Does the Harpin of Pseudomonas syringae Interact with a Host Protein?

Abstract

Harpin HrpZ is one of the pathogenesis-associated proteins secreted through the inducible type III secretion system of Pseudomonas syringae. HrpZ elicits a hypersensitive response (HR) in some non-host plants, but the function of this protein in the compatible interaction with host plants is not known. HrpZ functions as a HR-elicitor only when it is applied outside of the plant cell, and its putative binding site is likely to be on the plasma membrane. To study the ability of HrpZ to interact with other proteins, we screened peptides that bind to HrpZ by using phage display technique. All the peptides that had a strong affinity to HrpZ contained a similar hydrophobic amino acid motif. We also mutagenised hrpZ with 15bp insertions created by a mini-Mu transposon and screened for mutants whose protein product did not bind the phage-displayed peptides. The mutations that prevented HrpZ from binding to those peptides concentrated to the central region of the protein. This region is separate from the region previously shown to possess the HR-inducing activity, which resides at the C-terminal part. Interestingly, this peptide-binding region includes the only region with sequence homology shared by HrpZ and HrpN harpin of Erwinia species. We suggest that the peptide-binding region with the conserved motif might play a role in a protein-protein interaction involved in the bacterial pathogenesis in host plants.