HEPATOCYTE GROWTH-FACTOR AND ITS RECEPTOR, THE TYROSINE KINASE ENCODED BY THE C-MET PROTOONCOGENE

HGF is secreted by mesenchymal cells and regulates motogenesis, mitogenesis, and morphogenesis of epithelial and endothelial cells. HGF is a heterodimer of two glycosylated chains, alpha and beta, bound together by a disulfide bond. The molecule is synthesized as single chain precursor devoid of biological activity (pro-HGF). The critical step in pro-HGF activation is a proteolytic cleavage generating the two chain form. This step occurs in the extracellular enviroment, and is catalyzed by urokinase. Two alternative transcripts originate two HGF variants. One bears a deletion of five aminoacids in the alpha chain, and has the same properties of the full-size protein. The other one contains only the first portion of the alpha chain (two kringle HGF). Two kringle HGF binds the HGF receptor, triggers its tyrosine kinase activity and behaves as a partial agonist, inducing motogenesis but not mitogenesis in target cells. The HGF receptor is the tyrosine kinase encoded by the c-MET proto-oncogene, a tyrosine kinase receptor. This molecule is an heterodimer of an extracellular alpha chain disulfide linked to a transmembrane beta chain. The cytoplasmic portion of the beta chain contains the catalytic domain and critical sites for the regulation of its kinase activity. In the C-terminal tail, a bidentate motif containing two tyrosines associates the transducers responsible for HGF signalling.