CHARACTERIZATION OF A LIGAND FOR RECEPTOR PROTEIN-TYROSINE KINASE HTKEXPRESSED IN IMMATURE HEMATOPOIETIC-CELLS

HTK is a receptor tyrosine kinase that belongs to the Eph subfamily. A n extensive screening using BIAcore system revealed that a colon cance r cell line, C-l, expressed the ligand for HTK. From the conditioned m edium of C-l cells, a soluble form of ligand was purified by receptor affinity chromatography, and the isolation of full-length cDNA reveale d that this ligand is identical to the human HTK ligand (HTKL) previou sly reported. HTK receptor tyrosine phosphorylation was induced by mem brane-bound or clustered soluble HTKL but not by unclustered soluble H TKL, indicating that HTKL requires cell-to-cell interaction for recept or activation. Binding analysis demonstrated that HTKL binds to HTK wi th a much higher affinity (K-d: 1.23 nM) than the other transmembrane- type ligand for Eph family, LERK-2/ELKL (K-d: 135 nM). The expression of HTK in cord blood cells was upregulated after the culture in the pr esence of stem cell factor. Clustered soluble HTKL stimulated the prol iferation of sorted HTK+ cord blood cells and a hematopoietic cell lin e, UT-7/EPO from which HTK was isolated. These findings suggest the in volvement of HTK-HTKL system in the proliferation of HTK+ hematopoieti c progenitor cells in the hematopoietic environment.