**5.3 Virion assembly and budding**

Both assembly and budding of RSV occur at the apical side of ciliated cells [156]. RSV assembly is associated with lipid microdomain or lipid raft rich in cholesterol and sphingolipids; specifically, RSV filament formation observed in caveolin-1 and lipid-raft ganglioside GM1 rich regions of host cell surface membrane [157–159].

#### *Respiratory Syncytial Virus DOI: http://dx.doi.org/10.5772/intechopen.104771*

RSV assembly into viral filament occurs at the cell surface requiring the activity of F protein cytoplasmic tail and M protein and this process are not dependent on actin polymerization [160]. However, Mehedi et al., showed the depletion of ARP2 resulted in perturbation of RSV progeny virion on the infected cell surface, consequently reducing viral shedding [8]. Viral assembly requires the activity of F protein cytoplasmic tail and M protein because both proteins accumulate in inclusion bodies cytoplasmic tail of F protein enables the release of the complex of matrix and RNP from inclusion bodies [161]. Although previous studies showed that three proteins including M, P, and F proteins are enough to create virus-like particles, a recent nuclear magnetic resonance study suggests that three novel interaction sites of M on P including site I in αN2 region, site II in 115 to 125 region and oligomerization domain where oligomerization domain is necessary for virus-like structure formation and virus release [137]. The incorporation of RSV proteins into lipid microdomains during virus assembly can cause the interaction of F protein with host factors including caveolin-1, CD44, RhoA, causing microvillus-like projections essential for virus filament and syncytium formation [162, 163]. Actin cytoskeleton and actinassociated pathways linked with PI3K and Rac GTPase are involved in RSV assembly [164]. M protein can bind DNA as well as RNA and it localizes into the nucleus mediated by importin-β1 nuclear import receptor, which forms a complex with guanine nucleotide-binding protein Ran and binds M protein amino acid 100–183 [165, 166]. During the early phase of infection, nuclear accumulation of M protein was observed when M protein interacts with nuclear components mediated by its zinc finger domain resulting in the inhibition of host cell transcription [165]. During the later phase of infection, M protein undergoes phosphorylation inducing nuclear export mediated by Crm1 by unmasking the nuclear export signal [78]. Therefore, M protein is thought to play a regulatory role as a transcription inhibitory factor by inhibiting viral transcriptase to facilitate RSV assembly and budding [77, 167]. RSV glycoprotein and RNP vesicles combined together prior to the filamentous virus formation and G protein recycling has been observed via clathrinmediated endocytosis, which might be connected with filamentous RSV formation [168]. RSV budding preferentially appears at the apical membrane of epithelial cells by an apical recycling endosome (ARE)-mediated apical protein sorting pathway [169]. RSV budding is independent of the endosomal sorting complex necessary for transport (ESCRT) mechanism controlled by ARE-associated protein, Rab11 family interacting protein 2 (FIP2) [170]. Recently, ARP2 is identified as a novel host factor of RSV budding and cell-to-cell spread [8].
