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)