ALTERNATIVE SPLICING GENERATES ISOFORMS OF THE MET RECEPTOR TYROSINE KINASE WHICH UNDERGO DIFFERENTIAL PROCESSING

The met proto-oncogene is a member of the family of tyrosine kinase growth factor receptors. We describe the isolation and characterization of a cDNA clone (pOK) for the met receptor from a gastric carcinoma cell line. This clone differs from the published cDNA clone by the absence of 54 bp predicted to encode 18 amino acids in the extracellular domain. The pOK cDNA corresponds to the most abundant met RNA species of 8 kb expressed in human cell lines and tissue, and we show that there are in fact two 8-kb met receptor tyrosine kinase (RTK) isoforms that are generated by alternative splicing. This newly described met isoform when transiently expressed in COS cells encodes a protein of 190 kDa which corresponds in size to the p190 met alpha-beta-heterodimer expressed in human cell lines. Furthermore, we show that the 190-kDa product of pOK consists of the 140-kDa met beta-subunit associated with the 50-kDa met alpha-subunit. This finding suggests that both the alpha and beta-met chains are encoded by this construct and confirms the hypothesis that a single chain precursor is cleaved to produce both subunits of met. In contrast, the previously characterized met isoform corresponds to a minor met RNA species and encodes a protein of 170 kDa that is not cleaved yet is processed in a manner that allows cell surface expression. Both met RTK isoforms are autophosphorylated in the in vitro kinase assay. These results suggest that different isoforms of the met RTK may have distinct biological activities.