Post-translational modifications of coronavirus proteins: roles and function

Fung, To Sing; Liu, Ding Xiang

Nonetheless, in some instances, the antigenicity of coronavirus S protein does not depend on its glycosylation status. For example, when the S protein of TGEV was expressed by recombinant baculovirus in insect cells, the recombinant S protein acquired high mannose glycans, but the complete processing into complex glycans was not efficient. However, the recombinant TGEV S protein still exhibited antigenic properties and induced a high level of neutralizing antibodies [53] . Similarly, a potent neutralizing monoclonal antibody against the S1 protein of SARS-CoV could bind to the deglycosylated S1 protein, suggesting that the epitope was not glycosylationdependent [54] . In one early study, the RBD of SARS-CoV S protein was mapped to amino acid residues 319-518, which contained two potential glycosylation sites N330 and N357. However, mutation of N330 or N357 to either alanine or glutamine did not affect the binding ability of RBD-containing fragment to the cognate receptor ACE2 [55] . Later, the structure of RBD of SARS-CoV S protein complexed with human ACE2 was determined, and both N330 and N357 were not positioned in the interface where the two proteins interacted [56] . It was thus concluded that glycosylation did not always constitute neutralizing epitopes within the RBD. A later study exploring recombinant RBD of SARS-CoV S protein as a vaccine candidate found that yeast-expressed recombinant RBD (spanning amino acid residues 318-536) with glycosylation sites removed indeed induced a higher level of neutralizing antibody in immunized mice, compared with wild type RBD [57] .