Characterization of Alzheimer's beta-secretase protein BACE - A pepsin family member with unusual properties

The cerebral deposition of amyloid beta-peptide is an early and critical fe ature of Alzheimer's disease. Amyloid beta-peptide is released from the amy loid precursor protein by the sequential action of two proteases, beta-secr etase and gamma-secretase, and these proteases are prime targets for therap eutic intervention. We have recently cloned a novel aspartic protease, BACE , with all the known properties of beta-secretase. Here we demonstrate that BACE is an N-glycosylated integral membrane protein that undergoes constit utive N-terminal processing in the Golgi apparatus. We have used a secreted Fc fusion-form of BACE (BACE-IgG) that contains the entire ectodomain for a detailed analysis of posttranslational modifications, This molecule start s at Glu(46) and contains four N-glycosylation sites (Asn(153) Asn(172), As n(223), and Asn(354)). The six Cys residues in the ectodomain form three in tramolecular disulfide linkages (Cys(216)-Cys(420), Cys(278)-Cys(443), and Cys(330)-Cys(380)). Despite the conservation of the active site residues an d the 30-37% amino acid homology with known aspartic proteases, the disulfi de motif is fundamentally different from that of other aspartic proteases, This difference may affect the substrate specificity of the enzyme. Taken t ogether, both the presence of a transmembrane domain and the unusual disulf ide bond structure lead us to conclude that BACE is an atypical pepsin fami ly member.