CLONING, CHARACTERIZATION, AND DIFFERENTIAL EXPRESSION OF MDK2 AND MDK5, 2 NOVEL RECEPTOR TYROSINE KINASES OF THE ECK EPH FAMILY/

Using a polymerase chain reaction-based strategy for the cloning of de velopmentally regulated receptor tyrosine kinases, we identified two n ovel members of the eck/eph-related subfamily which, in analogy with t he recently identified mouse developmental kinase 1 (MDK1), were desig nated MDK2 and MDK5. MDK2 is highly homologous to the mouse kinase Myk -1 and the human kinase Htk, whereas MDK5 represents the mouse homolog ue of human Hek2. Northern blot analyses of adult mouse tissues reveal ed a 4.7 kb transcript of MDK2 and a 4.8 kb transcript of MDK5 in vari ous organ systems, including lung, liver, kidney, intestine, muscle, h eart, and, in the case of MDK5, also the brain, In addition to the ful l-length transcripts, smaller fragments were identified that probably represent truncated receptors. Northern blot analysis and in situ hybr idization of mouse embryos indicated abundant expression during embryo nic development, with preferential involvement of tissues of epithelia l and endothelial origin for both kinases and of the spinal cord gray matter for MDK5, Unlike most other members of the eck/eph-related subf amily, the expression of MDK2 and MDK5 is not primarily restricted to neuronal structures, and their abundant presence in various organ syst ems during embryonic development suggests an important role in gestati onal growth and differentiation.