Assembly and budding of influenza virus

Abstract

Influenza viruses are causative agents of an acute febrile respiratory disease called influenza (commonly known as "flu") and belong to the Orthomyxoviridae family. These viruses possess segmented, negative stranded RNA genomes (vRNA) and are enveloped, usually spherical and bud from the plasma membrane (more specifically, the apical plasma membrane of polarized epithelial cells). Complete virus particles, therefore, are not found inside infected cells. Virus particles consist of three major subviral components, namely the viral envelope, matrix protein (M1), and core (viral ribonucleocapsid [vRNP]). The viral envelope surrounding the vRNP consists of a lipid bilayer containing spikes composed of viral glycoproteins (HA, NA, and M2) on the outer side and M1 on the inner side. Viral lipids, derived from the host plasma membrane, are selectively enriched in cholesterol and glycosphingolipids. M1 forms the bridge between the viral envelope and the core. The viral core consists of helical vRNP containing vRNA (minus strand) and NP along with minor amounts of NEP and polymerase complex (PA, PB1, and PB2). For viral morphogenesis to occur, all three viral components, namely the viral envelope (containing lipids and transmembrane proteins), M1, and the vRNP must be brought to the assembly site, i.e. the apical plasma membrane in polarized epithelial cells. Finally, buds must be formed at the assembly site and virus particles released with the closure of buds. Transmembrane viral proteins are transported to the assembly site on the plasma membrane via the exocytic pathway. Both HA and NA possess apical sorting signals and use lipid rafts for cell surface transport and apical sorting. These lipid rafts are enriched in cholesterol, glycosphingolipids and are relatively resistant to neutral detergent extraction at low temperature. M1 is synthesized on free cytosolic polyribosomes. vRNPs are made inside the host nucleus and are exported into the cytoplasm through the nuclear pore with the help of M1 and NEP. How M1 and vRNPs are directed to the assembly site on the plasma membrane remains unclear. The likely possibilities are that they use a piggy-back mechanism on viral glycoproteins or cytoskeletal elements. Alternatively, they may possess apical determinants or diffuse to the assembly site, or a combination of these pathways. Interactions of M1 with M1, M1 with vRNP, and M1 with HA and NA facilitate concentration of viral components and exclusion of host proteins from the budding site. M1 interacts with the cytoplasmic tail (CT) and transmembrane domain (TMD) of glycoproteins, and thereby functions as a bridge between the viral envelope and vRNP. Lipid rafts function as microdomains for concentrating viral glycoproteins and may serve as a platform for virus budding. Virus bud formation requires membrane bending at the budding site. A combination of factors including concentration of and interaction among viral components, increased viscosity and asymmetry of the lipid bilayer of the lipid raft as well as pulling and pushi...

Influenza virus pleiomorphy characterized by cryoelectron tomography

“…The present observations afford insight into the molecular architecture of influenza virions, whose diverse forms represent the outcome of complex patterns of interactions among viral components: the glycoproteins, matrix protein, and the RNPs; and host factors (15). Because different viral strains propagated in the same cells produce different distributions of virion shapes (e.g., elongated vs. spherical), it follows that plasticity in the interactions among viral components is a major contributor to pleiomorphy.…”

“…In this process, the glycoproteins accumulate in lipid rafts, where they interact with the underlying matrix protein. Host factors also participate, particularly in the final stage of pinching off (15). During assembly, a dilemma common to all viruses with segmented genomes must be resolved: how to assign the various segments appropriately to nascent virions?…”

“…Their total number, eight, matches the number of distinct RNA segments in the influenza virus genome. However, it has not been determined whether each virion receives one copy of each segment or a random selection, nor how RNPs are recruited to the budding site (15).…”

Influenza infection modulates vesicular trafficking and induces Golgi complex disruption

“…However, another study [3] demonstrated that actin is not essential for vRNP transport as IFV replication was not affected by disruption of actin polymerization. In addition, there is evidence that vRNPs may be are transported to the plasma membrane through association with the cytoplasmic tails of HA and NA [24]. This latter ''piggy-backing'' transport mechanism is not mutually exclusive with transport in association with the cytoskeleton.…”

Mitogenic stimulation accelerates influenza-induced mortality by increasing susceptibility of alveolar type II cells to infection

“…There, host replication machinery is coopted to produce viral RNA and protein that are packaged into new viral particles and transported to the cell surface (16). Neuraminidase cleaves the sialic acid link between the viral HA and cell surface glycans that tether the virus to the host cell, liberating viral progeny to perpetuate the infectious cycle (17,18).…”