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Harpin of Pseudomonas syringae pv. phaseolicola Harbors a Protein Binding Site

¹General Microbiology, Department of Biological and Environmental Sciences, FI-00014 University of Helsinki, Finland; ²Department of Stress and Developmental Biology, Institute of Plant Biochemistry, Weinberg 3, D-06120 Halle/Saale, Germany; ³Eberhard-Karls-University Tübingen, Center of Molecular Biology of Plants, Plant Biochemistry, Auf der Morgenstelle 5, D-72076 Tübingen, Germany; ⁴Department of Ecological and Environmental Sciences, University of Helsinki, Niemenkatu 73, FI-15140 Lahti, Finland

Harpin HrpZ of plant-pathogenic bacterium Pseudomonas syringae elicits a hypersensitive response (HR) in some nonhost plants, but its function in the pathogenesis process is still obscure. HrpZ-interacting proteins were identified by screening a phage-display library of random peptides. HrpZ of the bean pathogen P. syringae pv. Phaseolicola (HrpZ_Pph) shows affinity to peptides with a consensus amino acid motif W(L)ARWLL(G/L). To localize the peptide-binding site, the hrpZ_Pph gene was mutagenized with randomly placed 15-bp insertions, and the mutant proteins were screened for the peptide-binding ability. Mutations that inhibited peptide-binding localized to the central region of hrpZ_Pph, which is separate from the previously determined HR-inducing region. Antiserum raised against one of the hrpZ_Pph-binding peptides recognized small proteins in bean, tomato, parsley, and Arabidopsis thaliana but none in tobacco. On native protein blots, hrpZ_Pph bound to a bean protein with similar pI as the protein recognized by the peptide antiserum. The result suggests a protein-protein interaction between the harpin and a host plant protein, possibly involved in the bacterial pathogenesis.

Additional keywords: functional domains, HrpN, type III secretion system.