Development and characterization of a novel membrane assay for full-length BACE-1 at pH 6.0

β-Site amyloid precursor protein cleaving enzyme-1 (BACE-1) is a transmembrane aspartic protease that mediates the initial cleavage of the amyloid precursor protein (APP), leading to the generation of amyloid-β (Aβ) peptides that are thought to be causative of Alzheimer's disease (AD). Consequently, inhibition of BACE-1 is an attractive therapeutic approach for the treatment of AD. In general, in vitro biochemical assays to monitor BACE-1 activity have used the extracellular domain of the protein that contains the catalytic active site. This form of BACE-1 is catalytically active at acidic pH and cleaves APP-based peptide substrates at the β-site. However, this form of BACE-1 does not mimic the natural physiology of BACE-1 and shows minimal activity at pH 6.0, which is more representative of the pH within the intracellular compartments where BACE-1 resides. Moreover, high-throughput screens with recombinant BACE-1 at pH 4.5 have failed to identify tractable leads for drug discovery, and hence, BACE-1 inhibitor development has adopted a rational drug design approach. Here we describe the development and validation of a novel membrane assay comprising full-length BACE-1 with measurable activity at pH 6.0, which could be used for the identification of novel inhibitors of BACE-1.