Functions as a stopper that is part of the head-tail connector and that locks the viral DNA in the capsid. Following tail attachment to the entry receptor, seems to open by a diaphragm-like motion, allowing the genome to exit the capsid through the tail tube to the host cell. During assembly, functions as a docking platform which the preassembled tail tapered by the head-tail joining protein gp17 can bind to. Homododecamer (PubMed:25991862). Interacts with the connector protein gp15 (PubMed:25991862). Interacts with the head-tail joining protein gp17 (PubMed:22072538, PubMed:25991862). Part of the connector between the portal and the tail.