1.C.65 The Type III Secretion System Plant Host Cell Membrane Pore-forming HrpF (HrpF) Family

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).

This family belongs to the Bacterial Bacteriocin (BB) Superfamily.



Büttner, D., D. Nennstiel, B. Klüsener, and U. Bonas. (2002). Functional analysis of HrpF, a putative type III translocon protein from Xanthomonas campestris pv. vesicatoria. J. Bacteriol. 184: 2389-2398.

Meyer, D., S. Cunnac, M. Guéneron, C. Declercq, F. Van Gijsegem, E. Lauber, C. Boucher, and M. Arlat. (2006). PopF1 and PopF2, two proteins secreted by the type III protein secretion system of Ralstonia solanacearum, are translocators belonging to the HrpF/NopX family. J. Bacteriol. 188: 4903-4917.

Petnicki-Ocwieja, T., K. van Dijk, and J.R. Alfano. (2005). The hrpK operon of Pseudomonas syringae pv. tomato DC3000 encodes two proteins secreted by the Type III (Hrp) protein secretion system: HopB1 and HrpK, a putative Type III translocator. J. Bacteriol. 187: 649-663.


TC#NameOrganismal TypeExample
1.C.65.1.1The plant host cell membrane pore-forming protein of type III protein secretion systems, HrpFGram-negative bacteriaHrpF of Xanthomonas campestris pv. vesicatoria
1.C.65.1.2The plant host membrane pore-forming translocator of type III secretion systems, PopF1 (Meyer et al., 2006)Gram-negative bacteriaPopF1 of Ralstonia solanacearum (Q8XPT2)
1.C.65.1.3The plant host membrane pore-forming translocater of type III secretion systems, PopF2 (Meyer et al., 2006)Gram-negative bacteria PopF2 of Ralstonia solanacearum (Q8XRF4)
1.C.65.1.4The plant host membrane pore-forming translocater of type III secretion systems, NopX (NolX)Gram-negative bacteriaNopX of Sinorhizobium fredii (Q5Y4S2)

TC#NameOrganismal TypeExample
1.C.65.2.1The putative type III translocator, HrpKGram-negative bacteriaHrpK of Pseudomonas syringae (AAF71489)