Bacteriophage T4-like capsid assembly protein (Gp20)
This family consists of several bacteriophage T4-like capsid assembly (or portal) proteins. The exact mechanism by which the double-stranded (ds) DNA bacteriophages incorporate the portal protein at a unique vertex of the icosahedral capsid is unknown. In phage T4, there is evidence that this vertex, constituted by 12 subunits of gp20, acts as an initiator for the assembly of the major capsid protein and the scaffolding proteins into a prolate icosahedron of precise dimensions. The regulation of portal protein gene expression is an important regulator of prohead assembly in bacteriophage T4. This family represents the protease responsible for the proteolysis of head proteins, a critical step in the morphogenesis of many tailed phages, Cleavage facilitates the conversion of the prohead to the mature capsid. All these cleavages are carried out by action at consensus S/A/G-X-E recognition sequences at 39 cleavage sites. Evidence of multiple processing sites in nine phiKZ proteins appears to represent a built-in mechanism by which the phage ensures that the majority of the propeptide regions are removed, and emphasizes the essential nature of processing in phiKZ-head morphogenesis. The family is classified by MEROPS as a serine peptidase.