The cytoplasmic carboxy-terminal amino acid determines the subcellular localization of proTGF-alpha and membrane type matrix metalloprotease (MT1-MMP)

Transforming growth factor alpha (TGF-alpha) is synthesized as a precursor transmembrane molecule (proTGF-alpha) whose ectodomain is shed from the cel l surface generating mature, soluble, growth factor. In agreement with rece nt reports, here we show that the structural determinant that targets proTG F-alpha to the cell surface maps to the very C-terminal cytoplasmic amino a cid, valine, The primary localization of proTGF-alpha C-terminal mutants is a perinuclear area that colocalizes with ER markers. Since the ectodomain shedding machinery that acts on proTGF-alpha is known to be located at the cell surface, deficient transport provides an explanation for the previousl y reported lack of PKC activated ectodomain shedding of proTGF-alpha C-term inal mutants. The transport of wild-type proTGF-alpha to the cell surface w as found to be mediated by a mechanism that includes a specific component s aturable by wild-type proTGF-alpha but not by cell surface transmembrane pr oteins whose trafficking is independent of their cytoplasmic tail such as b etaglycan. C-terminal valines are likely to be a general determinant of the subcellular location of cell surface transmembrane proteins since the matu ration and trafficking of MT1-MMP C-terminal mutants are severely impaired. Our data suggest the existence of a targeting mechanism that acts on cell surface transmembrane molecules as diverse as proTGF-alpha and MT1-MMP and that the interaction with such a mechanism depends on the identity of the C -terminal amino acid of the targeted molecules.