The translocon-associated protein (TRAP) complex regulates quality control of N-linked glycosylation during ER stress.

ABSTRACT

Asparagine (N)-linked glycosylation is required for endoplasmic reticulum (ER) homeostasis, but how this co- and posttranslational modification is maintained during ER stress is unknown. Here, we introduce a fluorescence-based strategy to detect aberrant N-glycosylation in individual cells and identify a regulatory role for the heterotetrameric translocon-associated protein (TRAP) complex. Unexpectedly, cells with knockout of SSR3 or SSR4 subunits restore N-glycosylation over time concurrent with a diminished ER stress transcriptional signature. Activation of ER stress or silencing of the ER chaperone BiP exacerbates or rescues the glycosylation defects, respectively, indicating that SSR3 and SSR4 enable N-glycosylation during ER stress. Protein levels of the SSR3 subunit are ER stress and UBE2J1 dependent, revealing a mechanism that coordinates upstream N-glycosylation proficiency with downstream ER-associated degradation and proteostasis. The fidelity of N-glycosylation is not static in both nontransformed and tumor cells, and the TRAP complex regulates ER glycoprotein quality control under conditions of stress.