Post-translational modifications of coronavirus proteins: roles and function

Fung, To Sing; Liu, Ding Xiang

N-linked glycosylation contributes significantly to the conformation of coronavirus S protein, and therefore profoundly affects the receptor binding and antigenicity of S protein. For example, early studies showed that the binding of IBV neutralizing antibodies was dependent on the glycosylation of the IBV S protein [47] . Consistently, mutations that introduced new N-linked glycosylation sites in the S1 domain were shown to contribute to antigenic shifting of IBV [48] . Also, when the S1 domain of BCoV S protein was cloned and expressed in insect cells, the mature protein was glycosylated and bound by neutralizing monoclonal antibodies [49] . In contrast, when cells were infected with TGEV in the presence of tunicamycin, an inhibitor of N-linked glycosylation, the antigenicity of both S and M protein was significantly reduced [50] . Similarly, when the overexpressed full-length homotrimeric SARS-CoV S protein was treated with PNGase F under a native condition, the protein was no longer recognized by neutralizing antisera raised against purified virions [51] . This finding suggests that N-linked glycosylation may play an important role in constituting the native structure of coronavirus S protein, thereby affecting its antigenicity. During its maturation in the ER, SARS-CoV S protein binds to the molecular chaperone calnexin [52] . Compared with control, SARS-CoV S-pseudotyped virions produced in calnexin-knockdown cells contained S protein with aberrant N-glycans and exhibited significantly lower infectivity [52] . As for IBV, we recently showed that N-D or N-Q mutations at the N-linked glycosylation site N212 or N276 abolished the function of S protein to induce cell-cell fusion and the infectivity of corresponding recombinant viruses [46] .