The major structural components of the contractile tail of bacteriophage P2 are proteins FI and FII, which are believed to be the tail sheath and tube proteins, respectively.

1: Nucleotide sequence of the genes encoding the major tail sheath and tail tube proteins of bacteriophage P2. Temple LM, Forsburg SL, Calendar R, Christie GE; Virology 1991;181:353-358. PMID:1825255

2: Tail sheath and tail tube genes of the temperate coliphage 186. Xue Q, Egan JB; Virology 1995;212:218-221. PMID:7676633

3: A conserved spiral structure for highly diverged phage tail assembly chaperones. Pell LG, Cumby N, Clark TE, Tuite A, Battaile KP, Edwards AM, Chirgadze NY, Davidson AR, Maxwell KL; J Mol Biol. 2013;425:2436-2449. PMID:23542344