Biosynthesis of the protein encoded by the c-met proto-oncogene

The proto-oncogene c-met encodes a transmembrane protein with structural features of a growth factor receptor. We have previously shown that the c-met protein (c-Met) is a heterodimer of two disulphide linked chains of 50 kd (α) and 145 kd (β). In this work we have studied the biosynthesis of the c-met product in a gastric carcinoma cell line (GTL-16) where the c-met gene is amplified and overexpressed. Following metabolic labelling of the cells in the presence of tunicamycin, anti-met antibodies immunoprecipitate a protein of 150 kd. In pulse-chase experiments carried out in the absence of tunicamycin, a 170 kd product appears first. Within the next few minutes, this precursor modifies its SDS migration, probably as a consequence of modification(s) of its intra-chain disulphide bonds. After 45 min of chase, this single polypeptide precursor is cleaved to form a 50 kd α subunit and a 145 kd β subunit that are joined by disulphide bonds in an αβ complex with an apparent molecular weight of 190 kd. The presence of N-linked oligosaccharides in both the precursor and the mature protein was shown by enzymatic de-glycosylation of the immunoprecipitated proteins. The half-life of the mature protein was calculated to be approximately 5 h. The c-met protein has similar structure and biosynthesis in other human cell lines.