected with M-CAT, pUUK-N, and pUUK-L (lane 1); with M-CAT, pUUK-N, pUUK-L, and wt GN/GC (lane 2); or with mutated pUUK-GN/wt GC (lanes 3 to 5). The two glycoproteins are detected with a polyclonal antibody recognizing both GN and GC, and the nucleoprotein was detected with polyclonal antibody recognizing the N protein. Data represent results from three independent experiments.

Mutagenesis screen of regions 21 to 25 and 46 to 50 in the G_N cytoplasmic tail. In the present study, we analyzed the regions encompassing amino acids 21 to 25 and 46 to 50 in more detail by a mutational screen, changing 1, 2, or 3 amino acids at a time to alanine. The mutants were tested in our VLP system for their ability to generate and release VLPs containing RNPs, as monitored by the transfer of CAT reporter activity to newly VLP-infected cells (Fig. 1B and C). BHK-21 cells transfected with the minigenome system of M-CAT, pUUK-L, and pUUK-N and either the wt or the mutated pUUK-G_N/G_C expression plasmids were analyzed for CAT activity 24 h posttransfection (Fig. 1B and C, upper panels). Both wt and all mutant glycoprotein transfected cells displayed strong CAT reporter activity in primary transfected cells, indicating efficient replication and expression of the minigenome M-CAT. Supernatants were collected 24 h posttransfection and were used to infect fresh BHK-21 cells, which were pretransfected with the L and the N expression plasmids to allow amplification of the UUK virus RNA segments and expression of the reporter protein (30). The VLP-infected cells were harvested 24 h after VLP infection, and reporter activity was determined (Fig. 1B and C, lower panels). No CAT activity could be detected in the negative control (Fig. 1B and C, lower panels, lanes 1), indicating that no minigenome can be transferred to new cells in the absence of the glycoprotein precursor expression plasmid. Maximum activity was detected in the positive control with wt G_N/G_C (Fig. 1B and C, lower panels, lanes 2). The mutant G_N48-50 showed a reduced level of VLP-mediated transfer of CAT activity to new cells compared to that of wt G_N/G_C (Fig. 1C, lower panel, compare lanes 2 and 4), and two mutants with alanine substitutions for residues 23 to 24 and 46 to 47 in the cytoplasmic tail of G_N showed only background levels (Fig. 1B, lower panel, compare lanes 1 and 4, and C, lower panel, compare lanes 1 and 3). Mutations of residues 21 to 22 and 25 did not have an effect on VLP-mediated transfer of CAT (Fig. 1B, lower panel, lanes 3 and 5). We have previously shown that mutating residues 26 to 30 did not affect VLP-mediated transfer of CAT activity or intracellular localization (29), suggesting that the palmitylation of residue 25 or 28 is not critical for the generation of infectious VLPs. To verify protein expression of the wt and mutated glycoproteins in the transfected cells, as well as the protein composition of wt and mutant VLPs in the supernatant, Western blot analysis was performed (Fig. 1D). No difference in glycoprotein or nucleoprotein expression levels could be detected between the wt and mutated glycoproteins in primary transfected cells (Fig. 1D, upper panels). The corresponding supernatants from these transfected cells were collected, concentrated through a sucrose cushion, and analyzed by Western blotting (Fig. 1D, lower panels). No UUK virus proteins were released from cells transfected with only the minigenome and the L and N expression plasmids (Fig. 1D, lane 1), and both glycoproteins and the nucleoprotein were detected when the wt G_N/G_C expression plasmid was present (lane 2). No UUK-virus-specific proteins were detected in the supernatants obtained from G_N23-24-, G_N46-47-, and G_N48-50-transfected cells, although all these mutated glycoproteins were expressed at the same level as wt glycoproteins, as was observed for the cell lysates (Fig. 1D). Even though the G_N48-50 mutant was able to transfer reduced amounts of CAT activity to VLP-infected cells (Fig. 1C, lane 4), no UUK virus proteins could be detected in the supernatant by Western blotting (Fig. 1D, lane 5), indicating that Western blotting is not sensitive enough to detect small amounts of secreted VLPs. These results indicate that these two regions in the G_N cytoplasmic tail between residues 23 to 24 and 46 to 50 are important for generating and releasing particles into the supernatant.

