Phi29 assembly intermediates reveal how scaffold interactions with capsid protein drive capsid construction and maturation.

The self-assembly of bacteriophage capsids from major capsid proteins (MCPs) and scaffolding proteins (SPs) and the subsequent expansion of these capsids are essential steps in bacteriophage life cycles. However, the mechanism by which assembly occurs remains poorly understood, and few intermediate states are available to illuminate the expansion of meta-stable procapsids into robust mature capsids. Here, we present the structure of a partially expanded phi29 procapsid that reveals distinct conformations of MCPs and allows visualization of SPs in multiple oligomeric states. These results suggest that formation of SP dimers, tetramers, and higher-order oligomers drives dissociation of SP from MCP to actuate capsid expansion. Hexons expand first, and we propose penton maturation is delayed by a symmetry match with SP oligomers. We further show that the prolate shape of phi29's capsid is possible due to concave hexons in the equatorial region of the capsid that may alter interactions with SP and explain the observed dependence of the prolate shape on SP.