Dark Proteome of Newly Emerged SARS-CoV-2 in Comparison with Human and Bat Coronaviruses

Rajanish Giri; Taniya Bhardwaj; Meenakshi Shegane; Bhuvaneshwari R. Gehi; Prateek Kumar; Kundlik Gadhave

Nsp4 has been reported to induce the formation of the double-membrane vesicles (DMVs) with the co-expression of full-length Nsp3 and Nsp6 proteins for optimal replication inside host cells [134] [135] [136] . It localizes itself in ERmembrane, when expressed alone but is demonstrated to be present in replication units in infected cells. It was observed that Nsp4 protein contains a tetraspanning transmembrane region having its N-and C-terminals in the cytosol [137] . No crystal or NMR solution structure is reported for this protein as of yet. Nsp4 protein of SARS-CoV-2 has multiple substitutions near the N-terminal region and has a quite conserved C-terminus (Supplementary Figure S2C) . It is found to be closer to Nsp4 of Bat CoV (81.40% identity) than to Human SARS Nsp4 (80%). Mean PPIDs of Nsp4s from SARS-CoV-2, Human SARS, and Bat CoV are estimated to be 0.80%, 0.60%, and 0.60% respectively. The low level of intrinsic disorder is further illustrated by Figures 22A, 22B , and 22C. With PPIDs around zero, Nsp4 were classified as highly structured proteins, which, however, contain some flexible regions. Likewise, Table 2 shows the presence of only Nand C-terminal MoRFs which possibly assist in cleavage of Nsp4 protein from long polyproteins 1a and 1ab.