Convertases other than furin cleave β‐secretase to its mature form

ABSTRACT An aspartyl protease, Beta‐Site APP cleaving enzyme (BACE), was identified as the β‐secretase responsible for generating the Amyloid β protein that is believed to cause Alzheimer's disease. BACE has a short propeptide domain that is absent in the mature enzyme because of proteolytic cleavage after the sequence RLPR. This sequence is a predicted substrate for proprotein convertases such as furin. To determine the role of furin and other proprotein convertases, we expressed proBACE in a furin‐deficient mutant Chinese hamster ovary (CHO‐K1) line, RPE.40. ProBACE signal was higher in RPE.40 than in the CHO‐K1 parent, which confirmed that furin plays a role in propeptide removal. However, two independent approaches showed that proBACE is cleaved to mature BACE in RPE.40: proBACE was rapidly turned over in RPE.40 although total BACE was stable, and decanoyl‐RVKR‐chloromethylketone, an inhibitor of the proprotein convertase family, substantially increased proBACE levels in both RPE40 and CHO‐K1. Transient transfection shows that furin, PACE4, PC5/6, and PC7 mediate BACE cleavage in vivo, at least when overexpressed. RPE.40 is proficient in BACE activity despite its furin deficiency. Therefore, our finding that proBACE is cleaved in this mutant leaves open the possibility that maturation is an important regulatory step and a therapeutic target.