HrpF of Xanthomonas campestris pv. vesicatoria is a component of the type III secretion system of this organism. It allows transfer of proteins from the bacterial cytoplasm directly into the cytoplasm of the host plant cell. HrpF is required for pathogenicity but is dispensable for type III secretion of effector proteins in vitro. Deletion analyses reveal that both the hydrophobic C-terminal region and the hydrophilic N-terminal region are essential for activity (Büttner et al., 2002). The N-terminal region of HrpF contains a signal for secretion. HrpF is a lipid-binding protein and forms pores in lipid bilayers. These observations lead to the suggestion that HrpF is the part of the bacterial translocon which mediates effector protein delivery across the host cell membrane (Büttner et al., 2002). HrpF is homologous to proteins from rhizobial species and Ralstonia solanacearum and shows short regions homologous to other proteins. It exhibits three striking C-terminal peaks of hydrophobicity at residues 500, 620 and 670 but is otherwise strongly hydrophilic throughout its 806 aa length. HrpF shows weak sequence similarity with HrpK of Pseudomonas syringae. The latter protein is similarly believed to be a type III translocator (Petnicki-Ocwieja et al., 2005).

The transport reaction presumably catalyzed by HrpF is:

proteins (bacterial cytoplasm) → proteins (plant cell cytoplasm).