**4. Palmitoylation**

Palmitoylation refers to the attachment of the palmitic fatty acid to the cysteine (S-palmitoylation) but less frequently to the serine and threonine amino acid residues (O-palmitoylation). In coronaviruses studies, the S protein undergone palmitoylation in infected cells and in the presence of tunicamycin it does not undergoes palmitoylation [37]. In another study conducted on MHV S protein reduces infectivity of MHV when treated with palmitoyl acyltransferase inhibitors 2-bromopalmitate [38]. The cytoplasmic part of the SARS-CoV S protein consists of four cysteine-rich clusters among them 2 clusters modified upon palmitoylation. However, cell surface expression of SARS-CoV S protein was unaffected due to this palmitoylation. In one of the previously published study, it was found that treatment of nitric oxide significantly leads to a reduction in the palmitoylation of the S protein of SARS-CoV [39].

In one of the study, it was found that there is three cysteines at position 40, 43 and 44 are found to undergo palmitoylation in the E protein of the SARS-CoV [40]. In another study, homologous cysteine of the E protein of MV-A59 at position C40, C44 and C47 were mutated to the alanine residues as resultant infectivity decreased [41]. It is therefore concluded that palmitoylation of the MHV E protein contributes to the stability and biological activity of the mature virions. Contrary, palmitoylation of the SARS CoV E protein is not mandatory for its interception with N protein.
