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The tight coupling of protein folding pathways with disposal mechanisms promotes the efficacy of protein production in the endoplasmic reticulum (ER). It has been hypothesized that the ER-resident molecular chaperone glucose-regulated protein 94 (GRP94) is part of this quality control coupling because it supports folding of select client proteins yet also robustly associates with the lectin osteosarcoma amplified 9 (OS-9), a component involved in ER-associated degradation (ERAD). To explore this possibility, we investigated potential functions for the GRP94/OS-9 complex in ER quality control. Unexpectedly, GRP94 does not collaborate with OS-9 in ERAD of misfolded substrates, nor is the chaperone required directly for OS-9 folding. Instead, OS-9 binds preferentially to a subpopulation of GRP94 that is hyperglycosylated on cryptic N-linked glycan acceptor sites. Hyperglycosylated GRP94 forms have nonnative conformations and are less active. As a result, these species are degraded much faster than the major, monoglycosylated form of GRP94 in an OS-9-mediated, ERAD-independent, lysosomal-like mechanism. This study therefore clarifies the role of the GRP94/OS-9 complex and describes a novel pathway by which glycosylation of cryptic acceptor sites influences the function and fate of an ER-resident chaperone.

Original publication

DOI

10.1091/mbc.E14-03-0805

Type

Journal article

Journal

Mol biol cell

Publication Date

01/08/2014

Volume

25

Pages

2220 - 2234

Keywords

Adenosine Triphosphate, Endoplasmic Reticulum-Associated Degradation, Glycosylation, HEK293 Cells, Humans, Kinetics, Lectins, Lysosomes, Membrane Glycoproteins, Neoplasm Proteins, Protein Binding, Protein Interaction Domains and Motifs, Protein Processing, Post-Translational, Proteolysis