Rapid and transient activation of a myelin basic protein kinase in tobacco leaves treated with harpin from Erwinia amylovora

Harpins are bacterial protein elicitors that induce hypersensitive response-like necrosis when infiltrated into nonhost plants such as tobacco (Nicotiana tabacura L.) (Z.-M. Wei, R.J. Laby, C.H. Zumoff, D.W. Bauer, S.Y. He, A. Collmer, S.V. Beer [1992] Science 257: 85-88). Activity of a 49-kD Mg²⁺-dependent and Ca²⁺-independent kinase in tobacco leaves increased 50-fold 15 min after infiltration of harpin from Erwinia amylovora (harpin(Ea)). Much less pronounced and more transient activation was detected in water-infiltrated leaves. Biochemical characteristics of the harpin(Ea)-activated protein kinase (HAPK) activity are consistent with those of the mitogen-activated protein kinase family. HAPK is cytosolic and phosphorylates myelin basic protein on serine/threonine residues. Treatment with a protein tyrosine phosphatase completely eliminated HAPK activity, suggesting that tyrosine phosphorylation is required for posttranslational activation. Sustained HAPK activation after cycloheximide treatment implies that HAPK may be negatively regulated by a translation-dependent mechanism. The extracellular Ca²⁺ chelator EGTA or the protein kinase inhibitor K252a, infiltrated in planta together with harpin(Ea), partially blocked HAPK activation. The Ca²⁺-channel blocker La³⁺ had no effect on HAPK activation, suggesting that phosphorylation events precede and/or do not depend on the entry of extracellular Ca²⁺ into the cell. These results suggest that early signal transduction events during harpin(Ea)induced hypersensitive response elicitation depend in part on the activation of HAPK.