Intracellular localization of the G_N cytoplasmic tail mutants. UUK virus buds into the Golgi complex and is transported in large vesicles to the plasma membrane, where they are released into the media (23). We wondered if the defect in VLP release of these three glycoprotein mutants is caused by mislocalization of the mutant glycoproteins, thereby preventing budding into the Golgi complex. BHK-21 cells transfected with wt G_N/G_C and the mutants G_N23-24, G_N46-47, and G_N48-50 were fixed 24 h posttransfection and were costained with a polyclonal antibody recognizing both G_N and G_C glycoproteins and the Golgi marker GM130, a cis-Golgi matrix protein (27) (Fig. 2A). In wt G_N/G_C-transfected and G_N23-24 mutant-transfected cells, the glycoproteins colocalized with the Golgi marker, demonstrating that the lack of reporter gene transfer and release of particles for this mutant was not caused by a defect in intracellular targeting (Fig. 2A). In contrast, the G_N46-47 and G_N48-50 mutants displayed more reticular staining patterns typically observed for proteins localized to the ER (Fig. 2A, lower panels).

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FIG. 2. Intracellular localization of GN glycoprotein mutants. (A) BHK-21 cells grown on coverslips were transfected with wt or mutant GN glycoprotein as described in the legend to Fig. 1, fixed, permeabilized, and stained 24 h posttransfection for immunofluorescence analysis. Cells were costained with monoclonal antibodies recognizing the Golgi marker GM130 (red) and a polyclonal antibody recognizing both GN and GC glycoproteins (green). Colocalization of the glycoproteins with GM130 is shown in yellow in the right panels. Numbers at the left side indicate the regions in the GN tail that were mutated to alanines. (B) Endo H and PNGase F treatment of wt and mutated glycoproteins. BHK-21 cells were transfected with the wt and mutated glycoproteins as indicated in the figure. The cells were lysed 24 h posttransfection, and the lysates were treated with Endo H (lanes 2, 4, 6, and 8), were left untreated (lanes 1, 3, 5, 7, and 9), or were treated with PNGase F (lane 10). The different forms of the GN protein were detected by Western blotting with an antibody recognizing GN. The fully glycosylated form of GN is indicated by an arrow, and the fully deglycosylated form is indicated by an arrowhead.

The wt G_N protein has been reported to be partially Endo H resistant, a modification that occurs in the Golgi complex (22, 32). We therefore analyzed whether the G_N mutants are processed in a manner similar to that of the wt G_N protein, an indication that the glycoprotein is able to reach the Golgi complex. Cell lysates from BHK-21 cells transfected with wt or mutant glycoproteins were left untreated or were treated with Endo H, and the glycosylation pattern of G_N was examined by Western blotting (Fig. 2B). We observed that the wt and G_N23-24 are more Endo H resistant than G_N46-47 and G_N48-50 (Fig. 2B, compare lanes 2 and 4 with 6 and 8). Treatment of wt G_N with PNGase F, a glycosidase that is able to remove both Endo H-senstitive and -resistant glycans, reveals that the lower band (Fig. 2B, indicated by an arrowhead) is the deglycosylated form of G_N. This demonstrates that, unlike the wt G_N, G_N46-47 and G_N48-50 are not as efficiently modified by the glycoprotein-modifying enzymes in the Golgi complex and, together with their intracellular distribution, suggests that these mutants are trafficking less efficiently to the Golgi complex. Together, these results suggest that the defect in VLP release observed for these two mutants is caused by mislocalization of the mutant glycoproteins.

Identification of residues important for the formation of VLPs. In order to identify which of the two amino acid residues, leucine 23 or leucine 24, is important for the generation and release of VLPs, single-amino-acid mutants G_NL23A and G_NL24A were generated. Both G_NL23A and G_NL24A displayed a clear Golgi staining pattern identical to that of wt G_N/G_C (data not shown), verifying their correct intracellular targeting. The mutants G_NL23A and G_NL24A were analyzed for their ability to generate and release VLPs and transfer CAT activity to new cells. Supernatants from wt and mutant glycoprotein-transfected cells were collected 24 h posttransfection, an aliquot was used for VLP infection, and the remainder of the supernatant was concentrated and used for Western blot analysis (Fig. 3). The primary transfected cells were harvested, and reporter protein expression was determined using the CAT assay (Fig. 3A, upper panel). All transfected cells showed strong reporter activity, verifying efficient transcription and replication of the UUK virus minigenome. Strong CAT activity also was detected in cells infected with wt VLPs (Fig. 3A, lower panel, lane 1). Transfer of CAT activity was greatly reduced compared to that of the wt for the G_NL24A mutant (compare lane 1 with lane 3), while only background levels could be observed with the G_NL23A mutant (lane 2). Both glycoproteins and the nucleoprotein were detected in VLPs generated from wt G_N/G_C-transfected cells. However, no glycoproteins or nucleoproteins were detected in the supernatant obtained from G_NL23A- or G_NL24A-transfected cells (Fig. 3B). These results show that both residues L23 and L24 are important for the generation and release of VLPs into the medium.

