Endoplasmic reticulum-localized amyloid beta-peptide is degraded in the cytosol by two distinct degradation pathways

The paradigm of endoplasmic reticulum (ER)-associated degradation (ERAD) holds that misfolded secretory and membrane proteins are translocated back to the cytosol and degraded by the proteasome in a coupled process. Analyzing the degradation of ER-localized amyloid β-peptide (Aβ), we found a divergence from this general model. Cell-free reconstitution of the export in biosynthetically loaded ER-derived brain microsomes showed that the export was mediated by the Sec61p complex and required a cytosolic factor but was independent of ATP. In contrast to the ERAD substrates known so far, the exported Aβ was degraded by both, a proteasome-dependent and a proteasome-independent pathway. RNA interference experiments in Aβ-transfected cells identified the protease of the proteasome-independent pathway as insulin-degrading enzyme (IDE). The IDE-mediated clearance mechanism for ER-localized Aβ represents an as yet unknown type of ERAD which is not entirely dependent on the proteasome.