This tail accessory factor of the P22 virus is also referred to as gene product 4 (Gp4). The proteins structure consists of 60% alpha helices. Gp4 is the first tail accessory factor to be added to newly DNA-filled capsids during P22-morphogenesis. In solution, the protein acts as a monomer and has low structural stability. The interaction of gp4 with the portal protein involves the binding of two non-equivalent sets of six gp4 proteins. Gp4 acts as a structural adaptor for gp10 and gp26, the other tail accessory factors.

The tail complex in bacteriophage P22, and related phages, mediates host cell attachement and DNA injection. Tail assembly is a complex event that is essential for the retention and stabilisation of the newly packaged DNA within the capsid. This process involves the the sequential addition of three tail accessory factors (GP4, GP10, GP26) and the adhesion tailspike protein (GP9) to the capsid-bound portal protein PMID:21310457.

GP4 is the first factor to attach after DNA packaging. Twelve monomeric subunits of GP4 bind to the exposed surface of the portal ring, beginning the tail assembly process. GP4 does not physically block the DNA entry/exit channel, but appears to act as a structural adaptor for the other tail accessory factors GP10 and GP26 PMID:16970964.

Gp4 has been shown to have peptidoglycan hydrolase activity PMID:14763988.

PFAM:PF11650,INTERPRO:IPR020362