Polymerizes to form the tail tube of the phage (Probable) (PubMed:29209037). Tail tube protein polymerization takes place around the tape measure protein (TMP-pb2) and is probably directed by chaperone proteins (Probable). The tail tube is involved in viral genome delivery during ejection (Probable). Homotrimer (PubMed:16876823, PubMed:29209037). The tail tube is made of 40 stacked trimeric rings (PubMed:16876823, PubMed:29209037). Main component of the tail tube. The C-terminus possesses an Ig-like fold of the Big-2 family.