Viral Reorganization of the Secretory Pathway Generates Distinct Organelles for RNA Replication

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Abstract

Summary Many RNA viruses remodel intracellular membranes to generate specialized sites for RNA replication. How membranes are remodeled and what properties make them conducive for replication are unknown. Here we show how RNA viruses can manipulate multiple components of the cellular secretory pathway to generate organelles specialized for replication that are distinct in protein and lipid composition from the host cell. Specific viral proteins modulate effector recruitment by Arf1 GTPase and its guanine nucleotide exchange factor GBF1, promoting preferential recruitment of phosphatidylinositol-4-kinase IIIβ (PI4KIIIβ) to membranes over coat proteins, yielding uncoated phosphatidylinositol-4-phosphate (PI4P) lipid-enriched organelles. The PI4P-rich lipid microenvironment is essential for both enteroviral and flaviviral RNA replication; PI4KIIIβ inhibition interferes with this process; and enteroviral RNA polymerases specifically bind PI4P. These findings reveal how RNA viruses can selectively exploit specific elements of the host to form specialized organelles where cellular phosphoinositide lipids are key to regulating viral RNA replication. PaperFlick

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Membrane recognition by phospholipid-binding domains.

Mark Lemmon (2008)

Many different globular domains bind to the surfaces of cellular membranes, or to specific phospholipid components in these membranes, and this binding is often tightly regulated. Examples include pleckstrin homology and C2 domains, which are among the largest domain families in the human proteome. Crystal structures, binding studies and analyses of subcellular localization have provided much insight into how members of this diverse group of domains bind to membranes, what features they recognize and how binding is controlled. A full appreciation of these processes is crucial for understanding how protein localization and membrane topography and trafficking are regulated in cells.

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The lipid droplet is an important organelle for hepatitis C virus production.

Yusuke Miyanari, Kimie Atsuzawa, Nobuteru Usuda … (2007)

The lipid droplet (LD) is an organelle that is used for the storage of neutral lipids. It dynamically moves through the cytoplasm, interacting with other organelles, including the endoplasmic reticulum (ER). These interactions are thought to facilitate the transport of lipids and proteins to other organelles. The hepatitis C virus (HCV) is a causative agent of chronic liver diseases. HCV capsid protein (Core) associates with the LD, envelope proteins E1 and E2 reside in the ER lumen, and the viral replicase is assumed to localize on ER-derived membranes. How and where HCV particles are assembled, however, is poorly understood. Here, we show that the LD is involved in the production of infectious virus particles. We demonstrate that Core recruits nonstructural (NS) proteins and replication complexes to LD-associated membranes, and that this recruitment is critical for producing infectious viruses. Furthermore, virus particles were observed in close proximity to LDs, indicating that some steps of virus assembly take place around LDs. This study reveals a novel function of LDs in the assembly of infectious HCV and provides a new perspective on how viruses usurp cellular functions.

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Modification of intracellular membrane structures for virus replication

Sven Miller, Jacomine Krijnse-Locker (2008)

Key Points Plus-stranded RNA viruses induce large membrane structures that might support the replication of their genomes. Similarly, cytoplasmic replication of poxviruses (large DNA viruses) occurs in associated membranes. These membranes originate from the endoplasmic reticulum (ER) or endosomes. Membrane vesicles that support viral replication are induced by a number of RNA viruses. Similarly, the poxvirus replication site is surrounded by a double-membraned cisterna that is derived from the ER. Analogies to autophagy have been proposed since the finding that autophagy cellular processes involve the formation of double-membrane vesicles. However, molecular evidence to support this hypothesis is lacking. Membrane association of the viral replication complex is mediated by the presence of one or more viral proteins that contain sequences which associate with, or integrate into, membranes. Replication-competent membranes might contain viral or cellular proteins that contain amphipathic helices, which could mediate the membrane bending that is required to form spherical vesicles. Whereas poxvirus DNA replication occurs inside the ER-enclosed site, for most RNA viruses the topology of replication is not clear. Preliminary results for some RNA viruses suggest that their replication could also occur inside double-membrane vesicles. We speculate that cytoplasmic replication might occur inside sites that are 'enwrapped' by an ER-derived cisterna, and that these cisternae are open to the cytoplasm. Thus, RNA and DNA viruses could use a common mechanism for replication that involves membrane wrapping by cellular cisternal membranes. We propose that three-dimensional analyses using high-resolution electron-microscopy techniques could be useful for addressing this issue. High-throughput small-interfering-RNA screens should also shed light on molecular requirements for virus-induced membrane modifications.