The Golgi protein ACBD3 facilitates Enterovirus 71 replication by interacting with 3A

Phosphatidylinositol 4-phosphate; A minor lipid with multiple personalities

Phosphorylated products of phosphatidylinositol (PI), named Diphosphoinositide (DPI) and triphosphoinositide (TPI) were identified long time ago and found to exhibit high turnover rates based on their rapid 32P-phosphate labeling. The PI kinase activities that were responsible for their production were subsequently identified and found to be associated with different organelle membranes, including the plasma membrane. These activities were then linked with a certain group of cell surface receptors that activated phospholipase C enzymes to hydrolyze PI and used calcium or cGMP as a second messenger. This visionary concept was introduced in the seminal BBA review written by Robert Michell, exactly 50 years ago. The enzymology and functional diversity of PI 4-phosphate (PI4P) (the term that has replaced DPI) has since underwent an expansion that could not have been foreseen. In this review I will attempt to revisit this expansion with some historical reflections celebrating the 50th anniversary of the Michell review.

Recruitment of PI4KIIIβ to the Golgi by ACBD3 is dependent on an upstream pathway of a SNARE complex and golgins

ACBD3 is a protein localised to the Golgi apparatus and recruits other proteins, such as PI4KIIIβ, to the Golgi. However, the mechanism through which ACBD3 itself is recruited to the Golgi is poorly understood. This study demonstrates there are two mechanisms for ACBD3 recruitment to the Golgi. First, we identified that an MWT374-376 motif in the unique region upstream of the GOLD domain in ACBD3 is essential for Golgi localization. Second, we use unbiased proteomics to demonstrate that ACBD3 interacts with SCFD1, a Sec1/Munc-18 (SM) protein, and a SNARE protein, SEC22B. CRISPR-KO of SCFD1 causes ACBD3 to become cytosolic. We also found that ACBD3 is redundantly recruited to the Golgi apparatus by two golgins: golgin-45 and giantin, which bind to ACBD3 through interaction with the MWT374-376 motif. Taken together, our results suggest that ACBD3 is recruited to the Golgi in a two-step sequential process, with the SCFD1-mediated interaction occurring upstream of the interaction with the golgins.

Allosteric regulation of Senecavirus A 3Cpro proteolytic activity by an endogenous phospholipid

Seneca virus A (SVA) is an emerging novel picornavirus that has recently been identified as the causative agent of many cases of porcine vesicular diseases in multiple countries. In addition to cleavage of viral polyprotein, the viral 3C protease (3Cpro) plays an important role in the regulation of several physiological processes involved in cellular antiviral responses by cleaving critical cellular proteins. Through a combination of crystallography, untargeted lipidomics, and immunoblotting, we identified the association of SVA 3Cpro with an endogenous phospholipid molecule, which binds to a unique region neighboring the proteolytic site of SVA 3Cpro. Our lipid-binding assays showed that SVA 3Cpro displayed preferred binding to cardiolipin (CL), followed by phosphoinositol-4-phosphate (PI4P) and sulfatide. Importantly, we found that the proteolytic activity of SVA 3Cpro was activated in the presence of the phospholipid, and the enzymatic activity is inhibited when the phospholipid-binding capacity decreased. Interestingly, in the wild-type SVA 3Cpro-substrate peptide structure, the cleavage residue cannot form a covalent binding to the catalytic cysteine residue to form the acyl-enzyme intermediate observed in several picornaviral 3Cpro structures. We observed a decrease in infectivity titers of SVA mutants harboring mutations that impaired the lipid-binding ability of 3Cpro, indicating a positive regulation of SVA infection capacity mediated by phospholipids. Our findings reveal a mutual regulation between the proteolytic activity and phospholipid-binding capacity in SVA 3Cpro, suggesting that endogenous phospholipid may function as an allosteric activator that regulate the enzyme's proteolytic activity during infection.

