MMK2, A NOVEL ALFALFA MAP KINASE, SPECIFICALLY COMPLEMENTS THE YEAST MPK1 FUNCTION

Mitogen-activated protein (MAP) kinases are serine/threonine protein k inases that are activated in response to a variety of stimuli. Here we report the isolation of an alfalfa cDNA encoding a functional MAP kin ase, termed MMK2. The predicted amino acid sequence of MMK2 shares 65% identity with a previously identified alfalfa MAP kinase, termed MMK1 . Both alfalfa cDNA clones encode functional kinases when expressed in bacteria, undergoing autophosphorylation and activation to phosphoryl ate myelin basic protein in vitro. However, only MMK2 was able to phos phorylate a 39 kDa protein from the detergent-resistant cytoskeleton o f carrot cells. The distinctiveness of MMK2 was further shown by compl ementation analysis of three different MAP kinase-dependent yeast path ways; this revealed a highly specific replacement of the yeast MPK1(SL T2) kinase by MMK2, which was found to be dependent on activation by t he upstream regulators of the pathway. These results establish the exi stence of MAP kinases with different characteristics in higher plants, suggesting the possibility that they could mediate different cellular responses.