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Journal of Virology, June 2002, p. 6364-6369, Vol. 76, No. 12
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.12.6364-6369.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Department of Biology, Arizona State University, Tempe, Arizona 85287,1 Department of Molecular Microbiology and Immunology, School of Medicine, University of Missouri, Columbia, Missouri 652122
Received 11 January 2002/ Accepted 18 March 2002
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Recently, a novel subnuclear compartment has been identified following infection by the two highly related parvoviruses MVM and H-1 (2, 9). This structure, termed autonomous parvovirus-associated replication (APAR) bodies, was identified at early time points postinfection (up to 15 h postinfection) and was found to be distinct from most of the classically described nuclear bodies, including Cajal bodies, promyelocytic leukemia oncogenic domains (PODs), and interachromatin granules or speckles (9). At later time points during infection, however, a dramatic nuclear reorganization occurs in which formerly distinct nuclear structures, such as Cajal bodies, PODs, speckles, and APAR bodies, merge into massive structures that contain NS1. These structures are likely active sites of viral genome replication and de novo capsid assembly (27), but their role during the infection process is not yet known. In addition, the survival motor neuron (Smn) gene product is present in these large structures (hence the term SMN-associated APAR bodies or SAABs), and the Smn protein and NS1 have been shown to interact in vitro and in vivo during infection. Spinal muscular atrophy, a common neuromuscular disease, is caused by homozygous mutations or deletions of the human SMN1 gene, and a knockout of the murine Smn gene results in preembryonic lethality (14, 23). The 294-amino-acid SMN protein has recently been shown to interact with the previously identified NS1 binding partner NSAP-1/hnRNP-Q (13, 20, 22).
Since NS1 and NS2 share 85 amino-terminal amino acids, we sought to determine whether NS2 could interact with the murine (Smn) and human (SMN) proteins, which are nearly identical (11, 14). Recombinant glutathione-S-transferase (GST)-tagged NS2 was captured by polyhistidine (6-His)-SMN, while the resin alone failed to capture NS2 (Fig. 1A). This interaction was confirmed using biomolecular interaction analysis (BIA) (30). GST-NS2, tethered on a sensor chip using an anti-GST antibody, interacted with wild-type SMN (Fig. 1B, shift F). In the absence of NS2 protein, recombinant SMN failed to bind to the chip or the GST antibody (Fig. 1B, shifts A and C) and SMN failed to interact with two other GST-tagged peptides (Fig. 1C and see below), demonstrating the specificity of the interaction. The binding levels between SMN and NS2 were similar to those observed between SMN and full-length NS1 (27). SMN binding by NS1 and NS2 suggested that the common amino-terminal domain of the two nonstructural proteins might mediate binding to SMN. To determine whether the amino-terminal domain common to NS1 and NS2 mediated SMN binding, BIA experiments were performed with recombinant GST-NS1, NS2, the carboxy terminus of NS1, or the common amino terminus (Fig. 1C). The full-length proteins and the common domain bound efficiently to SMN, while the large carboxy terminus of NS1 did not interact appreciably with SMN (Fig. 1C). The peptide corresponding to SMN exon 4 served as a control for nonspecific binding and, as expected, did not interact with SMN (Fig. 1C). Fusion proteins were similarly soluble and stable.
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FIG. 1. SMN and NS2 interact in vitro. (A) Western blot analysis of recombinant GST-tagged NS2 captured by polyhistidine (6-His)-tagged human SMN immobilized on histidine binding resin. The blot was developed using anti-GST monoclonal antibody. (B) BIA of the SMN/NS2 complex. Bound GST monoclonal antibody (shift B), NS2 (shift D), and wild-type SMN (shift F) are indicated. Vertical shifts in the horizontal baseline represent positive interactions as response units (RU) and are indicated by arrows (shifts B, D, and F). hTra2ß1 protein served as a negative control and does not bind to NS2 (E). (C) SMN binds to the N-terminal region common to NS1 and NS2. GST-tagged NS variants (NS1, NS2, amino acids 339 to 792, and the NS1 and -2 common domain) were captured using an anti-GST antibody and were then pulsed with wild-type SMN. GST-tagged SMN exon 4 was used as a negative control. Values represent binding of SMN constructs above a background control and the mean of three separate experiments.
