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1.C.56 The Pseudomonas syringae HrpZ Cation Channel (HrpZ) Family

The Harpin-PSS (HrpZ) protein is secreted by Pseudomonas syringae via the Hrp secretion system (IIISP; TC# 3.A.6) and elicits a hypersensitive response (HR) in non-host plants upon infection and pathogenicity in hosts (Haapalainen et al. 2011). It contains several repetitive regions and exhibits two extended (20 residue) regions of moderate hydrophobicity that might serve as α-helical TMSs. It is predicted to be largely of α-structure. HrpZ - a harpin  - is a highly thermostable protein that exhibits multifunctional abilities, e.g., it elicits the hypersensitive response (HR), enhances plant growth, acts as a virulence factor, and forms pores in plant plasma membranes as well as artificial membranes (Choi et al. 2013). When inserted into liposomes and synthetic bilayers at low concentrations (2 nM), it provokes a cation-selective ion current with large unitary conductance. Chloride is not transported (Lee et al. 2001). It has been hypothesized that such channels could allow nutrient release and/or delivery of virulence factors during bacterial colonization of host plants.  The leucine-zipper-like motifs may take part in the formation of oligomeric aggregates, and oligomerization could be related to HR elicitation (Tarafdar et al. 2014).

The generalized transport reaction thought to be catalyzed by HrpZ is:

Small molecules (in) →  Small molecules (out)

This family belongs to the: RTX-toxin Superfamily.

References associated with 1.C.56 family:

Choi, M.S., W. Kim, C. Lee, and C.S. Oh. (2013). Harpins, multifunctional proteins secreted by gram-negative plant-pathogenic bacteria. Mol. Plant Microbe Interact. 26: 1115-1122. 23745678
Haapalainen, M., S. Engelhardt, I. Küfner, C.M. Li, T. Nürnberger, J. Lee, M. Romantschuk, and S. Taira. (2011). Functional mapping of harpin HrpZ of Pseudomonas syringae reveals the sites responsible for protein oligomerization, lipid interactions and plant defence induction. Mol Plant Pathol 12: 151-166. 21199565
Lee, J., B. Klusener, G. Tsiamis, C. Stevens, C. Neyt, A.P. Tampakaki, N.J. Panopoulos, J. Nöller, E.W. Weiler, G.R. Cornelis, J.W. Mansfield, and T. Nürnberger. (2001). HrpZPsph from the plant pathogen Pseudomonas syringae pv. phaseolicola binds to lipid bilayers and forms an ion-conducting pore in vitro. Proc. Natl. Acad. Sci. USA 98: 289-294. 11134504
Tarafdar, P.K., L.V. Vedantam, R.S. Sankhala, P. Purushotham, A.R. Podile, and M.J. Swamy. (2014). Oligomerization, conformational stability and thermal unfolding of Harpin, HrpZPss and its hypersensitive response-inducing c-terminal fragment, C-214-HrpZPss. PLoS One 9: e109871. 25502017