Post-translational modifications of coronavirus proteins: roles and function

Fung, To Sing; Liu, Ding Xiang

Coronavirus nsp9 has been characterized as an ssRNA binding protein [152] . The colocalization of nsp9 with other replicase proteins [153] and its interaction with the coronavirus RdRP [154] suggested that the ssRNA-binding activity of nsp9 might play a role during coronavirus genome transcription/replication. Crystallography studies showed that SARS-CoV nsp9 formed homodimer [154] , and higher oligomers could be observed in solution using glutaraldehyde cross-linking [155] . Surprisingly, in spite of 45% sequence homology, nsp9 of HCoV-229E (but not SARS-CoV) was shown to form homodimer linked by a disulfide bond ( Figure 6 ) [155] . Mutation of the disulfide bond forming cysteine 69 to either alanine or serine significantly reduced the binding affinity of HCoV-229E nsp9 to ssRNA or ssDNA, as determined by surface plasmon resonance experiments. Although disulfide bonds are rare in cytosolic proteins, a disulfide-bonded form of nsp9 may be correlated with oxidative stress induced by HCoV-229E infection [155] .