Inner capsid protein that plays a role in viral DNA ejection into the host cell (PubMed:29590587). Ejected from the capsid after penetration the host outer membrane and has been released (PubMed:30886360). Assembles into a structure that extends the extracellular trans-envelope channel formed by the internal virion protein gp7 and probably gp16 (PubMed:30886360). This channel allows the delivery of the viral genome into the cell cytoplasm (PubMed:30886360).