We suggest that aromatic residues in the capsid protein serve to bind the side chain of the essential E2 tyrosine providing both specificity for spike incorporation and energy for budding. The same hydrophobic pocket also appears to play a role in capsid assembly. Furthermore, the results suggest that budding may occur in the absence of preformed nucleocapsids. This is the first demonstration of the molecular mechanisms of spike-nucleocapsid interactions during virus budding.

No abstract

Many enveloped viruses are released from infected cells by maturing and budding at the plasma membrane. During this process, viral core components are incorporated into membrane vesicles that contain viral transmembrane proteins, termed 'spike' proteins. For many years these spike proteins, which are required for infectivity, were believed to be incorporated into virions via a direct interaction between their cytoplasmic domains and viral core components. More recent evidence shows that, while such direct interactions drive budding of alphaviruses, this may not be the case for negative strand RNA viruses and retroviruses. These viruses can bud particles in the absence of spike proteins, using only viral core components to drive the process. In some cases the spike proteins, without the viral core, can be released as virus-like particles. Optimal budding and release may, therefore, depend on a 'push-and-pull' concerted action of core and spike, where oligomerization of both components plays a crucial role.