Hrp Translocon Formation in Pseudomonas syringae

The bacterial plant pathogen Pseudomonas syringae uses a type III secretion system (T3SS) to translocate, or inject, type III effector (T3E) proteins into plant cells. The T3SS is required for pathogenicity. For T3Es to be translocated, P. syringae must create an opening in the plant plasma membrane, using a group of proteins known as a translocon, through which the T3E proteins can be delivered. The translocon is composed of membrane proteins called translocators that span the plant plasma membrane. HrpK1 exhibits many of the characteristics of a translocator, including being a type III-secreted protein, possessing a transmembrane domain, and being crucial for pathogenicity. Most importantly, hrpK1 mutants are secretion competent but defective in T3E translocation. Here, we confirm that HrpK1 can act as a translocator for multiple T3Es and find that it interacts with liposomes, consistent with the properties of other established translocators. HrpK1 is more important to T3E translocation than other P. syringae proteins known as harpins, which have been implicated in translocation. Both HrpK1 and HrpZ1, one of the harpins, interact with phosphatidic acid, and when infiltrated into the apoplast, this compound can block T3E translocation in a HrpK1 and HrpZ1-specific manner. In contrast, injection of phospholipase D, an enzyme that increases phosphatidic acid in the membrane, allowed for increased T3E translocation. These experiments support that HrpK1 is a primary translocator for the P. syringae T3SS and that it may function by binding phospholipids in the plant plasma membrane.