A furin-like convertase mediates propeptide cleavage of BACE, the Alzheimer's beta-secretase

The novel transmembrane aspartic protease BACE (for Beta-site APP Cleaving Enzyme) is the beta -secretase that cleaves amyloid precursor protein to in itiate beta -amyloid formation. As such, RACE is a prime therapeutic target for the treatment of Alzheimer's disease. BACE, like other aspartic protea ses, has a propeptide domain that is removed to form the mature enzyme. RAC E propeptide cleavage occurs at the sequence RLPR down arrow E, a potential furin recognition motif. Here, we explore the role of furin in BACE propep tide domain processing. BACE propeptide cleavage in cells does not appear t o be autocatalytic, since an inactive D93A mutant of BACE is still cleaved appropriately. BACE and furin co-localize within the Gels apparatus, and pr opeptide cleavage is inhibited by brefeldin A and monensin, drugs that disr upt trafficking through the Golgi. Treatment of cells with the calcium iono phore A23187, leading to inhibition of calcium-dependent proteases includin g furin, or transfection with the alpha (1)-antitrypsin variant alpha (1)-P DX, a potent furin inhibitor, dramatically reduces cleavage of the RACE pro peptide. Moreover, the BACE propeptide is not processed in the furin-defici ent LoVo cell line; however, processing is restored upon furin transfection . Finally, in vitro digestion of recombinant soluble RACE with recombinant furin results in complete cleavage only at the established E46 site. Taken together, our results strongly suggest that furin, or a furin-like proprote in convertase, is responsible for cleaving the RACE propeptide domain to fo rm the mature enzyme.