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Abstract:
Assembly of tailed bacteriophages and herpesviruses starts with formation of procapsids(virion precursors without DNA). Scaffolding proteins (SP) drive assembly by chaperoningthe major capsid protein (MCP) to build an icosahedral lattice. Here we report near-atomicresolution cryo-EM structures of the bacteriophage SPP1 procapsid, the intermediateexpanded procapsid with partially released SPs, and the mature capsid with DNA. In theintermediate state, SPs are bound only to MCP pentons and to adjacent subunitsfrom hexons. SP departure results in the expanded state associated with unfolding of theMCP N-terminus and straightening of E-loops. The newly formed extensive inter-capsomerebonding appears to compensate for release of SPs that clasp MCP capsomeres together.Subsequent DNA packaging instigates bending of MCP A domain loops outwards,closing the hexons central opening and creating the capsid auxiliary protein binding interface.Thesefindings provide a molecular basis for the sequential structural rearrangements duringviral capsid maturation
