During the course of characterizing polymerase chain reaction products corresponding to protein kinases of a higher plant, Arabidopsis thaliana, we found a DNA fragment that potentially codes for a polypeptide with mosaic sequences of two classes of protein kinases, a tyrosine-specific and a serine/threonine-specific one. Overlapping complementary-DNA (cDNA) clones coinciding with this fragment were isolated from an A. thaliana cDNA library. From their sequence analyses a protein kinase was predicted composed of 410 amino acid residues (APK1, Arabidopsis protein kinase 1), in which the kinase domain was flanked by short non-kinase domains. Upon expression of APK1 in Escherichia coli cells, several bacterial proteins became reactive with anti-phosphotyrosine antibody but not with the same antibody preincubated with phosphotyrosine, convincing us that APK1 phosphorylated tyrosine residues. APK1 purified from an over-producing E. coli strain showed serine/threonine kinase activity, and no tyrosine kinase activity, towards APK1 itself, casein, enolase, and myosin light chains. APK1 was thus concluded to be a novel type of protein kinase, which could phosphorylate tyrosine, serine, and threonine residues, though tyrosine phosphorylation seemed to occur only on limited substrates. Since the structure of the APK1 N-terminal portion was indicative of N-myristoylation, APK1 might associate with membranes and thereby contribute to signal transduction. The A. thaliana genome contained two APK1 genes close to each other (APK1a and APK1b).