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FIG. 3. GNL23A and GNL24A glycoprotein mutants fail to generate VLPs. (A) CAT analysis of VLPs generated with the single-amino-acid mutants GNL23A and GNL24A. Cell lysates from the transfected cells were analyzed for CAT activity (upper panel), and the corresponding supernatants were harvested, used for VLP infection, and analyzed for CAT expression 48 h postinfection (lower panel). The asterisk indicates the cellular background. (B) Western blot analysis of supernatants containing VLPs used for infection shown in panel A. Both glycoproteins (GN/GC) and the nuclear protein (N) were detected using polyclonal antibodies as described in the legend to Fig. 1. Data are representative results from three independent experiments.

G_NL23A has a defect in budding into the Golgi membrane. In a previous report, it was shown by using TEM that a temperature-sensitive UUK virus mutant was unable to bud into the Golgi membrane and form particles at the restrictive temperature, while particles were observed to bud into the Golgi membrane at the permissive temperature (12). We explored the possibility that the G_NL23A glycoprotein mutant, as well as the G_NL24A glycoprotein mutant, which correctly localized to the Golgi complex, is defective in budding into the Golgi membrane and therefore cannot secrete VLPs into the supernatant. To address this, we examined the morphology of wt G_N/G_C- and G_NL23A-transfected cells by TEM. The G_NL23A mutant was chosen, since that mutant showed the strongest phenotype in VLP-mediated transfer of CAT activity. BHK-21 cells were cotransfected with a GFP-expressing plasmid together with a wt G_N/G_C- or mutant G_NL23A-expressing plasmid and were trypsinized 24 h posttransfection, and GFP-expressing cells were sorted and collected by FACS analysis. This procedure enriched the population of cells expressing the glycoproteins from 16% (primary transfected cells) to almost 100% (FACS-sorted cells), as shown by immunofluorescence microscopy performed on a small aliquot of the sorted cells (Fig. 4A). Both wt and mutant transfected cells showed fibroblast-specific morphology after sorting, and most of the sorted cells expressed the transfected glycoprotein, although the expression levels varied somewhat between cells (Fig. 4A). The majority of the FACS-sorted cells were fixed by modified Ito's fixative (35) and subsequently were prepared for thin-section TEM (Fig. 4B). Cells expressing wt G_N/G_C clearly contained VLPs inside Golgi vesicles (Fig. 4B, indicated by arrows), as was also shown previously (30). No visible particles or budding activity could be detected in the Golgi vesicles of cells expressing the G_NL23A mutant glycoprotein, strongly indicating that L23 in the G_N cytoplasmic tail is important for the initiation of VLP budding. Interestingly, although no particles were formed with G_NL23A, we observed a vacuolization of the Golgi complex similar to what was seen in wt G_N/G_C-transfected cells and UUK virus-infected cells (23, 30), indicating that the formation of particles is not driving this process. We conclude that the inability of G_NL23A, and most likely G_NL24A, to release VLPs is caused by a defect in budding into the Golgi membranes.

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FIG. 4. Morphological analysis of wt and GNL23A glycoprotein-transfected cells. (A) Immunofluorescence analysis of wt and GNL23A glycoprotein-transfected cells after enrichment by FACS. BHK-21 cells were cotransfected with GFP and wt GN/GC or GNL23A mutant glycoprotein in the presence of wt GC. GFP-expressing cells were selected by FACS 24 h posttransfection. An aliquot of the sorted cells was plated on coverslips and stained with a polyclonal antibody recognizing both glycoproteins (red) and 4',6'-diamidino-2-phenylindole (DAPI) (white). Almost all sorted cells express wt GN/GC or the L23A mutant, as shown in the merged images to the right. (B) TEM of FACS-sorted cells expressing the wt or GNL23A mutant. The cells expressing GFP were fixed and processed for ultrathin-section TEM. The Golgi region is indicated by a rectangle in both wt and GNL23A glycoprotein-transfected cells. Only in the wt GN/GC-expressing cells could VLPs be detected, as indicated by the arrows. Bars, 0.5 µm.

