Degradation of BACE by the ubiquitin-proteasome pathway.

The amyloid beta protein (Abeta) is derived from beta-amyloid precursor protein (APP). Cleavage of APP by beta-secretase generates a C-terminal fragment (APPCTFbeta or C99), which is subsequently cleaved by gamma-secretase to produce Abeta. BACE (or BACE1), the major beta-secretase involved in cleaving APP, has been identified as a Type 1 membrane-associated aspartyl protease. In this study, we found that treatment with proteasome inhibitors resulted in an increase in APP C99 levels, suggesting that APP processing at the beta-secretase site may be affected by the ubiquitin-proteasome pathway. To investigate whether the degradation of BACE is mediated by the proteasome pathway, cells stably transfected with BACE were treated with lactacystin. We found that BACE protein degradation was inhibited by lactacystin in a time- and dose-dependent manner. Non-proteasome protease inhibitors had no effect on BACE degradation. BACE protein is ubiquitinated. Furthermore, lactacystin increased APP C99 production and Abeta generation. Our data demonstrate that the degradation of BACE proteins and APP processing are regulated by the ubiquitin-proteasome pathway.