The carboxyl-terminal valine residues of ProTGF alpha are required for its efficient maturation and intracellular routing

Soluble forms of transforming growth factor-alpha (TGF alpha) are derived by proteolytic processing of an integral membrane glycoprotein precursor (proTGF alpha). Previous studies indicated that phorbol ester-induced cleavage of proTGF alpha in CHO cells is dependent on the presence of a valine residue located at the carboxyl terminus of the precursor's cytoplasmic domain. We reassessed this requirement with epitope-tagged constructs introduced into transformed rat liver epithelial cells that normally express and process TGF alpha. We found that proTGF alpha mutants lacking the terminal valine residues showed greatly reduced maturation to the fully glycosylated form. Additionally, they were present at substantially reduced levels on the cell surface and, instead, accumulated in the endoplasmic reticulum. Consistent with these results, enzyme-linked immunosorbant assay (ELISA) and Western blot analyses revealed little or no soluble TGF alpha in medium conditioned by cells expressing the mutant constructs. Finally, a truncated proTGF alpha mutant lacking most of the cytoplasmic domain but retaining a carboxyl-terminal valine was processed and cleaved in a near-normal manner. These results, some of which were reproduced in CHO cells, indicate that the predominant effect of the carboxyl-terminal valines is to ensure normal maturation and routing of the precursor.