Table 2.

Detailed view of structural genes of coronavirus

Structural genes	Composition	Functions	Reference
Nucleocaspid (N)	Two separate domains, i.e., N-terminal and C-terminal. Highly phosphorylated, TRSs and genomic packaging signals are two specific RNA substrates, consists of nsp3 (component of M protein and replicase complex)	Domains required to bind RNA, phosphorylation leads to a structural change which increases the affinity of viral RNA versus non-viral RNA and this viral genome binds with N protein forming beads like conformation. Genomic packaging signal bind specifically to the second or C-terminal RNA binding domain. Protein interaction helps in the formation of replicase-transcriptase complex (RTC) and packaging of encapsulated genome into the viral particles	[21–28]
Membrane (M)	Most abundant, small in size (~ 25–30 kDa), 3 transmembrane domain structural protein, contains small N- and large C-terminal glycosylated ectodomain which ranges from 6 to 8 nm inside the virus	Helps to give shape to virion and exists in dimer from different conformations, so that membrane curvatures and nucleocapsid can bind very well. Most of the M protein does not contain signal sequences although they are translationally inserted in the endoplasmic reticulum membrane	[29, 30]
Envelope (E)	Present in small amount (∼ 8–12 kDa), transmembrane protein in the virion. Highly divergent but consists of a common architect. Also consists of a N-ectodomain and C-endodomain terminal with ion channel activity	Membrane topology of E protein is not completely resolved. Helps in assembly and release of the virus and ion channel in SARS-CoV; this protein is required for pathogenesis but not for viral replication	[31–33]
Spike (S)	Heavily N-linked glycosylated protein of ~ 150 kDa which uses N-terminal sequence for the functioning to the ER, consists of S glyco-trimeric class I fusion protein in which S1 helps in the formation of the large receptor-binding domain and S2 in stalk of the spike molecule	Helps in the formation of homotrimers encoded by S protein spike like structure on the outer surface of the virion, Helps in attachment to the host receptor	[15–20]
Hemagglutinin-esterase (HE)	Subset of β-coronaviruses, acts as a hemagglutinin which binds sialic acids on the surface of glycoproteins and also contains acetyl-esterase activity	Enhances murine hepatitis virus (MHV) neurovirulence and the S protein-mediated cell entry through which virus spread through mucosa. It is also used against tissue culture whose function is still unknown	[29, 30]