Identification of single amino acids in the G_N cytoplasmic tail important for intracellular targeting. To evaluate which specific amino acid in the region of residues 46 to 50 of the G_N cytoplasmic tail is responsible for the observed mislocalization of the mutant glycoprotein and subsequent defect in VLP release, single-amino-acid mutants, in which individual amino acids were changed to alanines, were constructed. The mutant glycoproteins were first evaluated for their ability to generate VLPs and infect new cells (Fig. 5A). Both wt and the single-amino-acid-mutant G_N glycoproteins showed strong CAT activity in transfected cells (data not shown). G_NE48A and G_NG49A showed reporter activity in VLP-infected cells that was as strong as that of wt G_N/G_C (Fig. 5A, compare lanes 5 and 6 to lane 2). G_NL46A and G_NL50A (lanes 3 and 7) were unable to transfer any CAT activity to VLP-infected cells, while G_NE47A (lane 4) displayed a reduced ability to transfer CAT activity. To evaluate whether this defect in transfer of CAT activity was caused by mislocalization of the mutant glycoproteins, we determined the intracellular localization of these five mutant glycoproteins by immunofluorescence microscopy (Fig. 5B). The G_NE48A and G_NG49A mutants that were able to transfer CAT activity as efficiently as wt G_N/G_C colocalized with the Golgi marker GM130 (Fig. 5B), indicating correct targeting of these mutant glycoproteins. However, G_NL46A, G_NE47A, and G_NL50A, all being defective in transfer of CAT activity, displayed a more reticular staining pattern and only partially colocalized with the Golgi marker GM130 (Fig. 5B), suggesting that these mutants are partially localized to the ER. However, no difference in intracellular localization could be detected between mutant G_NE47A, with reduced CAT activity, and mutants G_NL46A and G_NL50A, without any detectable CAT activity. These results suggest that amino acid residues L46, E47, and L50 are important for correct targeting of the glycoproteins to the Golgi complex and subsequent formation of VLPs.

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FIG. 5. VLP generation is linked to the intracellular localization of the glycoproteins. (A) CAT activity in cells infected with VLPs collected from cells transfected with GN single-amino-acid mutants and wt GC. Point mutations in the region of residues 46 to 50 of the GN cytoplasmic tail were introduced by changing each amino acid to an alanine, generating the GN mutants GNL46A, GNE47A, GNE48A, GNG49A, and GNL50A. VLPs obtained from cells transfected without or with wt GN/GC served as negative (lane 1) and positive (lane 2) controls, respectively. (B) Intracellular localization of the GN single-amino-acid glycoprotein mutants that were assayed for CAT activity in panel A. Cells were costained for glycoprotein mutant GN and wt GC (GN/GC; green) and the Golgi marker GM130 (red). The inserts show higher magnifications of the Golgi areas. The panels to the right show the merged images, in which yellow indicates colocalization. (C) Colocalization of the wt GN and GNL46A and GNE47A mutants with the wt GC protein. Cells were transfected with wt GN, GNL46A, and GNE47A in the presence of wt GC and were costained with a polyclonal antibody specifically recognizing the cytoplasmic tail of the GN glycoprotein (green) and a monoclonal antibody specifically recognizing the GC protein (red). The panels to the right show the merged images.

G_C is known to be retained in the ER when expressed alone, while G_N, when expressed alone, is transported to and retained in the Golgi complex due to the Golgi complex targeting and retention signal located in its cytoplasmic tail (1). When coexpressed, G_N and G_C form heterodimers that are targeted to the Golgi complex (see Fig. 2) (31). It is unknown which part or parts of the glycoproteins are mediating this dimerization, either the ectodomain, the transmembrane domain, the cytoplasmic tails, or a combination of all three domains. Since G_NL46A, G_NE47A, and G_NL50A show typical ER and only partial Golgi staining, we speculated that mutation of these residues in the G_N cytoplasmic tail prevented this heterodimerization. We therefore transfected BHK-21 cells with wt and mutant glycoproteins and costained them with a G_N polyclonal antibody (42) and a G_C monoclonal antibody (31) in order to detect both glycoproteins separately (Fig. 5C). In wt G_N/G_C-transfected cells, both glycoproteins colocalize well in the Golgi area (Fig. 5C, upper panels). In cells cotransfected with G_NL46A, G_NE47A, and G_NL50A together with wt G_C, the G_N mutant glycoproteins colocalized with the wt G_C protein, confirming that both glycoproteins are localized to the same compartment (Fig. 5C, lower panels, and data not shown). In addition, coimmunoprecipitation experiments also showed that mutants G_NL46A, G_NE47A, and G_NL50A interacted as efficiently as wt G_N with a G_C construct containing a truncated ectodomain and a wt transmembrane domain and cytoplasmic tail (data not shown). This demonstrates that the localization of both glycoproteins G_N and G_C is dependent on the three amino acids L46, E47, and L50 in the G_N cytoplasmic tail but that these residues were not required for heterodimerization.

