cDNA cloning and characterization of a ligand for the Cek5 receptor protein-tyrosine kinase

We have isolated a murine cDNA encoding a ligand for the Cek5 receptor protein-tyrosine kinase (RPTK), a member of the Eph/Eck RPTK subfamily. Sequence analysis predicts an open reading frame of 345 amino acids with a predicted molecular mass of 38 kDa. Metabolic labeling and immunoprecipitation of cells transfected with a cDNA encoding the Cek5 ligand revealed the mature protein to have an apparent molecular mass of 45 kDa. The extracellular domain of the Cek5 ligand shows a 27% sequence identity at the protein level to B61, a ligand for the related Eck RPTK (Bartley, T. D., et al. (1994) Nature 368, 558-560). Consistent with the presence of a transmembrane domain, flow cytometry analysis revealed the Cek5 ligand to be expressed on the cell surface. The expressed Cek5 ligand is functionally active as it induces autophosphorylation of the Cek5 RPTK.