Cytoplasmic viral replication complexes

Abstract

Many viruses that replicate in the cytoplasm compartmentalize their genome replication and transcription in organelle-like structures that enhance replication efficiency and protection from host defenses. In particular, recent studies with diverse positive-strand RNA viruses have further elucidated the ultrastructure of membrane-bound RNA replication complexes and how these complexes function in close coordination with virion assembly and budding. The structure, function, and assembly of some positive-strand RNA virus replication complexes have parallels and potential evolutionary links with the replicative cores of double-strand RNA virus and retrovirus virions and more general similarities with the replication factories of cytoplasmic DNA viruses.

Figure 1

BMV-Induced Perinuclear ER RNA Replication Vesicles and Model of the BMV RNA Replication Complex (A and B) (A) and (B) show low- and high-magnification EM images, respectively, of BMV-induced spherular vesicles invaginated into the perinuclear ER membrane (adapted from Schwartz et al., 2002). (C) Diagram of the vesicular BMV RNA replication complex, illustrating the many copies of the membrane-bound, self-interacting 1a protein (blue) that induce vesicle formation, the ∼20-fold fewer copies of 2a polymerase (yellow), and replicative intermediate RNAs, including positive-strand RNA (red arrows) and negative-strand RNA (black dashed arrow).

Figure 2

FHV-Induced RNA Replication Vesicles on Outer Mitochondrial Membranes (A and B) (A) and (B) show two of a series of multiple EM tomographic images from different planes of a single thick section of FHV-modified mitochondria in an infected Drosophila cell. Labels 1 and 2 refer to the same mitochondria in both panels. (C) Three-dimensional reconstruction by EM tomography of the mitochondria in (A), showing outer (blue) and inner (yellow) mitochondrial membranes and FHV-induced replication vesicles (white). (D) Angled view of the side and top of RNA replication vesicles showing open neck-like connections with the cytoplasm (adapted from Kopek et al., 2007).

Figure 3

EM Tomographic Three-Dimensional Reconstruction of SARS Coronavirus-Induced, ER-Derived Double-Membrane Vesicles (A and B) Two-dimensional EM sectional view (A) and three-dimensional tomographic reconstruction (B) of SARS coronavirus-induced double-membrane vesicles (yellow/blue) and convoluted membrane structures (brown) (adapted from Knoops et al., 2008).

Figure 4

EM Tomographic Three-Dimensional Reconstruction of Dengue Virus-Induced, ER-Derived Vesicle Packets (A and B) Two-dimensional EM sectional view (A) and three-dimensional tomographic reconstruction (B) of DENV-induced vesicle packets. Virion particles associated with sites of virus assembly and budding are indicated with a black arrowhead in (A) and in red in (B) (adapted from Welsch et al., 2009).

Figure 5

Parallels between Positive-Strand RNA Virus, dsRNA Virus, and Retrovirus Genome Replication (A–C) Schematic representations of (A) the cytoplasmic replicative core of a dsRNA virus, (B) the invaginated, ER membrane-associated RNA replication complex of brome mosaic virus, and (C) a retrovirus virion in the midst of budding from the plasma membrane into the extracellular space. Red arrows represent the positive-strand (i.e., mRNA-sense) viral genomic RNA packaged into each structure to initiate genome replication. Dashed black arrows represent negative-strand RNA in (A) and (B) and negative-strand cDNA in (C), as is initiated prior to virion release for foamy retroviruses.