Molecular mechanisms of PI4K regulation and their involvement in viral replication

Lipid phosphoinositides are master signaling molecules in eukaryotic cells and key markers of organelle identity. Because of these important roles, the kinases and phosphatases that generate phosphoinositides must be tightly regulated. Viruses can manipulate this regulation, with the Type III phosphatidylinositol 4-kinases (PI4KA and PI4KB) being hijacked by many RNA viruses to mediate their intracellular replication through the formation of phosphatidylinositol 4-phosphate (PI4P)-enriched replication organelles (ROs). Different viruses have evolved unique approaches toward activating PI4K enzymes to form ROs, through both direct binding of PI4Ks and modulation of PI4K accessory proteins. This review will focus on PI4KA and PI4KB and discuss their roles in signaling, functions in membrane trafficking and manipulation by viruses. Our focus will be the molecular basis for how PI4KA and PI4KB are activated by both protein-binding partners and post-translational modifications, with an emphasis on understanding the different molecular mechanisms viruses have evolved to usurp PI4Ks. We will also discuss the chemical tools available to study the role of PI4Ks in viral infection.

Host neuronal PRSS3 interacts with enterovirus A71 3A protein and its role in viral replication

Enterovirus A71 (EV-A71) causes hand, foot, and mouth disease associated with neurological complications in young children. Currently, there is no specific treatment for EV-A71 infection due to the inadequate information on viral biology and neuropathogenesis. Among enteroviruses, nonstructural 3A protein mediates the formation of replication organelles which plays a major role in viral RNA synthesis and assembly. Although enteroviral 3A proteins have been intensively studied, the data on EV-A71 3A, especially in neuronal cells, are still limited. In this study, PRSS3 (mesotrypsinogen, also known as brain trypsinogen) was identified as EV-A71 3A-interacting counterpart from the transfected human neuroblastoma SH-SY5Y cells by pull-down assay and liquid chromatography tandem mass spectrometry. It was confirmed that PRSS3 variant 3 derived from human SH-SY5Y cells had the physical interaction with EV-A71 3A. Importantly, the role of PRSS3 in EV-A71 replication was verified by overexpression and siRNA-mediated gene silencing approaches. The detailed mechanism of the PRSS3 involved in EV-A71 replication and neuropathogenesis warrants further experimental elucidation. In conclusion, this study has discovered a novel EV-A71 3A interacting protein that offers the opportunity to study the neuropathogenesis of the infection which paves the way for developing a specific and effective treatment for the disease.

Gasdermin E is required for induction of pyroptosis and severe disease during enterovirus 71 infection

Pyroptosis is an inflammatory form of programmed cell death that is executed by the gasdermin (GSDM)-N domain of GSDM family proteins, which form pores in the plasma membrane. Although pyroptosis acts as a host defense against invasive pathogen infection, its role in the pathogenesis of enterovirus 71 (EV71) infection is unclear. In the current study, we found that EV71 infection induces cleavage of GSDM E (GSDME) by using western blotting analysis, an essential step in the switch from caspase-3-mediated apoptosis to pyroptosis. We show that this cleavage is independent of the 3C and 2A proteases of EV71. However, caspase-3 activation is essential for this cleavage, as GSDME could not be cleaved in caspase-3-KO cells upon EV71 infection. Further analyses showed that EV71 infection induced pyroptosis in WT cells but not in caspase-3/GSDME double-KO cells. Importantly, GSDME is required to induce severe disease during EV71 infection, as GSDME deficiency in mice was shown to alleviate pathological symptoms. In conclusion, our results reveal that GSDME is important for the pathogenesis of EV71 via mediating initiation of pyroptosis.

Enterovirus Infection Restricts Long Interspersed Element 1 Retrotransposition

Long interspersed element 1 (LINE-1 or L1) is the only active autonomous retrotransposon in the human genome that can serve as an endogenous upstream activator of cytoplasmic nucleic acid sensing pathways to elicit an antiviral immune response. In this study, we investigated the influence of enteroviral infection on L1 mobility. The results showed that infection with different enteroviruses, both EV-D68 and EV-A71, blocked L1 transposition. We screened diverse viral accessory proteins for L1 activity and identified EV-D68 2A, 3A, 3C, and EV-A71 ORF2p proteins as viral L1 inhibitors. EV-D68 2A suppressed L1 mobility by expression suppression of L1 proteins. Viral proteins 3A and 3C restricted ORF2p-mediated L1 reverse transcription in isolated L1 ribonucleoproteins. The newly identified enteroviral protein ORF2p inhibited the expression of L1 ORF1p. Altogether, our findings shed light on the strict modulation of L1 retrotransposons during enterovirus replication.

