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Journal of Virology, March 2004, p. 2648-2652, Vol. 78, No. 5
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.5.2648-2652.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Arbovirus Disease Branch, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, Public Health Service, U.S. Department of Health and Human Services, Fort Collins, Colorado 80522
Received 15 July 2003/ Accepted 31 October 2003
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FIG. 1. Location of SS bridges in the deduced crystal structure of the DEN2 virus E-protein dimer. Overhead (A) and lateral (B) view of the predicted three-dimensional backbone of the DEN2 16681 E protein. The mutated amino acid from each SS-bond pair is noted in space fill on one of the E-protein monomers and is defined in Table 1. Domains I (dark gray), II (white), and III (black) as defined by the DEN2 E-protein crystal structure (9) are shown.
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The antigenic structure of the DEN2 virus E protein has been investigated previously using monoclonal antibodies (MAbs) (8, 17). Not unexpectedly, the DEN2 virus E-protein antigenic structure was similar to the TBE virus E-protein antigenic structure. Three antigenic regions (A, B, and C) with unique biochemical characteristics were identified, and these regions correlated well with their analogous regions in the TBE virus E protein. The locations of many of the epitopes were mapped on the DEN2 E-protein three-dimensional structure using MAb competition binding assays, binding of MAb to peptide fragments, and serological testing of MAbs (17).
In this study we used site-directed mutagenesis of a chimeric plasmid expressing the prM and E proteins of DEN2 virus strain 16681 to determine the individual contributions of the six SS bonds to E-protein epitope expression. This chimeric plasmid, pCB8D2-2J-2-9-1, expressed modified DEN2 proteins (4). The DEN2 prM signal sequence was replaced by the Japanese encephalitis virus prM signal sequence. The last 20% (aa 397 to 450) of the DEN2, E-protein ectodomain, was replaced with the same region from the Japanese encephalitis virus E protein. These modifications in the prM and E proteins enhanced secretion of recombinant subviral particles from COS-1 cells transiently transformed with the pCB8D2-2J-2-9-1 plasmid compared with that from COS-1 cells transiently transfected with a plasmid containing the complete wild-type DEN2 prM and E-protein genes (pCBD2-14-6). Recombinant subviral particles secreted from transformed COS-1 cells contained prM and E proteins and very small amounts of M protein. The E-protein epitopes (A1, A2, A5, C1, C3, C4, B2, B3, and B4) expressed by acetone-fixed, plasmid-transfected COS-1 cells and probed with a panel of well-defined MAbs were identical to those expressed in DEN2 virus-infected cells and cells transformed with plasmid containing prM and 100% DEN2 E protein (4). The specific information on the construction and characterization of these DEN2 virus-derived plasmids has been described in a separate paper (4).
Site-directed mutagenesis of one Cys in each of the six SS bonds of the E protein expressed by the pCB8D2-2J-2-9-1 plasmid was performed as described in the QuikChange site-directed mutagenesis kit (Stratagene, La Jolla, Calif.). The primers were designed using the recommendations from Stratagene and Lasergene (DNASTAR, Madison, Wis.), and the primers used and the introduced mutations are listed in Table 1. The mutated plasmids were electroporated into XL-1 Blue Escherichia coli cells and grown on Luria broth plates containing antibiotic at 37°C overnight. Individual isolated colonies were picked and grown in Luria broth media containing antibiotic at 37°C overnight. The DNA from cells was purified using QIAfilter Plasmid Maxi Prep kit (Qiagen, Santa Clarita, Calif.). The purified plasmids were sequenced across the mutation to verify that only the engineered mutations were present. Ten micrograms of plasmid was electroporated into 5 x 106 LLC-MK2 cells by using a gene pulser (Bio-Rad, Hercules, Calif.) set to 2.5 kV, 950 µF, and infinite Ohms. The transformed cells were grown in 75-cm2 flats for 48 h, harvested to prepare spot slides, and used for end-point dilution epitope mapping by indirect fluorescent antibody tests as previously described (20). To determine the effects that SS-bond elimination had on the conformation and maturation of the E protein and the ability of the E protein to be transported to the cell surface, virus-transformed cells were tested either unfixed or following fixation for 15 min in ice-cold acetone.
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TABLE 1. Mutagenic oligonucleotide primers and location of the nucleotide and resultant amino acid mutational changes made in the DEN2 E-protein SS bridgesa
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TABLE 2. Effect on MAb binding to expressed DEN2 16681 prM/E protein after elimination of individual SS bonds
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The increased reactivity of epitopes A1 and A5 following acetone fixation may be related to their accessibility in prM-containing virions. The reactivity of MAbs specific for these epitopes is reduced in virions containing prM protein (6). It has been shown previously that the A1 and A5 epitopes cosegregate on a proteolytic or synthetic fragment of the E protein containing aa 1 to 120 (1, 17, 19). Recent cryoelectron-microscopic studies of prM-containing DEN2 virus particles confirm that this region of the E protein interacts with the prM protein (26). Our analysis would suggest that an increase in cell membrane permeability following acetone fixation better exposes these epitopes even though the prM cleavage in these transformed cells is not efficient (4). These binding differences could also be due to differences in epitope expression in virions versus virus-infected cell membranes. For either case it should be noted that the original epitope mapping studies of the DEN2 virus E protein used purified virus grown in C6/36 cells, a cell type in which the prM
M cleavage is not efficient, and most released virions contain large amounts of prM. Because of this the presence of prM alone in the membrane of cells transformed with pCB8D2-2J-2-9-1 plasmid should not alter the expression of the DEN2 E protein epitopes as previously defined in the C6/36-derived virus.
