Cell surface presenilin-1 participates in the gamma-secretase-like proteolysis of notch

Presenilin-1 (PS1), a polytopic membrane protein primarily localized to the endoplasmic reticulum, is required for efficient proteolysis of both Notch and beta-amyloid precursor protein (APP) within their transmembrane domain s. The activity that cleaves APP (called gamma-secretase) has properties of an aspartyl protease, and mutation of either of the two aspartate residues located in adjacent transmembrane domains of PS1 inhibits gamma-secretase processing of APP. We show here that these aspartates are required for Notc h processing, since mutation of these residues prevents PS1 from inducing t he gamma-secretase-like proteolysis of a Notch1 derivative. Thus PSI might function in Notch cleavage as an aspartyl protease or di aspartyl protease cofactor. However, the ER localization of PSI is inconsistent with that hyp othesis, since Notch cleavage occurs near the cell surface. Using pulse-cha se and biotinylation assays, we provide evidence that PS1 binds Notch in th e ER/Golgi and is then co-transported to the plasma membrane as a complex. PS1 aspartate mutants were indistinguishable from wild-type PS1 in their ab ility to bind Notch or traffic with it to the cell surface, and did not alt er the secretion of Notch. Thus, PS1 appears to function specifically in No tch proteolysis near the plasma membrane as an aspartyl protease or cofacto r.