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 [1].

1: Novel mutants in the 5' upstream region of the portal protein gene 20 overcome a gp40-dependent prohead assembly block in bacteriophage T4. Yap NL, Rao VB; J Mol Biol 1996;263:539-550. PMID:8918937