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.