PROKARYOTIC EXPRESSION CLONING OF A NOVEL HUMAN TYROSINE KINASE

Screening of a human embryonic lung fibroblast cDNA expression library with antiphosphotyrosine antibodies led to isolation of a novel prote in kinase. A clone, designated A6, contained a 3-kb cDNA insert with a predicted open reading frame of 350 amino acids. DNA sequence analysi s failed to reveal any detectable similarity with previously known gen es, and the predicted A6 protein lacked any of the motifs commonly con served in the catalytic domains of protein kinases. However, the bacte rially expressed beta-galactosidase-A6 fusion protein demonstrated bot h tyrosine and serine phosphorylation in an in vitro kinase assay and phosphorylated exogenous substrates including myelin basic protein spe cifically on tyrosine residues. The enzyme also displayed biochemical properties analogous to those of other protein tyrosine kinases. The A 6 gene was found to be expressed widely at the transcript level in nor mal tissues and was evolutionarily conserved. Thus, A6 represents a no vel tyrosine kinase which is highly divergent from previously describe d members of this important class of regulatory molecules.