The Hrp pilus of Pseudomonas syringae elongates from its tip and acts as a conduit for translocation of the effector protein HrpZ.

The type III secretion system (TTSS) is an essential requirement for the virulence of many Gram-negative bacteria infecting plants, animals and man. Pathogens use the TTSS to deliver effector proteins from the bacterial cytoplasm to the eukaryotic host cell, where the effectors subvert host defences. Plant pathogens have to translocate their effector proteins through the plant cell wall barrier. The best candidates for directing effector protein traffic are bacterial appendages attached to the membrane-bound components of the TTSS. We have investigated the protein secretion route in relation to the TTSS appendage, termed the Hrp pilus, of the plant pathogen Pseudomonas syringae pv. tomato. By pulse expression of proteins combined with immunoelectron microscopy, we show that the Hrp pilus elongates by the addition of HrpA pilin subunits at the distal end, and that the effector protein HrpZ is secreted only from the pilus tip. Our results indicate that both HrpA and HrpZ travel through the Hrp pilus, which functions as a conduit for the long-distance translocation of effector proteins.