ThePseudomonas syringaeHrpZ Cation Channel (HrpZ) Family (TC# 1.C.56) is a member of theRTX-toxin superfamily. The Harpin-PSS (HrpZ;TC# 1.C.56.1.1) protein is secreted byPseudomonas syringae via the Hrp secretion system (IIISP;TC# 3.A.6) and elicits ahypersensitive response (HR) in non-host plants upon infection and pathogenicity in hosts.[1] It contains several repetitive regions and exhibits two extended (20 residue) regions of moderate hydrophobicity that might serve as α-helical TMSs. The HrpZ cation channel 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.[2] Homologues are not found in organisms other thanP. syringae.[3][4]
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. 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.[3]
The generalized transport reaction thought to be catalyzed by HrpZ is:[4]