Compartmentalized replication organelle of flavivirus at the ER and the factors involved

Abstract

Flaviviruses are positive-sense single-stranded RNA viruses that pose a considerable threat to human health. Flaviviruses replicate in compartmentalized replication organelles derived from the host endoplasmic reticulum (ER). The characteristic architecture of flavivirus replication organelles includes invaginated vesicle packets and convoluted membrane structures. Multiple factors, including both viral proteins and host factors, contribute to the biogenesis of the flavivirus replication organelle. Several viral nonstructural (NS) proteins with membrane activity induce ER rearrangement to build replication compartments, and other NS proteins constitute the replication complexes (RC) in the compartments. Host protein and lipid factors facilitate the formation of replication organelles. The lipid membrane, proteins and viral RNA together form the functional compartmentalized replication organelle, in which the flaviviruses efficiently synthesize viral RNA. Here, we reviewed recent advances in understanding the structure and biogenesis of flavivirus replication organelles, and we further discuss the function of virus NS proteins and related host factors as well as their roles in building the replication organelle.

Keywords: Compartmentalization; ER rearrangement; Flavivirus; Host factors; Membrane remodeling; Nonstructural proteins; Replication organelle.

Fig. 1

Life cycle of flavivirus. Flavivirus enters cell through endocytosis pathway upon binding to some receptors on cell surface. Viral E protein drives the membrane fusion between viral envelope and endosome membrane to release the nucleocapsid. Flavivirus replication and virion assembly occur at the ER. Immature virion is then transported to the Golgi apparatus and the maturation happens in the trans-Golgi network. Viral particle is finally released outside the cell through exocytosis pathway

Fig. 2

Flavivirus replication organelles derived from the ER. Top panel, TEM image of DENV replication organelles derived from the ER. T tubes, Ve virus-induced vesicles, CM convoluted membranes. Middle panel, continuous slices through an EM tomogram (~ 2 nm thick). White arrowhead, continuity of vesicle and ER membranes; black arrowhead, virus particles. Bottom panel, three-dimensional architecture of flavivirus replication organelles (left) and the model of flavivirus replication, assembly and release (right). These images are reproduced with permission from Ref. [7]

Fig. 3

Localization and topology of flavivirus structural and NS proteins. Flavivirus genomic RNA is translated into a multitransmembrane polyprotein precursor on the ER and then cleaved into ten individual proteins by viral and host proteases, including three structural proteins (capsid (C), prM and E) and seven NS proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5). NS1 is located in the ER lumen. Capsid, NS3 and NS5 localize in the cytoplasm (some capsid and NS5 are transported into the nucleus). Other viral structural proteins and NS proteins are ER membrane-anchored proteins

Fig. 4

Flavivirus NS1 induces invaginated vesicles at the ER. Several hydrophobic residues in β-roll, greasy finger and wing flexible loop of NS1 can insert into lumenal leaflet of the ER membrane to induce the curvature. Thus, ER lumen located NS1-induced invaginated vesicles at the ER by the mechanism of asymmetrically insertion into the lipid membrane

Fig. 5

Flavivirus replication organelles at the ER. Membrane active NS proteins induce invaginated replication compartments at the ER, and RC (NS3, NS5 and viral RNA etc.) assemble in such compartments to carry out viral RNA synthesis. Host factors may also contribute to the establishment of flavivirus replication organelles