A cellular complex of BACE1 and γ-secretase sequentially generates Aβ from its full-length precursor.

Intramembrane proteolysis of transmembrane substrates by the presenilin-gamma-secretase complex is preceded and regulated by shedding of the substrate's ectodomain by alpha- or beta-secretase. We asked whether beta-and gamma-secretases interact to mediate efficient sequential processing of APP, generating the amyloid beta (A beta) peptides that initiate Alzheimer's disease. We describe a hitherto unrecognized multiprotease complex containing active beta- and gamma-secretases. BACE1 coimmunoprecipitated and cofractionated with gamma-secretase in cultured cells and in mouse and human brain. An endogenous high molecular weight (HMW) complex (similar to 5 MD) containing beta-and gamma-secretases and holo-APP was catalytically active in vitro and generated a full array of A beta peptides, with physiological A beta 42/40 ratios. The isolated complex responded properly to gamma-secretase modulators. Alzheimer's-causing mutations in presenilin altered the A beta 42/40 peptide ratio generated by the HMW beta/gamma-secretase complex indistinguishably from that observed in whole cells. Thus, A beta is generated from holo-APP by a BACE1-gamma-secretase complex that provides sequential, efficient RIP processing of full-length substrates to final products.