There are 80 spikes on the surface of Sindbis virus arranged as an icosahedral surface lattice. Each spike consists of three copies of each of the glycoproteins E1 and E2. There are two glycosylation sites on E1 and two on E2. These four sites have been located by removal of the glycosylation recognition motifs using site-specific mutagenesis, followed by cryoelectron microscopy. The positions of these sites have demonstrated that E2 forms the protruding spikes and that E1 must be long and narrow, lying flat on the viral surface, forming an icosahedral scaffold analogous to the arrangement of the E glycoprotein in flaviviruses. This arrangement of E1 leads to both dimeric and trimeric intermolecular contacts, consistent with the observed structural changes that occur on fusion with host cell membranes, suggesting a similar fusion mechanism for alpha- and flaviviruses.
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We are indebted to Felix Rey for information on the SFV E1 crystal structure before publication, and also to Steve Fuller and Erika Mancini for helpful and stimulating discussions on their cryoEM studies of the SFV and TBEV structures and fusion mechanisms. We thank Norm Olson and Rob Ashmore for technical advice regarding the cryoEM data gathering and analysis, respectively. We are grateful to Cheryl Towell and Sharon Wilder for help in the preparation of the manuscript. The work was supported by an NIH Program Project Grant AI45976 that includes T. S. B., R. J. K., and M. G. R., an NIH grant GM56279 to R. J. K., and an NSF shared instrumentation grant to T. S. B. and M. G. R. We also thank Purdue University for an instrumentation reinvestment grant to the Purdue Structural Biology faculty.