Pringle, M. A. | Search bioRxiv

bioRxiv2021

A Cytosolic Reductase Pathway is Required for Complete N-Glycosylation of an STT3B-Dependent Acceptor Site.

N-linked glycosylation of proteins entering the secretory pathway is an essential post-translational modification required for protein stability and function. Previously, it has been shown that there is a temporal relationship between protein folding and glycosylation, which influences the occupancy of specific glycosylation sites. Here we use an in vitro translation system that reproduces the initial stages of secretory protein translocation, folding and glycosylation under defined redox conditions. We found that the efficiency of glycosylation of hemopexin was dependent upon a robust NADPH-dependent cytosolic reductive pathway, which could also be mimicked by the addition of a membrane impermeable reducing agent. The identified hypoglycosylated acceptor site is adjacent to a cysteine involved in a short range disulfide bond, which has been shown to be dependent on the STT3B-containing oligosaccharyl transferase. We also show that efficient glycosylation at this site is dependent on the STT3A-containing oligosaccharide transferase. Our results provide further insight into the important role of the ER redox conditions in glycosylation site occupancy and demonstrate a link between redox conditions in the cytosol and glycosylation efficiency.

cell biology↗

bioRxiv2019

The Mammalian Cytosolic Thioredoxin Reductase Pathway Acts via a Membrane Protein to Reduce ER-localised Proteins

Folding of proteins entering the mammalian secretory pathway requires the insertion of the correct disulfide bonds. Disulfide formation involves both an oxidative pathway for their insertion and a reductive pathway to remove incorrectly formed disulfides. Reduction of these disulfides is critical for correct folding and degradation of misfolded proteins. Previously, we showed that the reductive pathway is driven by NADPH generated in the cytosol. Here, by reconstituting the pathway using purified proteins and ER microsomal membranes, we demonstrate that the thioredoxin reductase system provides the minimal cytosolic components required for reducing proteins within the ER lumen. In particular, saturation of the pathway and its protease sensitivity demonstrates the requirement for a membrane protein to shuttle electrons from the cytosol to the ER lumen. These results provide compelling evidence for the critical role of the cytosol in regulating ER redox homeostasis to ensure correct protein folding and to facilitate the degradation of misfolded ER proteins.

cell biology↗