title: Rapid and Transient Activation of a Myelin Basic Protein Kinase in Tobacco Leaves Treated with Harpin from Erwinia amylovora.
abstract: Harpins are bacterial protein elicitors that induce hypersensitive response-like necrosis when infiltrated into nonhost plants such as tobacco (Nicotiana tabacum 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 (harpinEa). Much less pronounced and more transient activation was detected in water-infiltrated leaves. Biochemical characteristics of the harpinEa-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 harpinEa, 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 harpinEa- induced hypersensitive response elicitation depend in part on the activation of HAPK.
authors.affiliation: Department of Plant Pathology, 108 Waters Hall (A.L.A., S.P., M.E.H., A.N.), and Division of Biological Sciences, 308 Tucker Hall (J.M.S., J.C.W.), University of Missouri, Columbia, Missouri 65211.
authors.forename: A. L.
authors.initials: AL
authors.lastname: Adam
authors.forename: S.
authors.initials: S
authors.lastname: Pike
authors.forename: M. E.
authors.initials: ME
authors.lastname: Hoyos
authors.forename: J. M.
authors.initials: JM
authors.lastname: Stone
authors.forename: J. C.
authors.initials: JC
authors.lastname: Walker
authors.forename: A.
authors.initials: A
authors.lastname: Novacky