A mature virion of the respiratory syncytial virus (RSV) consists of the ribonucleoprotein complex (RNP) surrounded by the protein matrix and a lipid bilayer envelope. The RNP is composed of the genomic negative sense single-stranded (-ssRNA) that is tightly associated with the N protein (nucleoprotein) and the RNA-dependent RNA polymerase complex (RdRP). The RdRP consists of the L protein subunit (large polymerase subunit), the P protein subunit (phosphoprotein polymerase cofactor), and the M2-1 protein, which acts as a transcription processivity factor. The matrix consists of the M (matrix) protein. The M2-1 protein serves as the bridge between the RNP and the M protein. The matrix supports the viral envelope. The viral envelope contains three embedded viral proteins: fusion protein (F), attachment protein (G), and a small hydrophobic protein (SH). The M protein associates with the cytoplasmic domain of the F protein. The SH protein forms a pentameric ion channel in the viral envelope and is thought to delay apoptosis of infected cells. The assembly and budding of RSV virions primarily occurs at the apical surface of ciliated airway epithelial cells where viral filaments containing RNPs form. The budding of RSV virions requires interactions between viral proteins, host cytoskeletal proteins, and membrane. For review, please refer to Shaikh and Crowe 2013, and Battles and McLellan 2019.

Based on the findings that P, M, and F proteins are sufficient for formation of viral‑like particles (VLPs), P protein, particularly its highly phosphorylated serine/threonine‑rich region between amino acids 39 and 57 that likely interacts with M and/or F proteins, may play an important role in the assembly (Meshram and Oomens 2019).