CLONING AND CHARACTERIZATION OF HTK, A NOVEL TRANSMEMBRANE TYROSINE KINASE OF THE EPH SUBFAMILY

Using a polymerase chain reaction based strategy, we identified a nove l transmembrane tyrosine kinase in CD34+ human bone marrow cells and a human hepatocellular carcinoma cell line, Hep3B. This protein, hepato ma transmembrane kinase or Htk, shares amino acid similarity with the EPH subfamily of tyrosine kinases. The HTK gene is located on human ch romosome 7. The predicted 987-amino acid sequence of Htk includes a tr ansmembrane region and signal sequence. In the predicted extracellular domain, a cysteine-rich region and tandem fibronectin type III repeat s are present while a single uninterrupted catalytic domain is present in the intracellular domain. These features are consistent with other members of the Eph subfamily. Antibodies raised against Htk extracell ular domain immunoprecipitated a 120-kDa protein from either in vitro translated HTK or Hep3B cells which localized primarily to the Hep3B m embrane subcellular fraction. Purified in vitro translated Htk was enz ymatically active and autophosphorylated on tyrosine in kinase assays. Furthermore, antibodies against Htk ECD were agonistic, inducing Htk tyrosine phosphorylation in transfected NIH3T3 cells. Northern blot an alysis demonstrated a single HTK transcript abundantly present in plac enta and in a range of primary tissues and malignant cell lines. HTK a ppears to be expressed in fetal but not adult brain and in primitive a nd myeloid but not lymphoid hematopoietic cells. The novel transmembra ne protein, Htk, may function as a receptor with an expression pattern suggesting a role in events mediating differentiation and development .