The pattern of epitope A1 sensitivity to reduction of the DII SS bonds is also of interest. This epitope is most sensitive to elimination of SS2, SS4, and SS5, which are the SS bonds necessary to maintain the overall conformation of DII. Epitope A1 is at least eightfold less sensitive to elimination of SS3, which is the SS bond that maintains the secondary structure in the immediate vicinity of the flavivirus fusion motif (aa 98 to 110). The location of SS3 (aa 74 to 105) suggests that a peptide as small as 30 aa might contain enough native structure of the fusion motif to successfully mimic this region. If this is true, then antipeptide antibodies specific for this region might successfully block fusion with some degree of specificity. A synthetic peptide corresponding to DEN2 aa 79 to 99 has been shown previously to elicit anti-DEN2 virus antibody and also low-level virus-neutralizing antibody to Murray Valley encephalitis virus (18, 19). Antipeptide antibodies reactive with this region of DII have also been used to demonstrate that the fusion motif of the E protein becomes exposed after low-pH-catalyzed conformational change (19). We are currently pursuing use of peptide mimicry and antipeptide antibodies to block virus-mediated cell membrane fusion.
Our results confirm that appropriate expression of DIII is most dependent on SS6 (12). The reactivity of MAbs 4B2B-4 and 10A1D-2, whose epitopes have not been completely defined but have biochemical characteristics similar to those of DIII epitopes, were very similar to the other DIII-reactive MAbs in their SS-bond sensitivities, providing additional evidence that 4B2B-4 and 10A1D-2 define DIII epitopes. The binding of MAb 1A6A-8 was not affected by any SS-bond elimination, which is consistent with its ability to bond well to the E protein in reduced immunoblots. A comparison of epitope sensitivity to SS-bond abolition and their other functional activities is shown in Fig. 2.
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FIG. 2. Effects on epitope reactivity in mutated plasmids. This balloon diagram was based on a similar epitope mapping diagram published previously (17). Overlapping circles indicate spatial proximity of epitopes as determined in antibody competitive binding assays. Antigenic regions containing epitopes A1 to A5 in DII, B1 to B4 in DIII, and C1 to C4 in DI are shown. Colors: light gray, type-specific epitopes; dark gray, subcomplex-reactive epitopes; stippled, subgroup-reactive epitopes; and white, group-reactive epitopes. Abbreviations: S, epitope accessible on protein surface; N, epitope that elicits virus-neutralizing antibody; B, epitope that elicits antibody that blocks virus adsorption (low-activity, B±; high activity, B++); E, epitope that elicits antibody that enhances virus adsorption; HI, epitope that elicits antibody that blocks hemagglutination; F = epitope that elicits antibody that blocks virus-mediated cell membrane fusion; SS1-6, SS bond (1 to 6) eliminated; N.T., not tested. Biological activities are as described in reference 17, and adsorption blocking activities are as described in reference 5.
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The results presented here correlate closely with previously published information concerning the antigenic structure of the flavivirus E protein; however, some alternative explanations for these results could be considered. It is possible that the reduction in some MAb activities with these mutant E proteins could be due to misfolding of the E protein due to novel interactions of the orphan sulfhydryl group remaining from the eliminated SS-bond with other regions of the protein or to simple degradation of the E protein. This concern for reactive orphan sulfydryl groups has not been substantiated in a number of other studies evaluating the effects of SS-bond elimination on protein function. This may be because the sulfhydryl R group in Cys has a pKa of 8.3. The orphan Cys sulfhydryl R group in our mutants should be fully hydrogenated, and less reactive, at the pH (7.4) at which these experiments are performed. We elected to minimize the possibilities of intrachain SS bond mismatches by mutating only one SS bond at a time. Even if misfolding occurred, eukaryotic cells have a system for retaining misfolded proteins in the endoplasmic reticulum and cycle these defective proteins back to the ubiquitin-proteosome system for hydrolysis (15). Since MAb-reactive (and therefore intact) E protein can be transported and identified on the cell surface with all of these SS-bond mutants, it is likely that any reduction in reactivity of MAbs with the mutant E proteins in these experiments was not the result of drastically misfolded or degraded protein.
The data presented here also support the concept that the prM and E proteins are normally associated in the SS-bond mutants. Previous studies with TBE virus have shown that there are two regions of the E protein important for oligimerization with prM protein: aa 431 to 449 and aa 450 to 472, which lie in the E-protein stem region (2). Neither of these stem regions contains SS bonds, and thus, they had no introduced Cys mutations in the present study. Further analysis of TBE virus E protein expressed without prM produced a protein with an authentic DI and DIII; however, an anti-A5 epitope was less reactive (11). Unfortunately, no other DII-reactive MAbs were used in this TBE study to better discern total DII conformation. In the present study, when a reduction of MAb binding was detected in one domain, cell surface binding was retained by MAbs defining the other two domains, and binding was frequently observed with other MAbs defining the primary affected domain as well. In addition, most MAbs (e.g., 1A6A-8) reacted similarly with all SS-bond mutants regardless of whether cells were tested acetone fixed or unfixed (data not shown), suggesting that maturation and transport of the prM/E heterodimer were occurring normally.
The results presented here help explain why attempts to mimic accurately flavivirus E-protein epitopes on short synthetic peptides have, in general, met with limited success (18, 19). The general conformational stability of the DII fusion motif and the DIII receptor binding domain, however, suggest possible targets in the E protein for the development of effective and specific antiflavivirus therapies.
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