CLONING AND BIOCHEMICAL-CHARACTERIZATION OF A PLANT PROTEIN-KINASE THAT PHOSPHORYLATES SERINE, THREONINE, AND TYROSINE

Phosphorylation of proteins on serine, threonine, or tyrosine residues represents an important biochemical mechanism to regulate the activit y of enzymes and is used in many cellular processes. In animals, prote in serine/threonine and protein tyrosine kinases are known to perform essential roles in many pathways that transmit external stimuli from t he cell surface to the cell inferior and the nucleus, In plants, altho ugh an increasing number of protein serine/threonine kinases have been cloned, the existence of protein tyrosine kinases remains yet to be d emonstrated, Here, we report the cloning and biochemical characterizat ion of a plant protein kinase, Arabidopsis dual specificity kinase 1 ( ADK1), using a functional screening method, namely by screening an Ara bidopsis expression Library with antiphosphotyrosine antibodies. Four independent cDNA clones that define a polypeptide of 319 amino acids l ength with homology to protein kinases were identified in this screen, Phosphoamino acid analysis of the autophosphorylated kinase shows tha t ADK1 phosphorylates serine, threonine, and tyrosine, Using poly(GIu/ Tyr) as a substrate, we confirm that ADK1 is capable of phosphorylatin g tyrosine residues.