Coronaviruses hijack human enzymes to assemble the sugar coat on their spike glycoproteins. The mechanisms by which human antibodies may recognize the antigenic viral peptide epitopes hidden by the sugar coat are unknown. Glycosylation by insect cells differs from the native form produced in human cells, but insect cell-derived influenza vaccines have been approved by the US Food and Drug Administration. In this study, we analyzed recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein secreted from BTI-Tn-5B1-4 insect cells, by trypsin and chymotrypsin digestion followed by mass spectrometry analysis. We acquired tandem mass spectrometry (MS/MS) spectrums for glycopeptides of all 22 predicted N-glycosylated sites. We further analyzed the surface accessibility of spike proteins according to cryogenic electron microscopy and homolog-modeled structures and available antibodies that bind to SARS-CoV-1. All 22 N-glycosylated sites of SARS-CoV-2 are modified by high-mannose N-glycans. MS/MS fragmentation clearly established the glycopeptide identities. Electron densities of glycans cover most of the spike receptor-binding domain of SARS-CoV-2, except YQAGSTPCNGVEGFNCYFPLQSYGFQPTNGVGYQ, similar to a region FSPDGKPCTPPALNCYWPLNDYGFYTTTGIGYQ in SARS-CoV-1. Other surface-exposed domains include those located on central helix, connecting region, heptad repeats and N-terminal domain. Because the majority of antibody paratopes bind to the peptide portion with or without sugar modification, we propose a snake-catching model for predicted paratopes: a minimal length of peptide is first clamped by a paratope and sugar modifications close to the peptide either strengthen or do not hinder the binding.
Keywords: antibody; cryogenic electron microscopy structure; crystal structures; epitope prediction; glycopeptide/SARS-CoV-2.
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