STT3B-Dependent Posttranslational N-Glycosylation as a Surveillance System for Secretory Protein

Nascent secretory proteins are extensively scrutinized at the encioplasmic reticulum (ER). Various signatures of client proteins, including exposure of hydrophobic patches or unpaired sulfhydryls, are coordinately utilized to reduce nonnative proteins in the ER. We report here the cryptic N-glycosylation site as a recognition signal for unfolding of a natively nonglycosylated protein, transthyretin (TTR), involved in familial amyloidosis. Folding and ER-associated degradation (ERAD) perturbation analyses revealed that prolonged TTR unfolding induces externalization of cryptic N-glycosylation site and triggers STT3B-depenclent posttranslational N-glycosylation. Inhibition of posttranslational N-glycosylation increases detergent-insoluble TTR aggregates and decreases cell proliferation of mutant TTR-expressing cells. Moreover, this modification provides an alternative pathway for degradation, which is EDErVI3-mediated N-glycan-dependent ERAD, distinct from the major pathway of Herpmediated N-glycan-independent ERAD. Hence we postulate that SIT3B-dependent posttranslational N-glycosylation is part of a triage-salvage system recognizing cryptic N-glycosylation sites of secretory proteins to preserve protein homeostasis.