Deciphering Protein O-Glycosylation: Solid-Phase Chemoenzymatic Cleavage and Enrichment.

Glycosylation plays a critical role in the biosynthetic-secretory pathway in the endoplasmic reticulum (ER) and Golgi apparatus. Over 50% of mammalian cellular proteins are typically glycosylated; this modification is involved in a wide range of biological functions such as barrier formation against intestinal microbes and serves as signaling molecules for selectins and galectins in the innate immune system. N-linked glycosylation analysis has been greatly facilitated owing to a range of specific enzymes available for their release. However, system-wide analysis on O-linked glycosylation remains a challenge due to the lack of equivalent enzymes and the inherent structural heterogeneity of O-glycans. Although O-glycosidase can catalyze the removal of core 1 and core 3 O-linked disaccharides from glycoproteins, analysis of other types of O-glycans remains difficult, particularly when residing on glycopeptides. Here, we describe a novel chemoenzymatic approach driven by a newly available O-protease and solid phase platform. This method enables the assignment of O-glycosylated peptides, N-glycan profile, sialyl O-glycopeptides linkage, and mapping of heterogeneous O-glycosylation. For the first time, we can analyze intact O-glycopeptides generated by O-protease and enriched using a solid-phase platform. We establish the method on standard glycoproteins, confirming known O-glycosites with high accuracy and confidence, and reveal up to 8-fold more glycosites than previously reported with concomitant increased heterogeneity. This technique is further applied for analysis of Zika virus recombinant glycoproteins, revealing their dominant O-glycosites and setting a basis set of O-glycosylation tracts in these important viral antigens. Our approach can serve as a benchmark for the investigation of protein O-glycosylation in diseases and other biomedical contexts. This method should become an indispensable tool for investigations where O-glycosylation is central.