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Nicastrin, a transmembrane glycoprotein, forms high molecular weight complexes with presenilin 1 and presenilin 2. Suppression of nicastrin expression in Caenorhabditis elegans embryos induces a subset of notch/glp-1 phenotypes similar to those induced by simultaneous null mutations in both presenilin homologues of C. elegans (sel-12 and hop-1). Nicastrin also binds carboxy-terminal derivatives of β-amyloid precursor protein (βAPP), and modulates the production of the amyloid β-peptide (Aβ) from these derivatives. Missense mutations in a conserved hydrophilic domain of nicastrin increase Aβ₄₂ and Aβ₄₀ peptide secretion. Deletions in this domain inhibit Aβ production. Nicastrin and presenilins are therefore likely to be functional components of a multimeric complex necessary for the intramembranous proteolysis of proteins such as Notch/GLP-1 and βAPP.