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FIG. 2. SMN and NS2 interact in vivo. (A) Anti-SMN (MANSMA3) coprecipitates endogenous SMN and hemagglutinin-tagged NS2 from A92L cells transiently expressing NS2. Anti-NS2 rabbit polyclonal sera were used to detect NS2. NS2 was not precipitated in the absence of MANSMA3 (-) or from nontransfected A92L cells (mock). mAb, monoclonal antibody; IP, immunoprecipitation. (B) BIA of NS2 coimmunoprecipitated with endogenous Smn from synchronized MVM-infected A92L cells (top panel). Total protein extracts from A92L cells 30 h postinfection were used. Anti-SMN (MANSMA3) coprecipitated SMN, and anti-NS2 rabbit polyclonal sera detected NS2 complexed with SMN. Captured levels of anti-SMN antibody (shift A), SMN/NS2 complex (shift B), and anti-NS2 rabbit polyclonal antibody (shift C) are indicated. NS2 is not coprecipitated with SMN from double-blocked, infected A92L cells 0 h postrelease (bottom panel). Total protein extracts were used from A92L cells 30 h after being released from the blocking process. Anti-SMN (MANSMA3)-precipitated SMN and anti-NS2 rabbit polyclonal sera detected no NS2 complexed with SMN. Captured levels of anti-SMN antibody (shift A), SMN (shift B), and anti-NS2 rabbit polyclonal antibody (shift C) are indicated as response units (RU).
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FIG. 3. (A) SMN exons 2 and 6 independently mediate NS2 binding. GST-NS2 immobilized on a rabbit anti-mouse sensor chip using anti-NS2 antibody was pulsed with GST-SMN fusion proteins (full-length SMN [SMN-FL], SMN exons 1 to 4, 5 to 7, 1, 2, 3, 4, and 6) and His exons 6 to 7 alone. Values represent binding of SMN constructs above a background control as response units (RU) and the mean of three separate experiments. (B) A schematic representation of the SMN protein. Previously reported physical and functional domains, amino acid positions, exon borders, and the relative positions of the three missense mutations (*) analyzed in Fig. 4 are indicated.
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7 to interact with NS2. SMN
7 is a naturally occurring, alternatively spliced isoform produced at high levels in spinal muscular atrophy patients. All recombinant mutant SMN proteins were soluble and stable at levels similar to those in the wild type, and an equivalent amount of purified recombinant proteins was used in each assay (data not shown). Compared to wild-type SMN, the SMN mutations and SMN
7 displayed a dramatically reduced ability to bind NS2 (Fig. 4, black bars). The defect in SMN self-association observed with the SMN mutations directly correlates with the extent of the NS2 binding defect (Fig. 4, white bars), suggesting that the dimerized form of SMN interacts with NS2. While the mutations that we analyzed are in the human form of the protein, murine and human SMN proteins are greater than 90% identical and share even greater degrees of identity in the self-association domain and the exon 2 region (11, 14).
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FIG. 4. Patient-derived SMN mutations decrease NS2 binding. Truncated and point-mutated SMN displays a reduced affinity for NS2. SMN wild type (WT), SMN point mutations (type I mutations, 274 and 279; type II mutation, 272), or SMN 7 was immobilized on a sensor chip using an anti-SMN monoclonal antibody and probed with recombinant NS2. Truncated and point-mutated SMN displays a reduced ability to self associate (open bars). Wild-type monomeric SMN covalently bound to a sensor chip was exposed to truncated and point-mutated SMN to determine the various proteins' ability to dimerize. Binding values are represented as response units (RU).
