IDENTIFICATION OF ELF-1 AND B61 AS HIGH-AFFINITY LIGANDS FOR THE RECEPTOR TYROSINE KINASE MDK1

Mouse Developmental Kinase 1 (MDK1) is a receptor tyrosine kinase of t he eck/eph subfamily expressed in a variety of tissues during early mo use embryogenesis. To obtain further insight into the function of MDK1 , we determined identity and localisation of its physiological ligand( s). Staining whole embryos,vith fusion proteins between the extracellu lar domain of MDK1 and human secreted alkaline phosphatase revealed ar eas of high receptor binding in the caudal mesencephalon, the frontal neocortex and the limb buds. This staining was sensitive to treatment with phosphatidylinositol-specific phospholipase C. Using Scatchard an alysis, high affinity binding of Elf-1 (1.7 x 10(-10) M) and B61 (2.2 x 10(-10) M) towards MDK1 could be demonstrated. However, the transmem brane ligand Lerk2 displayed no measurable affinity for MDK1. Elf-1 an d B61 bind to the three full-length MDK1 isoforms with similar dissoci ation constants. Slightly lower affinities were observed for the two t runcated receptors MDK1-T1 and MDK1-T2. The activation of MDK1 with El f-1 or B61 leads to the rapid autophosphorylation of MDK1 as well as t yrosine phosphorylation of an unknown 62 kDa phosphoprotein in Rat1 ce lls. These findings implicate MDK1 in patterning processes during earl y mouse embryogenesis and suggest MDK1 involvement in early organogene sis and midbrain development.