Role of Lipid Transfer Proteins (LTPs) in the Viral Life Cycle

Viruses are obligate parasites that depend on the host cell machinery for their replication and dissemination. Cellular lipids play a central role in multiple stages of the viral life cycle such as entry, replication, morphogenesis, and egress. Most viruses reorganize the host cell membranes for the establishment of viral replication complex. These specialized structures allow the segregation of replicating viral RNA from ribosomes and protect it from host nucleases. They also facilitate localized enrichment of cellular components required for viral replication and assembly. The specific composition of the lipid membrane governs its ability to form negative or positive curvature and possess a rigid or flexible form, which is crucial for membrane rearrangement and establishment of viral replication complexes. In this review, we highlight how different viruses manipulate host lipid transfer proteins and harness their functions to enrich different membrane compartments with specific lipids in order to facilitate multiple aspects of the viral life cycle.

Picornaviruses: A View from 3A

Picornaviruses are comprised of a positive-sense RNA genome surrounded by a protein shell (or capsid). They are ubiquitous in vertebrates and cause a wide range of important human and animal diseases. The genome encodes a single large polyprotein that is processed to structural (capsid) and non-structural proteins. The non-structural proteins have key functions within the viral replication complex. Some, such as 3D^pol (the RNA dependent RNA polymerase) have conserved functions and participate directly in replicating the viral genome, whereas others, such as 3A, have accessory roles. The 3A proteins are highly divergent across the Picornaviridae and have specific roles both within and outside of the replication complex, which differ between the different genera. These roles include subverting host proteins to generate replication organelles and inhibition of cellular functions (such as protein secretion) to influence virus replication efficiency and the host response to infection. In addition, 3A proteins are associated with the determination of host range. However, recent observations have challenged some of the roles assigned to 3A and suggest that other viral proteins may carry them out. In this review, we revisit the roles of 3A in the picornavirus life cycle. The 3AB precursor and mature 3A have distinct functions during viral replication and, therefore, we have also included discussion of some of the roles assigned to 3AB.

A Crucial Role of ACBD3 Required for Coxsackievirus Infection in Animal Model Developed by AAV-Mediated CRISPR Genome Editing Technique

Genetic screens using CRISPR/Cas9 have been exploited to discover host–virus interactions. These screens have identified viral dependencies on host proteins during their life cycle and potential antiviral strategies. The acyl-CoA binding domain containing 3 (ACBD3) was identified as an essential host factor for the Coxsackievirus B3 (CVB3) infection. Other groups have also investigated the role of ACBD3 as a host factor for diverse enteroviruses in cultured cells. However, it has not been tested if ACBD3 is required in the animal model of CVB3 infection. Owing to embryonic lethality, conventional knockout mice were not available for in vivo study. As an alternative approach, we used adeno-associated virus (AAV)-mediated CRISPR genome editing to generate mice that lacked ACBD3 within the pancreas, the major target organ for CVB3. Delivery of sgRNAs using self-complementary (sc) AAV8 efficiently induced a loss-of-function mutation in the pancreas of the Cas9 knock-in mice. Loss of ACBD3 in the pancreas resulted in a 100-fold reduction in the CVB3 titer within the pancreas and a noticeable reduction in viral protein expression. These results indicate a crucial function of ACBD3 in CVB3 infection in vivo. AAV-mediated CRISPR genome editing may be applicable to many in vivo studies on the virus–host interaction and identify a novel target for antiviral therapeutics.

Regulation of the proteostasis network during enterovirus infection: A feedforward mechanism for EV-A71 and EV-D68

The proteostasis network guarantees successful protein synthesis, folding, transportation, and degradation. Mounting evidence has revealed that this network maintains proteome integrity and is linked to cellular physiology, pathology, and virus infection. Human enterovirus A71 (EV-A71) and EV-D68 are suspected causative agents of acute flaccid myelitis, a severe poliomyelitis-like neurologic syndrome with no known cure. In this context, further clarification of the molecular mechanisms underlying EV-A71 and EV-D68 infection is paramount. Here, we summarize the components of the proteostasis network that are intercepted by EV-A71 and EV-D68, as well as antivirals that target this network and may help develop improved antiviral drugs.