The heterodimer of wt G_N and G_C is transported from the ER to the Golgi complex 45 min after the synthesis of the G_C protein (31). To analyze trafficking and transport from the ER to the Golgi complex, wt G_N/G_C-transfected and mutant G_N/wt G_C-transfected BHK-21 cells were treated with cycloheximide to block new protein synthesis 5 h posttransfection. The intracellular localization of the transfected proteins was examined by immunofluorescent microscopy 0, 0.5, 1, 2, and 4 h after cycloheximide treatment (Fig. 6 and data not shown). The wt G_N/G_C heterodimer colocalized well with the Golgi marker at all time points examined, reflecting the efficient transport of the G_N/G_C heterodimer to the Golgi complex (Fig. 6). The mutants G_NL46A, G_NE47A, and G_NL50A showed partial ER and Golgi complex staining at the beginning of the cycloheximide chase, and the localization of the mutant glycoproteins did not change significantly during the 4-h chase (Fig. 6 and data not shown). These results demonstrate that these three glycoprotein mutants are less efficient in transport to the Golgi complex than the wt glycoprotein.

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FIG. 6. Glycoprotein mutants GNL46A, GNE47A, and GNL50A are partially retained in the ER. BHK-21 cells were transfected with wt and mutant GN, and 5 h posttransfection they were treated with cycloheximide. Cells were fixed, and glycoproteins were detected by immunofluorescence at the beginning of the cycloheximide treatment (time zero) and 4 h posttreatment.

Analysis of the G_C cytoplasmic tail. The intracellular tail of UUK virus G_C is not as well characterized as the G_N tail, but it has been suggested that the G_C cytoplasmic tail of the members of the Phlebovirus genus, being rich in lysines, contains a lysine-based ER retrieval signal (15). Indeed, an amino acid alignment of the extreme C terminus of G_C from viruses belonging to the genera Phlebovirus, Hantavirus, and Orthobunyavirus revealed that the lysine at position –3 is conserved across the genera in this family (Fig. 7A, indicated by a gray box). However, this amino acid was not conserved in the Nairovirus and Tospovirus genera (data not shown). UUK virus has a very short G_C cytoplasmic tail of only 5 amino acids (Fig. 1A), and a mutational screen was performed to analyze the importance of these residues in VLP generation and transfer of CAT activity. The last four residues were all changed to alanines (G_C2-5), the cytoplasmic tail of G_C was inverted (G_C1-5 invert), and finally the last four amino acids were changed to alanines, one residue at a time (G_CV2A, G_CK3A, G_CK4A, and G_CS5A) (Fig. 7B). These mutants were, in the presence of wt G_N, analyzed for their ability to generate and release infectious VLPs (Fig. 7C). All the G_C mutant proteins showed strong reporter activity in primary transfected cells, verifying minigenome replication and transcription (data not shown). In VLP-infected cells, strong reporter activity could be detected with wt G_N/G_C (Fig. 7C, lane 2), and similar transfer of activity was seen with two of the mutants, G_CV2A and G_CK4A (Fig. 7C, lanes 5 and 7). The other G_C mutants, G_C2-5, G_C1-5 invert, G_CK3A, and G_CS5A (Fig. 7C, lanes 3, 4, 6, and 8), displayed reduced CAT activity compared to that of wt G_C. Next, the protein composition of the G_C mutant VLPs generated and released from the transfected cells was analyzed by Western blotting (Fig. 7D). Both glycoproteins, G_N and G_C, and the nucleoprotein were readily detected in the supernatants of wt G_N/G_C-transfected cells (Fig. 7D, lane 2) and in the supernatants of the two CAT-positive mutants G_CV2A and G_CK4A (Fig. 7D, lanes 5 and 7). Reduced amounts of both glycoproteins and the nucleoprotein were detected in the supernatant from cells transfected with the mutants G_C2-5, G_C1-5 invert, G_CK3A, and G_CS5A (Fig. 7D, lanes 3, 4, 6, and 8), indicating that in these G_C cytoplasmic tail mutants efficient generation and release of infectious VLPs were compromised. The primary transfected cells also were analyzed for protein expression, and both the glycoprotein and nucleoprotein were expressed at similar levels (Fig. 7D, lower panel, and data not shown).

