Virus factories, double membrane vesicles and viroplasm generated in animal cells

Abstract

Many viruses reorganise cellular membrane compartments and the cytoskeleton to generate subcellular microenvironments called virus factories or 'viroplasm'. These create a platform to concentrate replicase proteins, virus genomes and host proteins required for replication and also protect against antiviral defences. There is growing interest in understanding how viruses induce such large changes in cellular organisation, and recent studies are beginning to reveal the relationship between virus factories and viroplasm and the cellular structures that house them. In this review, we discuss how three supergroups of (+)RNA viruses generate replication sites from membrane-bound organelles and highlight research on perinuclear factories induced by the nucleocytoplasmic large DNA viruses.

Figure 1

Models for formation of sperules and double membrane vesicles during replication of (+)strand RNA viruses. Panel 1: Spherule produced by alphaviruses. Replicase proteins (red spheres) are recruited to the cytoplasmic face of membrane-bound organelles. Assembly of replicase proteins induces membrane curvature and invagination forming a spherule. The spherule remains connected to the limiting membrane of the organelle and a pore allows new genomes to enter the cytosol (adapted from [2]). Panel 2: Virus-induced vesicles and double membrane vesicles generated by DENV flavivirus. Replicase proteins (red spheres) are recruited to the cytoplasmic face of membrane-bound organelles. Assembly of replicase proteins induces membrane curvature and invagination into the ER forming a large spherule. The invagination remains connected to the limiting membrane of the organelle and a pore allows new genomes to enter the cytosol. Close apposition of ER membranes leads to the formation of DMVs connected to the ER by convoluted membranes (CM) (adapted from [10^••]). This may close the pore leading to the cytosol. Panel 3: Virus-induced vesicles and double membrane vesicles generated by SARS-CoV coronavirus. Replicase proteins (red spheres) are recruited to the cytoplasmic face of membrane-bound organelles. Assembly of replicase proteins induces membrane curvature and invagination into the ER forming a large spherule. It has been difficult to find evidence for a pore connecting invaginations to cytosol. Close apposition of ER membranes leads to the formation of DMVs connected to the ER by CM. These may exclude replicase proteins and become sites for storage of viral RNA. In some cases the close apposition of ER membranes is lost and single membraned vesicles arranged in vesicle packets (VP) appear within membrane networks connected to the ER (adapted from [11^••]).

Figure 2

Sites of replication generated by plant and animal viruses. The diagram is in two halves. The left represents a plant cell and is surrounded by a cell wall (green). Viral replication complexes (VRC) are generated by plant viruses at the ER, chloroplast and peroxisomes. Tobamovirus VRCs are initiated at the tonoplast. VRCs of many plant viruses can be transported within the cell along the ER and between cells using the plasmodesmata. This movement is directed by plant movement proteins (MP); see accompanying article by Jeanmarie Verchot for details. The right represents an animal cell. Alphavirus and nodaviruses form spherules at the endosomal–lysosomal system and mitochondria respectively. Coronavirus, arterivirus and flaviviruses form double membrane vesicles (DMVs) from the ER and coronaviruses and arteriviruses also form vesicle packets (VPs). The membranes used to generate sites of picornavirus replication are unclear but may involve the Golgi, ER and/or autophagosomes. Nucleo-cytoplasmic large DNA viruses (NCLDV) factories are formed after microtubule-mediated delivery of incoming viruses to the microtubule organising centre next to the nucleus. Thereafter they recruit host chaperones, mitochondria and intermediate filaments.