Functional mapping of harpin HrpZ of Pseudomonas syringae reveals the sites responsible for protein oligomerization, lipid interactions and plant defence induction
Harpin HrpZ is one of the most abundant proteins secreted through the pathogenesis-associated type III secretion system of the plant pathogen Pseudomonas syringae. HrpZ shows membrane-binding and pore-forming activities in vitro, suggesting that it could be targeted to the host cell plasma membrane. We studied the native molecular forms of HrpZ and found that it forms dimers and higher order oligomers. Lipid binding by HrpZ was tested with 15 different membrane lipids, with HrpZ interacting only with phosphatidic acid. Pore formation by HrpZ in artificial lipid vesicles was found to be dependent on the presence of phosphatidic acid. In addition, HrpZ was able to form pores in vesicles prepared from Arabidopsis thaliana plasma membrane, providing evidence for the suggested target of HrpZ in the host. To map the functions associated with HrpZ, we constructed a comprehensive series of deletions in the hrpZ gene derived from P. syringae pv. phaseolicola, and studied the mutant proteins. We found that oligomerization is mainly mediated by a region near the C-terminus of the protein, and that the same region is also essential for membrane pore formation. Phosphatidic acid binding seems to be mediated by two regions separate in the primary structure. Tobacco, a nonhost plant, recognizes, as a defence elicitor, a 24-amino-acid HrpZ fragment which resides in the region indispensable for the oligomerization and pore formation functions of HrpZ.
Document Type: Research Article
Affiliations: 1: Eberhard-Karls-University Tübingen, Centre of Molecular Biology of Plants, Plant Biochemistry, Auf der Morgenstelle 5, D-72076 Tübingen, Germany 2: General Microbiology, Department of Biological and Environmental Sciences, FI-00014 University of Helsinki, Helsinki, Finland 3: Department of Stress and Developmental Biology, Leibniz Institute of Plant Biochemistry, Weinberg 3, D-06120 Halle/Saale, Germany 4: Department of Ecological and Environmental Sciences, University of Helsinki, Niemenkatu 73, FI-15140 Lahti, Finland
Publication date: 2011-02-01