IDENTIFICATION AND CHARACTERIZATION OF A NOVEL RELATED ADHESION FOCALTYROSINE KINASE (RAFTK) FROM MEGAKARYOCYTES AND BRAIN

We have isolated a cDNA encoding a novel human intracytoplasmic tyrosi ne kinase, termed RAFTK (for a related adhesion focal tyrosine kinase) , In addition, we have cloned and characterized the murine homolog of the human RAFTK cDNA, Comparison of the deduced amino acid sequences o f human RAFTK and murine Raftk cDNAs revealed 95% homology, indicating that RAFTK is highly conserved between these species, The RAFTK cDNA clone, encoding a polypeptide of 1009 amino acids, has closest homolog y (48% identity, 65% similarity) to the focal adhesion kinase (pp125(F AK)). Comparison of the deduced amino acid sequences also indicates th at RAFTK, like pp125(FAK), lacks a transmembrane region, myristylation sites, and SH2 and SH3 domains, In addition, like pp125(FAK), RAFTK c ontains a kinase domain flanked by large N-terminal (426 residues) and C-terminal (331 residues) domains, and the C-terminal region contains a predicted proline-rich stretch of residues, In fetal tissues, RAFTK expression was abundant in brain, and low levels were observed in lun g and liver. In adult tissues, it was less restricted, indicating that RAFTK expression is developmentally up-regulated, Expression of RAFTK was also observed in human CD34(+) marrow cells, primary bone marrow megakaryocytes, platelets, and various areas of brain, The human RAFTK gene was assigned to human chromosome 8 using genomic DNAs from human /rodent somatic cell hybrid lines, The mouse Raftk gene was mapped to chromosome 14, closely linked to gonadotropin-releasing hormone, Using specific antibodies for RAFTK, a similar to 123-kDa protein from the human megaIzaryocytic CMK cell line was immunoprecipitated. Treatment of the megakaryocytic CMK cells with thrombin caused a rapid induction of tyrosine phosphorylation of RAFTK protein, The structural features of RAFTK suggest that it is a member of the focal adhesion kinase gen e family and may participate in signal transduction in human megakaryo cytes and brain as well as in other cell types.