XBP1s Activation Globally Remodels N-Glycan Structure Distribution Patterns

The unfolded protein response (UPR), as its name implies, safeguards secretory pathway proteostasis. The most ancient arm of the UPR, the IRE1-activated, XBP1s-mediated transcriptional response, has roles in secretory pathway maturation beyond resolving proteostatic stress. Understanding the consequences of XBP1s' transcriptional output for cellular processes is critical for elucidating mechanistic connections between XBP1s and development, immunity, and disease. Here, we show that a key functional consequence of XBP1s activation is a cell type-dependent shift in the distribution of N-glycan structures on endogenous membrane and secreted proteomes. XBP1s activity decreases sialylation of tri- and tetra-antennary N-glycans in the HEK293 membrane proteome and secretome, while substantially increasing the population of high mannose N-glycans only in the secretome. Related, but distinctive, signatures in the HEK293 N-glycome are observed when the entire UPR is activated in a stress-dependent manner using thapsigargin. In HeLa cells, stress-independent XBP1s activation increases the population of cell surface high mannose N-glycans and tetra-antennary N-glycans. mRNA profiling experiments suggest that the XBP1s-mediated remodeling of the N-glycome may re-flect a coordinated consequence of transcriptional resculpting of the N-glycan maturation pathway by XBP1s. The discovery of XBP1s-induced N-glycan structural remodeling on a glycome-wide scale suggests that XBP1s is a master regulator of N-glycan maturation. Moreover, because the sugars on cell surface proteins or on those proteins secreted from an XBP1s-activated cell can be molecularly distinct from those of an unactivated cell, these findings reveal a potential new mechanism for translating intracellular stress signaling pathways into al-tered interactions with the extracellular environment. Overall design: Three biological replicates of HeLaXBP1s cells treated with DMSO vehicle, 1 ug/ml dox or 750 nM Thapsigargin.

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Wong MY, Chen K, Antonopoulos A, Kasper BT, Dewal MB, Taylor RJ, Whittaker CA, Hein PP, Dell A, Genereux JC, Haslam SM, Mahal LK, Shoulders MD