Functional analysis of HrpF, a putative type III translocon protein from Xanthomonas campestris pv. vesicatoria

Type III secretion systems (TTSSs) are specialized protein transport systems in gram-negative bacteria which target effector proteins into the host cell. The TTSS of the plant pathogen Xanthomonas campestris pv. vesicatoria, encoded by the hrp (hypersensitive reaction and pathogenicity) gene cluster, is essential for the interaction with the plant. One of the secreted proteins is HrpF, which is required for pathogenicity but dispensable for type III secretion of effector proteins in vitro, suggesting a role in translocation. In this study, complementation analyses of an hrpF null mutant strain using various deletion derivatives revealed the functional importance of the C-terminal hydrophobic protein region. Deletion of the N terminus abolished type III secretion of HrpF. Employing the type III effector AvrBs3 as a reporter, we show that the N terminus of HrpF contains a signal for secretion but not a functional translocation signal. Experiments with lipid bilayers revealed a lipid-binding activity of HrpF as well as HrpF-dependent pore formation. These data indicate that HrpF presumably plays a role at the bacterial-plant interface as part of a bacterial translocon, which mediates effector protein delivery across the host cell membrane.