# O-mannosylation of misfolded ER proteins promotes ERAD

## TL;DR

A new glycan-dependent mechanism for degrading misfolded proteins in the endoplasmic reticulum is discovered, involving O-mannosylation by the Pbn1-Gpi14 enzyme complex.

## Contribution

The discovery that Pbn1-Gpi14 O-mannosylates misfolded proteins to promote ERAD, especially in the absence of N-glycosylation, is novel.

## Key findings

- The Pbn1-Gpi14 complex catalyzes O-mannosylation of misfolded ER proteins.
- O-mannosylation promotes ERAD for proteins lacking N-glycans.
- This mechanism acts as a fail-safe pathway for protein quality control.

## Abstract

Protein quality control (PQC) in the secretory pathway, a process critically linked to numerous human diseases, begins in the endoplasmic reticulum (ER) and involves ER-associated degradation (ERAD) of terminally misfolded proteins. In this study, we conducted genome-wide screens in baker’s yeast (Saccharomyces cerevisiae) to investigate the degradation of Gas1*, a misfolded version of an O-mannosylated, glycosylphosphatidylinositol (GPI)-anchored protein. In combination with detailed biochemical and genetic analyses, these screens revealed an unexpected bifunctionality of the evolutionarily conserved heteromeric enzyme complex Pbn1-Gpi14: while it has been previously recognized as a GPI-mannosyltransferase, we here find that it catalyzes the O-mannosylation of misfolded proteins, thereby promoting their ERAD. This process is particularly relevant for misfolded proteins that lack N-glycans. Our results suggest that protein O-mannosylation constitutes a distinct type of glycan-dependent mechanism for promoting ERAD.

As newly synthesized proteins mature in the ER, the dynamic modification of N-linked glycans encodes information on their folding status. This study shows that the yeast Pbn1–Gpi14 complex (PIGX-PIGM in mammals) O-mannosylates a specific set of misfolded secretory proteins, creating a parallel quality-control pathway.

PBN1 was identified in genome-wide complementary yeast screens linked to ER protein quality control.Gpi14 is phylogenetically related to protein mannosyltransferases (PMTs).Pbn1-Gpi14 participates in the O-mannosylation of scattered and clustered hydroxy amino acids in misfolded model proteins.This protein-O-mannosylation can provide a fail-safe mechanism for ERAD in the absence of N-glycosylation.

PBN1 was identified in genome-wide complementary yeast screens linked to ER protein quality control.

Gpi14 is phylogenetically related to protein mannosyltransferases (PMTs).

Pbn1-Gpi14 participates in the O-mannosylation of scattered and clustered hydroxy amino acids in misfolded model proteins.

This protein-O-mannosylation can provide a fail-safe mechanism for ERAD in the absence of N-glycosylation.

A yeast screen reveals a distinct type of glycan-dependent ERAD mechanism for misfolded secretory substrates lacking N-glycosylation.