Forms the portal vertex of the capsid. This portal plays critical roles in head assembly, genome packaging, neck/tail attachment, and genome ejection. The portal protein multimerizes as a single ring-shaped homododecamer arranged around a central channel. Binds to the terminase subunits to form the packaging machine. Homododecamer. Interacts with the terminase complex composed of two small and one large terminase subunits. Proteolytically cleaved by the viral protease during capsid maturation. Belongs to the siphoviridae portal protein family.