Angiotensin-converting enzyme and the amyloid precursor protein secretases.

Introduction Several integral membrane proteins are now known to be released from the surface of cells through a post-translational proteolytic cleavage event. Examples of proteins existing as both membrane-bound and soluble forms as a result of such proteolytic cleavage include angiotensinconverting enzyme (ACE), the Alzheimer's amyloid precursor protein (APP), the tumour necrosis factor ligand and receptor superfamily, transforming growth factor x , Fas ligand and certain cytokine receptors (reviewed in [l]). The biological function of the proteolytic release of a membrane protein varies. In some cases it might be a process for rapidly down-regulating the protein from the surface of the cell, in others it might be to generate a soluble form of the protein that has properties identical with, or subtly different from, those of the membrane-bound