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

The novel transmembrane aspartic protease BACE (for Beta-site APP Cleaving Enzyme) is the beta -secretase that cleaves amyloid precursor protein to initiate beta -amyloid formation. As such, RACE is a prime therapeutic target for the treatment of Alzheimer's disease. BACE, like other aspartic proteases, has a propeptide domain that is removed to form the mature enzyme. RACE propeptide cleavage occurs at the sequence RLPR down arrow E, a potential furin recognition motif. Here, we explore the role of furin in BACE propeptide domain processing. BACE propeptide cleavage in cells does not appear to be autocatalytic, since an inactive D93A mutant of BACE is still cleaved appropriately. BACE and furin co-localize within the Golgis apparatus, and propeptide cleavage is inhibited by brefeldin A and monensin, drugs that disrupt trafficking through the Golgi. Treatment of cells with the calcium ionophore A23187, leading to inhibition of calcium-dependent proteases including furin, or transfection with the alpha (1)-antitrypsin variant alpha (1)-PDX, a potent furin inhibitor, dramatically reduces cleavage of the RACE propeptide. Moreover, the BACE propeptide is not processed in the furin-deficient 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 proprotein convertase, is responsible for cleaving the RACE propeptide domain to form the mature enzyme.