This repeat composes the C-terminal part of the bacteriophage T4 baseplate protein Gp5. This region of the protein forms a needle like projection from the baseplate that is presumed to puncture the bacterial cell membrane. Structurally three copies of the repeated region trimerise to form a beta solenoid type structure [1]. This family also includes repeats from bacterial Vgr proteins.

1: Structure of the cell-puncturing device of bacteriophage T4. Kanamaru S, Leiman PG, Kostyuchenko VA, Chipman PR, Mesyanzhinov VV, Arisaka F, Rossmann MG; Nature 2002;415:553-557. PMID:11823865