Harpin(Pss), a proteinaceous elicitor from Pseudomonas syringae pv. syringae (Pss), can induce a hypersensitive response (HR) in certain plants (e.g. tobacco and tomato). In this report, we show that an amphipathic hypersensitive response assisting protein (HRAP), purified from sweet pepper intensified the harpin(Pss)-mediated HR in sweet pepper, a plant normally not strongly affected by harpin(Pss). The molecular weight of HRAP was approximately 29 kDa, indicated by SDS-PAGE analysis, and the isoelectric point 8.6. The activity of HRAP was highly dependent on the ratio of harpin(Pss) to HRAP. In FPLC analysis we found that harpin(Pss) was present as monomer, dimer, trimer, tetramer, and octamer forms in neutral pH buffer systems. Among these native forms, only the monomer and dimer had HR eliciting activities. In the presence of HRAP, multimeric harpin(Pss) dissociated into dimeric and monomeric forms. HRAP had no proteolytic activity against harpin(Pss). The possible significance of the interaction between HRAP and the harpin(Pss) elicitor is discussed.
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