Locations of carbohydrate sites on alphavirus glycoproteins show that E1 forms an icosahedral scaffold

There are 80 spikes on the surface of Sindbis virus arranged as an icosahed ral surface lattice. Each spike consists of three copies of each of the gly coproteins 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 recogn ition motifs using site-specific mutagenesis, followed by cryoelectron micr oscopy. The positions of these sites have demonstrated that E2 forms the pr otruding spikes and that E1 must be long and narrow, lying flat on the vira l surface, forming an icosahedral scaffold analogous to the arrangement of the E glycoprotein in flaviviruses. This arrangement of E1 leads to both di meric and trimeric intermolecular contacts, consistent with the observed st ructural changes that occur on fusion with host cell membranes, suggesting a similar fusion mechanism for alpha- and flaviviruses.