HrpA is an essential component of the type III secretion system (TTSS) which pathogens use to inject virulence factors directly into their host cells, and to cause disease. The TTSS has an Hrp pilus appendage for channelling effector proteins through the plant cell wall and this pilus elongates by the addition of HrpA pilin subunits at the distal end [1].

1: The Hrp pilus of Pseudomonas syringae elongates from its tip and acts as a conduit for translocation of the effector protein HrpZ. Li CM, Brown I, Mansfield J, Stevens C, Boureau T, Romantschuk M, Taira S; EMBO J. 2002;21:1909-1915. PMID:11953310