Evidence for non-covalent clusters of the c-met proto-oncogene product

The met proto-oncogene is a member of the tyrosine kinase growth factor receptor family and is the receptor for hepatocyte growth factor (HGF) or scatter factor. The primary met product is a 150 kDa precursor that is glycosylated to generate a 170 kDa (p170met) proreceptor protein. The mature form of the receptor is generated by cleavage of p170met to yield a disulfide-linked 140 kDa beta-subunit (p140met) and a 45 kDa alpha-subunit (p45met). Both products are glycosylated. Under non-reducing conditions both p170met and the alpha, beta-disulfide-linked protein are detected as a 185 kDa product (p185met), but only alpha-beta heterodimeric p185met is cross-linked and rendered resistant to disulfide reduction with membrane-impermeable 6.4 A linker length cross-linking reagents. These data indicate that the p170 precursor is not on the cell surface. Cross-linking experiments using 12-A linker reagents yield multimeric forms of alpha-beta heterodimeric p185met greater than 500 kDa in size. These multimeric forms are produced in all cell lines tested regardless of the levels of protein expressed. These data suggest that alpha-beta heterodimeric p185met occurs in clusters or patches on the cell surface. Immunohistochemical analysis of met protein in the absence of ligand reveals a clustered staining pattern on the cell surface.