Nucleocapsid and glycoprotein organization in an enveloped virus

Alphaviruses are a group of icosahedral, positive-strand RNA, enveloped viruses. The membrane bilayer, which surrounds the ∼ 400 Å diameter nucleocapsid, is penetrated by 80 spikes arranged in a T = 4 lattice. Each spike is a trimer of heterodimers consisting of glycoproteins Ell and E2. Cryoelectron microscopy and image reconstruction of Ross River virus showed that the T=4 quaternary structure of the nucleocapsid consists of pentamerand hexamer clusters of the capsid protein, but not dimers, as have been observed in several crystallographic studies. The E1-E2 heterodimers form one-to-one associations with the nucleocapsid monomers across the lipid bilayer. Knowledge of the atomic structure of the capsid protein and our reconstruction allows us to identify capsid-protein residues that interact with the RNA, the glycoproteins, and adjacent capsid-proteins.