Intramembrane Proteolysis Mediated by the gamma-Secretase Complex : Nicastrin Functions as a Substrate Receptor

The proteolytic processing of proteins within the lipid bilayer, and release of their membrane tethered biologically active fragments, fundamentally controls a growing list of cell signaling events. The gamma -secretase, one of a small family of independently evolved proteases, performs this enigmatic hydrolysis of a peptide bond within the membrane. Remarkably atypical, gamma-secretase activity: (1) requires a complex of proteins that include presenilin, nicastrin, Aph1, and Pen-2; (2) catalyzes the intramembrane cleavage of a broad range of substrates, regulating physiology from neurodevelopment to neurodegeneration. The aim of this thesis is to elucidate the mechanism by which the gamma -secretase recognizes its substrates. I provide evidence that nicastrin, in addition to being a critical component of the complex, plays a major function in substrate recognition. The ectodomain of nicastrin binds the new amino terminus that is generated upon the prerequisite 'shedding' of substrates, thereby recruiting substrates into the gamma -secretase complex. The gamma -secretase complex has been traditionally viewed as a hub for signal transduction of substrates such as Notch and APP. The mechanism by which a broad range of substrates may be recognized and subsequently cleaved, as demonstrated in this thesis, supports a mutually inclusive function as a protease that has evolved to simply dispose transmembrane domains thus controlling the repertoire of a class of proteins present in the membrane.