Protein Glycosylation Patterns Shaped By the IRE1-XBP1s Arm of the Unfolded Protein Response

Protein Glycosylation

Protein post-translational modifications, including phosphorylation, acetylation, ubiquitylation, and more, confer key levels of regulation and can dramatically alter the structure and function of proteins, acting as molecular switches or rheostats for tuning activity.¹ Many post-translational modifications are specialized to specific subcellular compartments and clientele, such as the sophisticated pathways for protein N-glycosylation in the endoplasmic reticulum (ER) and Golgi mediated by a suite of glycosyltransferase and glycosidase enzymes. Protein glycosylation involves covalent modification of amino acid sidechains with sugars to yield linear or branched structures (glycans; Figure 1). The consequences of glycosylation shape protein function, cell–cell recognition, cell–matrix interactions, and more.²

Figure 1

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Protein N-linked glycosylation is a co- and post-translational modification that involves the installation of glycans on asparagine side chains in specific amino acid sequons in proteins traversing the secretory pathway. A: A 14-residue precursor oligosaccharide is first synthesized in a step-wise fashion while attached to a dolichol pyrophosphate molecule on the ER membrane. Monosaccharide substrates in the form of nucleotide sugars are each added to the growing sugar chain by their respective transferase enzymes. The dolichol-linked precursor then requires the action of flippase enzymes, prior to being added to a nascent ER client protein by the oligosaccharyltransferase (OST) complex as the polypeptide translocates from the ribosome to the ER. Note that N-glycans can also be installed post-translationally by OST. After installation of the precursor, folding and initial trimming occurs in the ER and the nascent glycoprotein is trafficked to the Golgi for further processing. B: Glycan-modifying enzymes in the ER and Golgi process the N-glycan via sequential removal and addition of monosaccharides by specific enzymes, ultimately yielding a vast array of potential glycan structures, including hybrid glycans, complex glycans, core fucosylated glycans, and sialylated glycans. The specific identity of the glycan has important and varied consequences for cellular communication and the function of N-glycoproteins.

Unlike other biomacromolecules, such as DNA, RNA, and proteins, glycans are synthesized without templates, instead relying on the availability/synthesis of nucleotide-activated monosaccharides as building blocks, their associated transporters,³ and enzymes that mediate the addition and removal of saccharides.⁴ While several forms of protein glycosylation occur in cells (including but not limited to N-linked, O-linked, C-linked, and S-linked forms of glycosylation), N-linked glycosylation of asparagine is perhaps the most common.⁵ N-Glycosylation features step-wise synthesis of a 14-membered precursor oligosaccharide, en bloc transfer of that precursor onto (typically) Asn-Xaa-Ser/Thr (where Asn=asparagine; Xaa=any amino acid except proline; Ser=serine; Thr=threonine) sequons in ER client proteins, and then further step-wise processing by ER- and Golgi-localized enzymes (Figure 1),⁶ ultimately yielding an enormous diversity of highly branched structures.

N-Glycosylation is evolutionarily conserved,⁷ and has wide-ranging impacts on health and disease.⁸ Indeed, all kingdoms of life feature N-glycosylation, although they may utilize specialized building blocks depending on the organism.⁹