Chemokine (C-X-C Motif) Ligand 4 Is a Restrictor of Respiratory Syncytial Virus Infection and an Indicator of Clinical Severity

Rationale: Respiratory syncytial virus (RSV) is the leading cause of childhood respiratory infections worldwide; however, no vaccine is available, and treatment options are limited. Identification of host factors pivotal to viral replication may inform the development of novel therapies, prophylaxes, or diagnoses.Objectives: To identify host factors involved in RSV replication and to evaluate their potential for disease management.Methods: A gain-of-function screening was performed on the basis of a genome-wide human complementary DNA library screen for host factors involved in RSV replication. The antiviral mechanism of CXCL4 (chemokine [C-X-C motif] ligand 4) was analyzed. Its clinical role was evaluated via nasopharyngeal aspirates and plasma samples from patients with RSV infection and different disease severities.Measurements and Main Results: Forty-nine host factors restricting RSV replication were identified by gain-of-function screening, with CXCL4 showing the strongest antiviral effect, which was secretion dependent. CXCL4 blocked viral attachment through binding to the RSV main receptor heparan sulfate, instead of through interacting with RSV surface proteins. Intranasal pretreatment with CXCL4 alleviated inflammation in RSV-infected mice, as shown by decreased concentrations of tumor necrosis factor and viral load in BAL fluid samples as well as by viral nucleocapsid protein histological staining in lungs. Compared with non-RSV infections, RSV infections induced elevated CXCL4 concentrations both in plasma and airway samples from mice and pediatric patients. The airway CXCL4 concentration was correlated with viral load and disease severity in patients (P < 0.001).Conclusions: Our results suggest that CXCL4 is an RSV restriction factor that can block viral entry and serve as an indicator of clinical severity in RSV infections.

PI4KB on Inclusion Bodies Formed by ER Membrane Remodeling Facilitates Replication of Human Parainfluenza Virus Type 3

Many positive-strand RNA viruses remodel the endomembrane to form specialized replication organelles. However, knowledge regarding whether negative-strand RNA viruses take advantage of intracellular membranes for replication is limited. Here we show that a negative-strand RNA virus, human parainfluenza virus type 3 (HPIV3), remodels the endoplasmic reticulum (ER) membrane to form inclusion bodies (IBs), whereby the phosphoprotein (P) of HPIV3 recruits phosphatidylinositol 4-kinase beta (PI4KB) to IBs to generate PI4P, creating a PI4P-enriched microenvironment to promote HPIV3 replication. In addition, we find that human respiratory syncytial virus (HRSV) also takes advantage of the ER to form IBs and that these IBs are also enriched with PI4P. The nucleoprotein of HRSV recruits PI4KB to IBs. These results suggest that paramyxoviruses also exploit the host endomembrane to form IBs and that PI4KB is recruited by viral proteins to enrich IBs with PI4P to facilitate viral replication.

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Inclusion bodies (IBs) of HPIV3 induce membrane rearrangement of ER

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PI4P generated by PI4KB on IBs facilitates replication of HPIV3

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PI4KB is recruited to IBs via interaction with the HPIV3 phosphoprotein, P

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Remodeling ER is a general mechanism for IBs of negative-strand RNA viruses

Enterovirus Replication Organelles and Inhibitors of Their Formation

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Emerging Role for Acyl-CoA Binding Domain Containing 3 at Membrane Contact Sites During Viral Infection

Acyl-coenzyme A binding domain containing 3 (ACBD3) is a multifunctional protein residing in the Golgi apparatus and is involved in several signaling pathways. The current knowledge on ACBD3 has been extended to virology. ACBD3 has recently emerged as a key factor subverted by viruses, including kobuvirus, enterovirus, and hepatitis C virus. The ACBD3-PI4KB complex is critical for the role of ACBD3 in viral replication. In most cases, ACBD3 plays a positive role in viral infection. ACBD3 associates with viral 3A proteins from a variety of Picornaviridae family members at membrane contact sites (MCSs), which are used by diverse viruses to ensure lipid transfer to replication organelles (ROs). In this review, we discuss the mechanisms underlying the involvement of ACBD3 in viral infection at MCSs. Our review will highlight the current research and reveal potential avenues for future research.