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FIG. 7. Analysis of the GC cytoplasmic tail. (A) Alignment of the last seven C-terminal amino acids of the GC proteins from members in the Phlebovirus, Hantavirus, and Orthobunyavirus genera. The conserved lysine is indicated by a gray box. (B) Schematic representation of the GC cytoplasmic tail sequence of the wt GC and the six GC cytoplasmic tail mutants. Residues that were mutated or inverted are underlined. TM, transmembrane domain. (C) CAT activity in cells infected with the VLPs collected from cells transfected with GC cytoplasmic tail mutants and wt GN. (D) Western blot analysis of the supernatants used for VLP infection shown in panel B. VLPs were concentrated and analyzed for the presence of both glycoproteins (GN and wt or mutant GC) and for nucleoprotein (N) (upper panels). The lysates from the corresponding transfected cells also were analyzed for nucleoprotein expression (lower panel).

Localization of the G_C cytoplasmic tail mutants. To determine the mechanism that caused the G_C mutants G_C2-5, G_C1-5 invert, G_CK3A, and G_CS5A to result in reduced particle formation and release, we decided first to analyze the intracellular localization of these glycoproteins by immunofluorescence microscopy (Fig. 8). The two glycoprotein mutants G_CV2A and G_CK4A, which both displayed a wt phenotype as judged from VLP-mediated transfer of CAT activity as well as the protein composition of the VLPs, colocalized with the Golgi marker (Fig. 8A), a pattern identical to that observed with wt G_N/G_C-expressing cells (Fig. 2A). In cells transfected with the G_CS5A mutant, the glycoproteins did not colocalize with the Golgi marker but instead displayed a reticular staining pattern, including staining of the nuclear rim, typically observed for proteins localized to the ER (Fig. 8A, bottom panel). It appears that this G_C mutant, together with the wt G_N protein, forms a heterodimer (Fig. 8B), which is effectively retained in the ER. For cells transfected with the three other mutants, G_C2-5, G_C1-5 invert, and G_CK3A, a different staining pattern was observed (Fig. 8A). The glycoproteins in these cells only partially colocalized with the Golgi marker and, in addition, displayed staining at the plasma membrane. This suggests a defect in Golgi complex retention for the glycoprotein mutants G_C2-5, G_C1-5 invert, and G_CK3A, resulting in the plasma membrane expression of both glycoproteins.

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FIG. 8. Intracellular localization of the GC cytoplasmic tail mutants. (A) BHK-21 cells were transfected with mutant GC and wt GN glycoproteins and analyzed 24 h posttransfection by immunofluorescence microscopy. Cells were costained with polyclonal antibodies recognizing mutant GC and wt GN (green) and with monoclonal antibodies recognizing the Golgi marker GM130 (red). The insert shows a higher magnification of the Golgi area. (B) Colocalization of the wt GN with mutant GCS5A protein. Cells were transfected with GCS5A and costained with a polyclonal antibody specifically recognizing the cytoplasmic tail of the GN glycoproteins (green) and a monoclonal antibody specifically recognizing the GC protein (red). Panels to the right show the merged images.

Colocalization of G_N and G_C in the plasma membrane. To study the cell surface staining of the G_C mutants in more detail, we performed immunofluorescence analysis on nonpermeabilized glycoprotein-transfected cells. Cells were transfected with wt G_N/G_C or the different G_C mutants in the presence of wt G_N and were analyzed 24 h posttransfection. Nonpermeabilized cells were stained with monoclonal antibodies that specifically recognize the G_N or the G_C protein (31). Neither G_N nor G_C could be detected at the cell surface in wt G_N/G_C-transfected cells, demonstrating that both glycoproteins are not present at the cell surface (Fig. 9, top panels). However, in cells transfected with the mutants G_C2-5, G_C1-5 invert, and G_CK3A, both G_N and G_C were detected at the cell surface (Fig. 9, middle panels, and data not shown), while no glycoprotein was detected at the cell surface in G_CV2A, G_CK4A, and G_CS5A mutant-transfected cells (Fig. 9, lower panels, and data not shown). These data suggest that the G_C tail and, more specifically, residue K3 are important for retention of the G_N/G_C heterodimer in the Golgi complex, allowing efficient formation of particles.