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FIG. 5. SMN and NS2 colocalize in SAABs at 30 h post-MVM infection. (A) SMN (primary, anti-SMN rabbit polyclonal sera; secondary, fluorescein isothiocyanate/green) and NS2 (primary: mouse monoclonal antibody; secondary: tetramethyl rhodamine isothiocyanate/red) are indicated. Synchronized infections were obtained by performing an isoleucine-aphidicolin double block on A92L cells prior to infection. Immunofluorescence experiments were performed on MVM-infected cells 0, 15, and 30 h postrelease (27). Nuclear Cajal bodies (white arrows) and NS2-positive/SMN-negative nuclear bodies (yellow arrows) are indicated. (B) SAAB formation is a consequence of viral infection. Smn localization was examined in synchronized, noninfected A92L cells 30 h postrelease. SMN (primary: anti-SMN rabbit polyclonal sera; secondary: tetramethyl rhodamine isothiocyanate/red) and NS2 (primary: mouse monoclonal antibody; secondary: fluorescein isothiocyanate/green) are indicated. Cell nuclei were counterstained with 4',6'-diamidino-2-phenylindole (blue). The bar represents 30 µm.
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The human and murine SMN proteins are highly conserved and are greater than 90% identical and share even greater degrees of identity in regions such as their self-association domains (exons 6 and 2b), the nucleic acid binding region (exon 2), and the NS2-interacting region (exon 2) (11, 14). The mutations within Smn exon 6 that inhibit binding to NS2 may do so through two mechanisms: (i) direct disruption of the exon 6 NS2 binding domain or (ii) inhibition of SMN self-association, thereby preventing the proper quaternary structure required for the formation of the NS2 binding domain. The active form of the SMN protein is a higher-order complex, consisting of at least a dimer (18, 30). SMN self-association is a prerequisite for SMN biochemical properties, including SMN protein stability and efficient interaction with nearly all SMN-interacting proteins (references 18, 22, and 30 and references within). Consistent with this, single-amino-acid substitution mutations that disrupt SMN self-association disrupted the efficiency of NS2:SMN complex formation. The degree to which self-association was decreased was reflected in the levels of NS2 binding, suggesting that the NS2:Smn interaction is not simply a nonspecific interaction.
The NS2 polypeptides are 25-kDa phosphoproteins. Nonphosphorylated forms are present in the nucleus and cytoplasm of MVM-infected cells, while phosphorylated NS2 isoforms are found exclusively in the cytoplasm (3, 5, 6). Phosphorylation of NS2 has previously been shown to influence its ability to interact with cellular factors: nonphosphorylated NS2 binds Crm1, while 14-3-3 isoforms exclusively bind phosphorylated NS2 (3, 4). Our results suggest that Smn can at least bind the nonphosphorylated NS2. Smn efficiently interacted with the nonphosphorylated recombinant NS2 that was expressed and purified from bacteria, and at 30 h postrelease, when Smn and NS2 are nearly exclusively nuclear, an NS2/Smn complex was detected. Further work will be required to determine whether Smn preferentially binds to nonphosphorylated NS2 and to characterize the temporal accumulation of the NS2:Smn complex. Future work is aimed at identifying the role of the NS2:Smn interaction during the viral life cycle. Although we have identified SAABs as sites of genome replication and capsid assembly (27), it is not known whether these structures are required for efficient viral replication or, alternatively, whether they are a manifestation of a cellular response to the late stages of viral infection. A better understanding of the nature of the parvovirus:cell interaction at later times of infection may help clarify this issue.
P.J.Y. was supported by a fellowship from Families of SMA, and K.T.J. was supported by a postdoctoral fellowship from the University of Missouri Life Science Mission Enhancement Program. Funding for these studies was provided by Families of SMA (C.L.L.), Andrew's Buddies (C.L.L.), the Muscular Dystrophy Association (C.L.L), and the National Institutes of Health (C.L.L., RO1 NS41584-01; and D.J.P., RO1 AI21302 and RO1 AI46458).
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