Structural basis for hijacking of the host ACBD3 protein by bovine and porcine enteroviruses and kobuviruses

Picornaviruses infect a wide range of mammals including livestock such as cattle and swine. As with other picornavirus genera such as Aphthovirus, there is emerging evidence of a significant economic impact of livestock infections caused by members of the genera Enterovirus and Kobuvirus. While the human-infecting enteroviruses and kobuviruses have been intensively studied during the past decades in great detail, research on livestock-infecting viruses has been mostly limited to the genomic characterization of the viral strains identified worldwide. Here, we extend our previous studies of the structure and function of the complexes composed of the non-structural 3A proteins of human-infecting enteroviruses and kobuviruses and the host ACBD3 protein and present a structural and functional characterization of the complexes of the following livestock-infecting picornaviruses: bovine enteroviruses EV-E and EV-F, porcine enterovirus EV-G, and porcine kobuvirus AiV-C. We present a series of crystal structures of these complexes and demonstrate the role of these complexes in facilitation of viral replication.

Structural insights into Acyl-coenzyme A binding domain containing 3 (ACBD3) protein hijacking by picornaviruses

Many picornaviruses hijack the Golgi resident Acyl-coenzyme A binding domain containing 3 (ACBD3) protein in order to recruit the phosphatidylinositol 4-kinase B (PI4KB) to viral replication organelles (ROs). PI4KB, once recruited and activated by ACBD3 protein, produces the lipid phosphatidylinositol 4-phosphate (PI4P), which is a key step in the biogenesis of viral ROs. To do so, picornaviruses use their small nonstructural protein 3A that binds the Golgi dynamics domain of the ACBD3 protein. Here, we present the analysis of the highly flexible ACBD3 proteins and the viral 3A protein in solution using small-angle X-ray scattering and computer simulations. Our analysis revealed that both the ACBD3 protein and the 3A:ACBD3 protein complex have an extended and flexible conformation in solution.

A Redundant Mechanism of Recruitment Underlies the Remarkable Plasticity of the Requirement of Poliovirus Replication for the Cellular ArfGEF GBF1

The replication of many positive-strand RNA viruses [(+)RNA viruses] depends on the cellular protein GBF1, but its role in the replication process is not clear. In uninfected cells, GBF1 activates small GTPases of the Arf family and coordinates multiple steps of membrane metabolism, including functioning of the cellular secretory pathway. The nonstructural protein 3A of poliovirus and related viruses has been shown to directly interact with GBF1, likely mediating its recruitment to the replication complexes. Surprisingly, viral mutants with a severely reduced level of 3A-GBF1 interaction demonstrate minimal replication defects in cell culture. Here, we systematically investigated the conserved elements of GBF1 to understand which determinants are important to support poliovirus replication. We demonstrate that multiple GBF1 mutants inactive in cellular metabolism could still be fully functional in the replication complexes. Our results show that the Arf-activating property, but not the primary structure of the Sec7 domain, is indispensable for viral replication. They also suggest a redundant mechanism of recruitment of GBF1 to the replication sites, which is dependent not only on direct interaction of the protein with the viral protein 3A but also on determinants located in the noncatalytic C-terminal domains of GBF1. Such a double-targeting mechanism explains the previous observations of the remarkable tolerance of different levels of GBF1-3A interaction by the virus and likely constitutes an important element of the resilience of viral replication.IMPORTANCE Enteroviruses are a vast group of viruses associated with diverse human diseases, but only two of them could be controlled with vaccines, and effective antiviral therapeutics are lacking. Here, we investigated in detail the contribution of a cellular protein, GBF1, in the replication of poliovirus, a representative enterovirus. GBF1 supports the functioning of cellular membrane metabolism and is recruited to viral replication complexes upon infection. Our results demonstrate that the virus requires a limited subset of the normal GBF1 functions and reveal the elements of GBF1 essential to support viral replication under different conditions. Since diverse viruses often rely on the same cellular proteins for replication, understanding the mechanisms by which these proteins support infection is essential for the development of broad-spectrum antiviral therapeutics.

