Mutant analysis links the translocon and BiP to retrograde protein transport for ER degradation

Proteins enter the secretory pathway through the endoplasmic reticulum1, which delivers properly folded proteins to their site of action2 and contains a quality-control system to monitor and prevent abnormal proteins from being delivered3. Many of these proteins are degraded by the cytoplasmic proteasome4,5,6,7,8, which requires their retrograde transport to the cytoplasm5,6. Based on a co-immunoprecipitation of major histocompatibility complex (MHC) class I heavy-chain breakdown intermediates with the translocon subunit Sec61p (refs 9, 10), it was speculated that Sec61p may be involved in retrograde transport11. Here we present functional evidence from genetic studies that Sec61p mediates retrograde transport of a mutated lumenal yeast carboxypeptidase ycsY (CPY*) in vivo. The endoplasmic reticulum lumenal chaperone BiP (Kar2p) and Sec63p, which are also subunits of the import machinery10,12, are involved in export of CPY* to the cytosol. Thus our results demonstrate that retrograde transport of proteins is mediated by a functional translocon. We consider the export of endoplasmic reticulum-localized proteins to the cytosol by the translocon for proteasome degradation to be a general process in eukaryotic cell biology.