Decoding ERAD glycans
Misfolded proteins are cleared from the endoplasmic reticulum in a process called ER-associated degradation (ERAD). Recognition of ERAD substrates depends on the presence of both misfolded regions and specific N-linked glycans, but many of the molecular details of this process are unknown. Yos9p is a putative lectin found on the luminal side of the multiprotein ERAD complex; disruption of Yos9p's sugar binding domain leads to stabilization of a model ERAD substrate, which suggests that Yos9p has a specific role in recognizing the ERAD glycan signal. To determine Yos9p's ligand, Quan et al. obtained purified, functional protein by regenerating its disulfide bonds in vitro. Frontal affinity chromatography then demonstrated that Yos9p binds to sugars with a terminal α(1−6)-linked mannose. Though this epitope is not present in the preassembled oligosaccharides that are initially transferred to glycoproteins, use of a mutant strain that could directly produce the modified sugars confirmed that Yos9p recognized these substrates in vivo. Surprisingly, under these circumstances, ERAD no longer depended on Htm1p, a mannosidase-like protein, which indicates that Htm1p acts upstream of Yos9p and suggests that Htm1p generates the terminal α(1−6)-linked mannose on ERAD substrates that allows subsequent recognition by Yos9. Though many questions remain, these results highlight two likely ERAD checkpoints and provide new glycan determinants to probe ERAD function. (Mol. Cell 32, 870–877, 2008) CG
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