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

Abstract

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 proteasome^4,5,6,7,8, 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.