γ-secretase exists on the plasma membrane as an intact complex that accepts substrates and effects intramembrane cleavage

Abstract

Research on Alzheimer's disease led to the identification of a novel proteolytic mechanism in all metazoans, the presenilin/γ-secretase complex. This unique intramembrane-cleaving aspartyl protease is required for the normal processing of Notch, Jagged, β-amyloid precursor protein (APP), E-cadherin, and many other receptor-like proteins. We recently provided indirect evidence of γ-secretase activity at the cell surface in HeLa cells following inhibition of receptor-mediated endocytosis. Here, we directly identify and isolate γ-secretase as an intact complex (Presenilin, Nicastrin, Aph-1, and Pen-2) from the plasma membrane, both in overexpressing cell lines and endogenously. Inhibition of its proteolytic activity allowed cell surface γ-secretase to be captured in association with its plasma membrane-localized APP substrates (C83 and C99). Moreover, non-denaturing isolation of the intact enzyme complex revealed that cell surface γ-secretase can specifically generate amyloid β-protein from an APP substrate and similarly cleave a Notch substrate. These data directly establish the proteolytic function of γ-secretase on the plasma membrane, independent of a hypothesized substrate trafficking role. We conclude that presenilin/γ-secretase exists as a mature complex at the cell surface, where it interacts with and can cleave its substrates, consistent with an essential function in processing many adhesion molecules and receptors required for cell-cell interaction or intercellular signaling.