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FIG. 9. Both GN and GC glycoproteins are located at the cell surface in cells transfected with GC cytoplasmic tail mutant GCK3A. BHK-21 cells were transfected with wt GN/GC and mutant GCK3A or GCK4A together with wt GN and were fixed 24 h posttransfection. The cell surface staining of the two glycoproteins GN and GC was detected in nonpermeabilized cells stained with monoclonal antibodies specifically recognizing only the GN or the GC protein.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
In a recent report, we identified two regions in the G_N cytoplasmic tail of the UUK virus that were important for VLP-mediated transfer of reporter activity (29). In the present study, we have analyzed both regions in more detail and explored the mechanism behind this defect in generating and releasing VLPs. We found two different mechanisms responsible for this lack of VLP-mediated transfer of reporter protein activity. Two residues in the G_N cytoplasmic tail, L23 and L24, were demonstrated to be important for the generation and budding of VLPs into the Golgi membrane. Three other residues in the G_N tail, L46, E47, and L50, were found to be important for the proper intracellular localization of both glycoproteins, G_N as well as G_C, and mutation of these residues prevented efficient targeting of these two glycoproteins to the Golgi complex and subsequent formation of VLPs. In addition, we also revealed that the short cytoplasmic tail of G_C harbors important information for the proper intracellular targeting of both glycoproteins and the release of VLPs.

Until now, little was known about the budding mechanism of bunyaviruses. Here we have identified two amino acids (L23 and L24) in the cytoplasmic tail of G_N that, when changed to alanines, disrupt the release of VLPs. The fact that both glycoproteins, mutant G_N as well as wt G_C, were correctly targeted and located to the Golgi complex suggested to us that these G_N mutants possibly interfered with either the budding or release of particles. Indeed, morphological analysis by high-resolution TEM revealed that the Golgi complex of cells expressing G_NL23A were devoid of particles, while the Golgi complex of cells expressing wt G_N/G_C contained VLPs (Fig. 4B) (30). This was corroborated by the fact that we have been unable to extract any infectious UUK virus VLPs from G_NL23A-transfected cells using freeze-thaw (data not shown), indicating the absence of infectious VLPs inside the cell. Previously, a temperature-sensitive mutant of UUK virus (ts12) was reported to be unable to generate and release virus particles at the restrictive temperature (39°C) (11, 12). Both glycoproteins accumulated in the Golgi membrane but were unable to initiate budding, resulting in large vacuolization of the Golgi complex. The function of the Golgi complex remained intact, and at a lower temperature (32°C) virus particles could be produced (13, 14). However, we did not observe an increase in VLP formation at lower temperatures, indicating that our G_N mutants are not temperature sensitive. This indicates that although the G_NL23A mutant morphologically resembles the ts12 mutant, the mechanism by which particle formation in the Golgi complex is blocked appears to be different.

The cytoplasmic tail of UUK virus G_N was shown to be palmitoylated at its two cysteines (C25 and C28), but the functional significance of this modification has remained unknown (2). Palmitoylation was reported to be important for the budding process of other viruses, such as Sindbis virus (19), retroviruses (36), rubella virus (43), coronavirus (41), and influenza virus (5). We were unable to detect any changes in intracellular distribution, particle formation, and release when both cysteines in the G_N cytoplasmic tail of UUK virus were changed to alanines (reference 29 and the present study), demonstrating that palmitoylation of G_N is not important for the formation of infectious VLPs.

Moreover, we found that the proper intracellular targeting of the two glycoproteins is critical for the budding and release of particles. Particles were not generated when the G_N/G_C proteins were localized to the ER. It is known that the UUK virus already starts to bud in the ER-Golgi intermediate compartment, but no UUK virus budding was observed to occur in the ER (20). Three G_N glycoprotein mutants showing similar intracellular distributions located to both the ER and Golgi complex did not have the same ability to generate VLPs as the wt Golgi complex-located glycoproteins. Although we demonstrated that all three G_N mutants were less efficient in ER exit, only two mutants, G_NL46A and G_NL50A, could not generate and release VLPs at all, while the G_NE47A mutant could transfer some CAT activity. At the moment, we cannot rule out that the introduction of some of these mutations might have an additional effect on the formation or release of VLPs, resulting in the complete absence of VLPs. It was hypothesized that mutation of these residues in the G_N cytoplasmic tail disrupted heterodimer formation with the wt G_C, resulting in ER retention of G_C, while G_N was still able to exit the ER and was transported to the Golgi complex, as is known to occur when these proteins are expressed alone (26). Simultaneous detection of the mutated G_N and wt G_C clearly showed that both glycoproteins are always colocalized, in the ER as well as in the Golgi membrane, implying that they still are able to form heterodimers. These results indicate that mutation of residues L46, E47, and L50 in the G_N cytoplasmic tail interferes with the correct targeting of the G_N/G_C heterodimers to the Golgi complex, the site where budding occurs, resulting in no or strongly reduced release of infectious particles. The G_C was proposed to contain an ER retention signal in its short cytoplasmic tail (15, 37), and masking of this signal through heterodimerization with the G_N might be a requirement for its exit out of the ER and entry into the secretory pathway. Our results suggest that the masking of this G_C ER retention signal may be disrupted when residues L46, E47, and L50 are mutated, leading to the ER retention of the G_N/G_C heterodimers. No conservation of amino acids 46 to 50 was observed between the cytoplasmic tail of UUK virus G_N and other phleboviruses (reference 17 and data not shown).

