Interestingly, Hcp and the valine–glycine repeat (VgrG) proteins
VgrG1–VgrG3 are both secreted and required for the secretion
of other T6SS substrates. The VgrG proteins are homologous
to the cell-puncturing needle of T4 bacteriophage and are
presumed to form a complex at the tip of the T6SS apparatus (3).
Hcp is structurally similar to the lambda phage major tail protein
gpV, which serves as a conduit for bacteriophage genome delivery(7, 8, 11). Another essential T6SS component, VasK, is predicted
to be a component of the inner membrane complex. V. cholerae
cells lacking vasK are unable to export either Hcp or VgrG proteins;
as a result these cells do not engage in T6SS-mediated
toxicity (3).