Convergent evolution in the mechanisms of ACBD3 recruitment to picornavirus replication sites

Enteroviruses, members of the family of picornaviruses, are the most common viral infectious agents in humans causing a broad spectrum of diseases ranging from mild respiratory illnesses to life-threatening infections. To efficiently replicate within the host cell, enteroviruses hijack several host factors, such as ACBD3. ACBD3 facilitates replication of various enterovirus species, however, structural determinants of ACBD3 recruitment to the viral replication sites are poorly understood. Here, we present a structural characterization of the interaction between ACBD3 and the non-structural 3A proteins of four representative enteroviruses (poliovirus, enterovirus A71, enterovirus D68, and rhinovirus B14). In addition, we describe the details of the 3A-3A interaction causing the assembly of the ACBD3-3A heterotetramers and the interaction between the ACBD3-3A complex and the lipid bilayer. Using structure-guided identification of the point mutations disrupting these interactions, we demonstrate their roles in the intracellular localization of these proteins, recruitment of downstream effectors of ACBD3, and facilitation of enterovirus replication. These structures uncovered a striking convergence in the mechanisms of how enteroviruses and kobuviruses, members of a distinct group of picornaviruses that also rely on ACBD3, recruit ACBD3 and its downstream effectors to the sites of viral replication.

Enteroviruses are the most common viruses infecting humans. They cause a broad spectrum of diseases ranging from common cold to life-threatening diseases, such as poliomyelitis. To date, no effective antiviral therapy for enteroviruses has been approved yet. To ensure efficient replication, enteroviruses hijack several host factors, recruit them to the sites of virus replication, and use their physiological functions for their own purposes. Here, we characterize the complexes composed of the host protein ACBD3 and the ACBD3-binding viral proteins (called 3A) of four representative enteroviruses. Our study reveals the atomic details of these complexes and identifies the amino acid residues important for the interaction. We found out that the 3A proteins of enteroviruses bind to the same regions of ACBD3 as the 3A proteins of kobuviruses, a distinct group of viruses that also rely on ACBD3, but are oriented in the opposite directions. This observation reveals a striking case of convergent evolutionary pathways that have evolved to allow enteroviruses and kobuviruses (which are two distinct groups of the Picornaviridae family) to recruit a common host target, ACBD3, and its downstream effectors to the sites of viral replication.

ACBD3 Is an Essential Pan-enterovirus Host Factor That Mediates the Interaction between Viral 3A Protein and Cellular Protein PI4KB