The five-amino-acid G_C cytoplasmic tail postulated to contain an ER retention signal (15, 37) was also analyzed by a mutagenesis screen. We found that two residues, the third residue, a lysine, and the last residue, a serine, were important for the proper localization of both glycoproteins. Mutation of the residue K3 in G_CK3A resulted in the expression of both G_C and G_N at the cell surface, in addition to its Golgi complex localization. Mutation of residue S5 effectively prevented both glycoproteins from exiting the ER, suggesting that mutation of this residue interferes with the ER-to-Golgi complex trafficking. Although we observed a similar phenotype for the G_CK3A and G_C invert mutants, a lysine is present at position 3 in the G_C invert mutant, indicating that the effect of this amino acid also is context dependent. This indicates that residue K3 plays a role in the retention of the glycoprotein heterodimer in the Golgi membrane, thereby allowing the efficient formation of infectious particles. In addition to its proposed role in ER retention, K3 also is involved in the retention of the glycoprotein heterodimers in the Golgi membrane. We hypothesize that mutation of this conserved lysine residue affects the interaction with the G_N cytoplasmic tail and possibly alters the secondary structure of the Golgi retention motif, previously found to be localized between residues 10 and 40. This would lead to inefficient Golgi retention of the heteromeric glycoprotein complex, leading to transport to the plasma membrane. Interestingly, this residue is absolutely conserved among the Phlebovirus, Hantavirus, and Orthobunyavirus genera. Most G_C proteins of the different members in the Bunyaviridae family are retained in the ER when expressed alone, although Punta Toro virus G_C has been shown to reach the plasma membrane despite the presence of the conserved lysine at position –3 (6).

In the present study we demonstrate, using a VLP system for UUK virus, that the cytoplasmic tails of both G_N and G_C contain specific motifs essential for the virus life cycle. In a previous study, four amino acids in the G_N cytoplasmic tail were shown to be involved in the packaging through a direct interaction between the RNPs and the viral spike proteins (29). Here we continued our analysis of the glycoprotein cytoplasmic tails and showed that leucine at position 23 and, to a lesser extent, at position 24 in the G_N tail are important for the initiation of VLP budding in the Golgi membrane. For many viruses it has been established that the matrix protein is the major determinant for budding (16, 18, 21, 28, 40), and specific motifs in the matrix proteins, so-called late domains, have been identified that are known to interact with cellular factors to enable the budding of enveloped viruses (4, 10). We have not been able to identify a late domain in the G_N or G_C cytoplasmic tail that could be involved in VLP formation. Also, bunyaviruses do not contain a matrix protein, and we have shown that the two glycoproteins alone can generate particles (29, 30). So far, the exact mechanism for how bunyaviruses bud into the Golgi membrane and if additional cellular factors are required for this process are not known. We also show that the cellular localization of the two glycoproteins G_N and G_C is important for generation of VLPs and that specific residues, other than the previously identified Golgi complex retention motif (residues 10 to 40), play an important role in intracellular targeting of the viral spike proteins. These results, together with our identification of the packaging signal, give a better understanding of the biogenesis of UUK virus and other, more clinically relevant members of the Bunyaviridae family.

 
We thank Anita Bergström for excellent assistance with cell cultures and Violet C. Han and Julie W. Wen for expert assistance in electron microscopy. We are also grateful to Åsa-Lena Dackland for excellent assistance with the FACS analysis and to Rene Rijnbrand for critically reading the manuscript.

 
* Corresponding author. Mailing address: Ludwig Institute for Cancer Research, Stockholm Branch, Karolinska Institute, Box 240, SE-17177 Stockholm, Sweden. Phone: 46 (0)8 524 871 01. Fax: 46 (0)8 33 28 12. E-mail: rpet{at}licr.ki.se

Published ahead of print on 1 August 2007.

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Journal of Virology, October 2007, p. 11381-11391, Vol. 81, No. 20
0022-538X/07/$08.00+0     doi:10.1128/JVI.00767-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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