Component of the phage injection machinery. Required for DNA injection in the membrane transformation event. Involved in the formation of the membrane tail tube to connect the virus interior with the host cytosol. Essential for viral infectivity. Part of the capsid inner membrane. PRD1 virions are composed of a tail-less icosahedral capsid of diameter 63 nm, an inner protein-lipid membrane, and a dsDNA genome which is located inside the lipid vesicle. The DNA is packaged into a preformed procapsid. The internal membrane plays an active role in DNA delivery to the host cell by forming a tubular structure used for injecting the DNA into the host cytoplasm.