**4.3 Phosphoprotein (P)**

The P protein (241 amino acids) is a homotetrameric protein consisting of N-terminal domain, oligomerization domain, and C-terminal domain and it functions as a cofactor of RdRp and plays a significant role in transcription and replication by networking with other RSV proteins [31–35]. P protein functions as a multimodular adaptor for RNA synthesis by interacting with N-RNA, L, and M2–1 [36]. P can act as a chaperone for newly synthesized N (N0 ) protein by forming an N0 -P complex that prevents the association of N0 with host RNA [37]. This protein is heavily phosphorylated by host kinase enzymes and it has 41 serine and threonine residues as potential phosphorylation sites; specifically, phosphorylation at residues T105, T188, T210, and S203 are essential for replication, and phosphorylation at residue S156 is vital for viral RNA synthesis [38].

### **4.4 RSV glycoproteins**

As an enveloped virus, the RSV lipid envelope contains three transmembrane glycoproteins including a fusion (F) protein, an attachment glycoprotein (G), and a small hydrophobic (SH) protein; F and G proteins are essential for viral attachment and entry whereas SH protein is less likely involved in viral entry and budding [39, 40].

### *4.4.1 Fusion (F) protein*

Fusion protein is a type 1 transmembrane protein (574 amino acids including a cytoplasmic tail domain of approximately 24 residues) involved in viral entry and assembly [39, 41]. Initially, F protein is synthesized as F0 protein and subsequently, F0 undergoes post-translational modification with multiple N-linked glycosylations depending on RSV strains [42]. To obtain fusion competence, precursor F0 protein (approximately 68–75 KDa) undergoes proteolytic cleavage by furin-like protease which cleaves two polybasic sites and removes a glycosylated peptide of 27 amino acids (Peptide 27 or Pep27) [43, 44]. This cleavage process occurs in the trans-Golgi network and then fusion protein transport to plasma membrane generating two subunits: one is amino-terminal F2 subunit (approximately 15–20 KDa) and another is carboxy-terminal F1 subunit (approximately 50–55 KDa) [45, 46]. A heterodimeric protomer is formed by F1 and F2 subunits covalently connected by disulfide bonds and three protomers combinedly form the matured trimeric form of F protein [47]. After trimerization, F protein exists as a prefusion conformation remaining approximately 12 nm above the membrane of the virus [48]. This prefusion conformation is not a stable form and undergoes a refolding process [6, 49]. This refolding process creates a more stable post-fusion conformation of F protein remaining approximately 17 nm above the viral membrane [50, 51]. The sequence difference of F ectodomains is almost 5% between RSV A and RSV B and therefore, F protein undergoes less antigenic drift and gets preference for suitable vaccine candidates [52].
