Post-translational modifications of coronavirus proteins: roles and function

Fung, To Sing; Liu, Ding Xiang

Palmitoylation of coronavirus S protein was initially identified in cells infected with MHV-A59, as 3 H-palmitate was found to be incorporated in unglycosylated S protein in MHV-infected cells treated with tunicamycin [22] . Treatment of palmitoyl acyltransferase inhibitor 2-bromopalmitate at a nontoxic dose reduced palmitoylation of MHV S protein and led to a significant reduction in the infectivity of MHV [72] . Reduction of S palmitoylation correlated with a decreased level of S associated with M protein and subsequent exclusion of S from virions. However, underpalmitoylated S protein could still be expressed on the cell surface to induce cell-cell fusion. The C1347F/C1348S mutant virus harboring mutations in the putative palmitoylation sites exhibited reduced infectivity, further supporting the importance of palmitoylation in virion assembly and infectivity [72] . Using antiviral heptad repeat peptides that only bind to folding intermediates of the fusion process, it was found that MHV S mutants lacking the palmitoylated cysteines were trapped in translational folding states almost ten-times longer than wild-type MHV S protein, leading to slower cell entry and reduced infectivity [73] . In a later study using reverse genetics, the nine cytoplasmic cysteines in MHV S protein were singly or doubly substituted to alanine [74] . Interestingly, no single specific cysteine in the MHV S endodomain was essential for viral replication, but a minimum of three cysteines within the motif independent of position was required for the recovery of viable recombinant MHV [74] .