Transforming growth factor-α and β-amyloid precursor protein share a secretory mechanism

Abstract

Cleavage and release of membrane protein ectodomains, a regulated process that affects many cell surface proteins, remains largely uncharacterized. To investigate whether cell surface proteins are cleaved through a shared mechanism or through multiple independent mechanisms, we mutagenized Chinese hamster ovary (CHO) cells and selected clones that were unable to cleave membrane-anchored transforming growth factor α (TGF-α). The defect in TGF- α cleavage in these clones is most apparent upon cell treatment with the protein kinase C (PKC) activator PMA, which stimulates TGF-α cleavage in wild-type cells. The mutant clones do not have defects in TFG-α expression, transport to the cell surface or turnover. Concomitant with the loss of TGF- α cleavage, these clones have lost the ability to cleave many structurally unrelated membrane proteins in response to PMA. These proteins include β- amyloid precursor protein (β-APP), whose cleavage into a secreted form avoids conversion into the amyloidogenic peptide Aβ, and a group of cell surface proteins whose release into the medium is stimulated by PMA in wild type CHO cells but not in mutants. The mutations prevent cleavage by PKC- dependent as well as PKC-independent mechanisms, and thus affect an essential component that functions downstream of these various signaling mechanisms. We propose that regulated cleavage and secretion of membrane protein ectodomains is mediated by a common system whose components respond to multiple activators and act on susceptible proteins of diverse structure and function.