HCP is a family of proteins which are expressed in up to 1000 copies in Gram-negative bacteria. Together these copies aggregate into a needle-like shaft or tube that will penetrate other bacteria via a puncturing protein attached to its head [1]. Initially Hcp forms a hexameric structure with a central channel of 40 Angstroms. These hexamers pile up one on top of each other forming nanotubes resembling the gp19 tail phage tube [2].

1: The rise of the Type VI secretion system. Filloux A; F1000Prime Rep. 2013;5:52. PMID:24381728

2: A virulence locus of Pseudomonas aeruginosa encodes a protein secretion apparatus. Mougous JD, Cuff ME, Raunser S, Shen A, Zhou M, Gifford CA, Goodman AL, Joachimiak G, Ordonez CL, Lory S, Walz T, Joachimiak A, Mekalanos JJ; Science. 2006;312:1526-1530. PMID:16763151