Author(s): Hirayama T, Oka A, Hirayama T, Oka A
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Abstract 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).
This article was published in Plant Mol Biol
and referenced in Journal of Biodiversity, Bioprospecting and Development