The architect of virus assembly: the portal protein complex nucleates procapsid assembly in bacteriophage P22

Abstract

The genetic material of tailed dsDNA bacteriophages, herpesviruses and adenoviruses is packaged into a precursor capsid through a 12-mer ring-shaped protein complex called the portal protein, located at a unique 5-fold vertex. In several phages and viruses, including T4, {Phi}29, and HSV-1, the dodecameric portal protein forms a nucleation complex with scaffolding proteins to initiate procapsid assembly, thereby ensuring incorporation of only one portal complex per capsid. However, for bacteriophage P22, the role of its portal protein in initiation of procapsid assembly is unclear. We recently developed an in vitro P22 assembly assay where portal protein is co-assembled into procapsid-like particles. We also showed that scaffolding protein catalyzes oligomerization of monomeric portal protein into 12-mer rings, and possibly forming a scaffolding-protein nucleation complex that results in one portal complex per P22 procapsid. Here, we present evidence substantiating that P22 portal protein, similar to the other dsDNA viruses, can act as an assembly nucleator. We find that the presence of P22 portal protein is able to increase the rate of particle assembly. Additionally, we show that P22 portal protein proper contributes to proper morphology of the assembled particles. Our results highlight a key function of portal protein as an assembly initiator, a feature likely conserved among these classes of dsDNA viruses.