Misfolded BiP is degraded by a proteasorne-independent endoplasmic-reticulum-associated degradation pathway

Misfolded proteins are removed from the ER (endoplasmic reticulum) by retrotranslocation to the cytosol and degradation by the ubiquitin-proteasome system in a process designated ERAD (ER-associated degradation). Analysing the turnover of a misfolded form of the ER-resident chaperone BiP (heavy-chain binding protein) (BiP Delta A), we found that the degradation of BiP Delta A did not follow this general ERAD pathway. In transfected cells, BiP Delta A was degraded, although proteasome-dependent ERAD was inactivated either by proteasome inhibitors or by ATP depletion. In semi-permeabilized cells, which did not support the degradation of the proteasomal substrate alpha 1-antitrypsin, the degradation of BiP Delta A was still functional, excluding the Golgi apparatus or lysosomes as the degradative compartment. The degradation of BiP Delta A was recapitulated in biosynthetically loaded brain microsomes and in an extract of luminal ER proteins. In contrast with proteasome-dependent ERAD, degradation fragments were detectable inside the microsomes and in the extract, and the degradation was prevented by a serine protease inhibitor. These results show that the degradation of BiP Delta A was initiated in the ER lumen by a serine protease, and support the view that proteasome-independent ERAD pathways exist.