Bromovirus-induced remodeling of host membranes during viral RNA replication
Introduction
The RNA replication compartments of positive-strand RNA viruses are mini-organelles that feature the close association of both viral proteins and host cellular components [1]. Viral RNA replication takes place in close association with cellular membranes and although the architecture of these compartments differs among viral families, they usually involve membrane vesicle formation or other membrane rearrangements. Cellular membranes provide a scaffold for the enrichment of the viral replication factors as well as exploited cellular proteins, provide an environment to protect the viral RNA from cellular defense mechanisms and help to separate and coordinate the various stages of the viral life cycle. Because of the limited coding capacity of positive-strand RNA viruses, the formation and function of the viral replication compartments require complex interactions between the viral proteins, viral genome, and co-opted host factors. Recent work has shown that cellular host proteins that either structurally induce and/or support membrane curvature are required to form the replication compartments [2, 3, 4••]. Moreover, cellular lipid synthesis and appropriate lipid composition are essential for viral replication [5], implying that the membrane is an essential, functional component of the RNA replication compartments. Host membrane remodeling and the role of host genes in viral replication of other positive-strand RNA viruses have been summarized in the following reviews [1, 3, 6, 7•]. This review will focus on recent advances on the formation of the BMV-induced RNA replication complexes.
Brome mosaic virus (BMV) is the type member of the Bromoviridae family and a representative member of the alphavirus-like superfamily of human, animal, and plant positive-strand RNA viruses. With its high yield, tripartite genome and ability to replicate in the yeast Saccharomyces cerevisiae, BMV has served as a productive model to study some of the general features of positive-strand RNA virus infection, including viral genomic RNA replication, gene expression, recombination, and virus–host interactions [8]. In the following sections we will summarize recent work that has enhanced our understanding of BMV replication complex assembly and structure, both of which are probably applicable to other positive-strand RNA viruses as seemingly diverse membrane rearrangements may represent topologically and functionally related structures.
Section snippets
General features of spherule formation
BMV has three genomic RNAs and one subgenomic mRNA. RNA1 encodes the multifunctional replication factor 1a, which contains an N-proximal RNA capping domain that is separated from a C-terminal NTPase/RNA helicase-like domain by a short proline-rich sequence that may be a flexible spacer [9, 10, 11, 12]. 2apol is encoded from RNA2 and contains a central polymerase domain and an N-terminal domain that interacts with the helicase-like domain of 1a [13, 14]. Genomic RNA3 encodes the 3a movement
Host genes involved in spherule formation
Genetic and genomic screens have identified >100 genes whose loss inhibits or enhances BMV RNA replication [8, 27, 28]. The following two sections will cover recent in-depth analysis showing that host factors that regulate proper membrane composition and fluidity or affect the generation and/or maintenance of virus-induced membrane rearrangements are essential for proper spherule formation and function.
Membrane remodeling in plants infected by Bromoviruses
In BMV-infected barley protoplasts, 1a, 2apol, and newly synthesized RNAs show a nearly complete co-localization with ER markers and are found adjacent to or surrounding the nucleus [19•]. Various ER-derived vesicular structures have been documented in natural plant infections by BMV and its close relatives Cowpea chlorotic mosaic virus and Broad bean mosaic virus [39, 40]. Similar invaginations in other membranes are present in plants infected by other Bromoviridae [41], Tombusviridae [42],
Conclusions and future perspectives
The work summarized herein has led to a working model where multiple 1a–1a, 1a–membrane, and 1a–host factor interactions mediate the invagination of the ER membrane as well as regulate spherule size (Figure 1). Nevertheless, new questions about our understanding of the membrane deformation process provide opportunities for further research and development. Obtaining a 1a crystal structure may provide molecular details of how 1a self-interacts and may reveal new functional domains of 1a that are
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We would like to thank Dr. Paul Ahlquist for critical discussions on the review subjects. Research in AD laboratory is supported by a startup fund from La Sierra University. Research in XW lab is supported by NSF grant IOS1265260 and by funds from Virginia Tech University. We apologize to all colleagues whose work could not be cited due to space limitations.
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