Post-translational modifications of coronavirus proteins: roles and function

Fung, To Sing; Liu, Ding Xiang

The cytoplasmic portion of SARS-CoV S protein contains four cysteine-rich clusters. Mutational analysis showed that cysteine clusters I and II were modified by palmitoylation. Although cell surface expression of SARS-CoV S protein was not significantly affected by mutations in cysteine clusters I and II, S-mediated cell fusion was markedly reduced compared with wild-type protein, suggesting that palmitoylation in the endodomain may be required for the fusogenic activity of SARS-CoV S protein [75] . In a later study, a recombinant nonpalmitoylated SARS-CoV S protein was generated by mutating all nine cytoplasmic cysteines to alanines [76] . Using this nonpalmitoylated mutant, it was shown that similar to MHV S protein, palmitoylation of the SARS-CoV S protein was required for its partitioning into detergent-resistant membranes and for cell-cell fusion. However, unlike MHV S protein, palmitoylation of SARS-CoV S protein was not required for S-M interaction [76] . Interestingly, treatment of nitric oxide or its derivatives led to a reduction in the palmitoylation of SARS-CoV S protein, which affected its binding to the cognate receptor ACE2 [77] . The S protein of the Alphacoronavirus TGEV is also modified by palmitoylation, and inhibition of palmitoylation by 2-bromopalmitate treatment reduced TGEV replication in cell culture [78] . Although palmitoylation of TGEV S protein was essential for its incorporation into virus-like particles (VLP), the interaction between TGEV S and M proteins was not affected by the lack of palmitoylation [78] . Therefore, dependent on the coronavirus in question, palmitoylation may differentially affect the folding, fusogenic activity and/or protein-protein interaction of S protein. Palmitoylation of S protein has not been characterized for other coronaviruses.