Placement of the structural proteins in Sindbis virus

The structure of the lipid-enveloped Sindbis virus has been determined by fitting atomic resolution crys-tallographic structures of component proteins into an 11-A ̊ resolution cryoelectron microscopy map. The virus has T4 quasisymmetry elements that are accurately maintained between the external glycoproteins, the transmembrane helical region, and the internal nucleocapsid core. The crystal structure of the E1 glycoprotein was fitted into the cryoelectron microscopy density, in part by using the known carbohydrate positions as restraints. A difference map showed that the E2 glycoprotein was shaped similarly to E1, suggesting a possible common evolutionary origin for these two glycoproteins. The structure shows that the E2 glycoprotein would have to move away from the center of the trimeric spike in order to expose enough viral membrane surface to permit fusion with the cellular membrane during the initial stages of host infection. The well-resolved E1-E2 transmembrane regions form -helical coiled coils that were consistent with T4 symmetry. The known structure of the capsid protein was fitted into the density corresponding to the nucleocapsid, revising the structure published earlier. Three-dimensional, near-atomic resolution studies of viruses have been possible for a wide variety of nonenveloped icosa-hedral viruses because of their ability to form well-diffractin