Abstract

The densely glycosylated spike (S) proteins that are highly exposed on the surface of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) facilitate viral attachment, entry, and membrane fusion. We have previously reported all the 22 N-glycosites and site-specific N-glycans in the S protein protomer. Herein, we report the O-glycosylation landscapes of SARS-CoV-2 S proteins, which were characterized through high-resolution mass spectrometry. Following digestion with trypsin and trypsin/Glu-C, and de-N-glycosylation using PNGase F, we determined the GalNAc-type O-glycosylation pattern of S proteins, including O-glycosites and the six most common O-glycans occupying them, via Byonic identification and manual validation. Finally, 255 intact O-glycopeptides composed of 50 peptides sequences and 43 O-glycosites were discovered by higher energy collision-induced dissociation (HCD), and three O-glycosites were confidently identified by electron transfer/higher energy collision-induced dissociation (EThcD) in the insect cell-expressed S protein. Most glycosites were modified by non-sialylated O-glycans such as HexNAc(1) and HexNAc(1)Hex (1). In contrast, in the human cell-expressed S protein S1 subunit, 407 intact O-glycopeptides composed of 34 peptides sequences and 30 O-glycosites were discovered by HCD, and 11 O-glycosites were unambiguously assigned by EThcD. However, the measurement of O-glycosylation occupancy hasn’t been made. Most glycosites were modified by sialylated O-glycans such as HexNAc(1)Hex (1)NeuAc (1) and HexNAc(1)Hex (1)NeuAc (2). Our results reveal that the SARS-CoV-2 S protein is an O-glycoprotein; the O-glycosites and O-glycan compositions vary with the host cell type. These comprehensive O-glycosylation landscapes of the S protein are expected to provide novel insights into the viral binding mechanism and present a strategy for the development of vaccines and targeted drugs.

Highlights

  • The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an extensively N-glycosylated protein (Watanabe et al, 2020) that protrudes from the virus surface and binds to the angiotensin-converting enzyme 2 (ACE2) receptor on host cells to mediate cell entry (Wrapp et al, 2020)

  • Most O-glycosites of the human cell–produced S1 subunit contained HexNAc(1)Hex (1)NeuAc (2), HexNAc(1) Hex (1)NeuAc (1), and HexNAc(1)Hex (1) (Figure 4D). These results implied that the O-glycosite and O-glycan compositions varied with the host cell type, which could be taken into account when using the recombinant proteins for vaccine and drug development

  • HexNAc(2)Hex (1) and HexNAc(2) were not verified by EThcD in the S expressed in insect cells, which suggested that the two O-glycans might be misidentified by higher energy collision-induced dissociation (HCD) due to fragmentation of glycans. All of these results indicate that with both HCD and EThcD, we can identify a large number of intact O-glycopeptides of spike proteins

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Summary

Introduction

The spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an extensively N-glycosylated protein (Watanabe et al, 2020) that protrudes from the virus surface and binds to the angiotensin-converting enzyme 2 (ACE2) receptor on host cells to mediate cell entry (Wrapp et al, 2020). O-glycosylation of SARS-CoV-2 Spike Proteins high-resolution liquid chromatography–tandem mass spectrometry (LC-MS/MS) (Lenza et al, 2020; Rosenbalm et al, 2020; Walls et al, 2020; Xu et al, 2020; Yan et al, 2020; Zhang et al, 2020; Wang et al, 2021; Zhou et al, 2021) These N-glycosites are preferentially distributed in two functional subunits responsible for receptor binding (S1 subunit) and membrane fusion (S2 subunit) (Zhang et al, 2020). The comprehensive O-glycosylation analysis cannot be performed without appropriate sample preprocessing, analysis methods, and software (King et al, 2017; Qin et al, 2017; Yang et al, 2017; Yang et al, 2018; Ye et al, 2019; Park et al, 2020; Dong et al, 2021)

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