Polymerizes to make up the central tail tube that is surrounded by the tail sheath protein (TSP). Tail tube protein polymerization takes place around the tape measure protein (TMP) and is probably directed by chaperone proteins. Upon binding to host cell, the tail sheath contracts and the tail tube penetrates the host envelope. The tail tube is involved in viral genome delivery during ejection. Homomultimer. Tail. Expressed in the late phase of the viral replicative cycle. Expression of late genes is activated by the viral late transcription activator C. No tail is synthesized.