The enterovirus genus of the picornavirus family includes a large number of important human pathogens such as poliovirus, coxsackievirus, enterovirus A71, and rhinoviruses. Like all other positive-strand RNA viruses, genome replication of enteroviruses occurs on rearranged membranous structures called replication organelles (ROs). Phosphatidylinositol 4-kinase IIIβ (PI4KB) is required by all enteroviruses for RO formation. The enteroviral 3A protein recruits PI4KB to ROs, but the exact mechanism remains elusive. Here, we investigated the role of acyl-coenzyme A binding domain containing 3 (ACBD3) in PI4KB recruitment upon enterovirus replication using ACBD3 knockout (ACBD3^KO) cells. ACBD3 knockout impaired replication of representative viruses from four enterovirus species and two rhinovirus species. PI4KB recruitment was not observed in the absence of ACBD3. The lack of ACBD3 also affected the localization of individually expressed 3A, causing 3A to localize to the endoplasmic reticulum instead of the Golgi. Reconstitution of wild-type (wt) ACBD3 restored PI4KB recruitment and 3A localization, while an ACBD3 mutant that cannot bind to PI4KB restored 3A localization, but not virus replication. Consistently, reconstitution of a PI4KB mutant that cannot bind ACBD3 failed to restore virus replication in PI4KB^KO cells. Finally, by reconstituting ACBD3 mutants lacking specific domains in ACBD3^KO cells, we show that acyl-coenzyme A binding (ACB) and charged-amino-acid region (CAR) domains are dispensable for 3A-mediated PI4KB recruitment and efficient enterovirus replication. Altogether, our data provide new insight into the central role of ACBD3 in recruiting PI4KB by enterovirus 3A and reveal the minimal domains of ACBD3 involved in recruiting PI4KB and supporting enterovirus replication.IMPORTANCE Similar to all other positive-strand RNA viruses, enteroviruses reorganize host cellular membranes for efficient genome replication. A host lipid kinase, PI4KB, plays an important role in this membrane rearrangement. The exact mechanism of how enteroviruses recruit PI4KB was unclear. Here, we revealed a role of a Golgi-residing protein, ACBD3, as a mediator of PI4KB recruitment upon enterovirus replication. ACBD3 is responsible for proper localization of enteroviral 3A proteins in host cells, which is important for 3A to recruit PI4KB. By testing ACBD3 and PI4KB mutants that abrogate the ACBD3-PI4KB interaction, we showed that this interaction is crucial for enterovirus replication. The importance of specific domains of ACBD3 was evaluated for the first time, and the domains that are essential for enterovirus replication were identified. Our findings open up a possibility for targeting ACBD3 or its interaction with enteroviruses as a novel strategy for the development of broad-spectrum antienteroviral drugs.

Recent Progress on Functional Genomics Research of Enterovirus 71

Enterovirus 71 (EV71) is one of the main pathogens that causes hand-foot-and-mouth disease (HFMD). HFMD caused by EV71 infection is mostly self-limited; however, some infections can cause severe neurological diseases, such as aseptic meningitis, brain stem encephalitis, and even death. There are still no effective clinical drugs used for the prevention and treatment of HFMD. Studying EV71 protein function is essential for elucidating the EV71 replication process and developing anti-EV71 drugs and vaccines. In this review, we summarized the recent progress in the studies of EV71 non-coding regions (5' UTR and 3' UTR) and all structural and nonstructural proteins, especially the key motifs involving in viral infection, replication, and immune regulation. This review will promote our understanding of EV71 virus replication and pathogenesis, and will facilitate the development of novel drugs or vaccines to treat EV71.

Host lipidome analysis during rhinovirus replication in HBECs identifies potential therapeutic targets

In patients with asthma or chronic obstructive pulmonary disease, rhinovirus (RV) infections can provoke acute worsening of disease, and limited treatment options exist. Viral replication in the host cell induces significant remodeling of intracellular membranes, but few studies have explored this mechanistically or as a therapeutic opportunity. We performed unbiased lipidomic analysis on human bronchial epithelial cells infected over a 6 h period with the RV-A1b strain of RV to determine changes in 493 distinct lipid species. Through pathway and network analysis, we identified temporal changes in the apparent activities of a number of lipid metabolizing and signaling enzymes. In particular, analysis highlighted FA synthesis and ceramide metabolism as potential anti-rhinoviral targets. To validate the importance of these enzymes in viral replication, we explored the effects of commercially available enzyme inhibitors upon RV-A1b infection and replication. Ceranib-1, D609, and C75 were the most potent inhibitors, which confirmed that FAS and ceramidase are potential inhibitory targets in rhinoviral infections. More broadly, this study demonstrates the potential of lipidomics and pathway analysis to identify novel targets to treat human disorders.

Arrayed CRISPR screen with image-based assay reliably uncovers host genes required for coxsackievirus infection

Pooled CRISPR screens based on lentiviral systems have been widely applied to identify the effect of gene knockout on cellular phenotype. Although many screens were successful, they also have the limitation that genes conferring mild phenotypes or those essential for growth can be overlooked, as every genetic perturbation is incorporated in the same population. Arrayed screens, on the other hand, incorporate a single genetic perturbation in each well and could overcome these limitations. However, arrayed screens based on siRNA-mediated knockdown were recently criticized for low reproducibility caused by incomplete inhibition of gene expression. To overcome these limitations, we developed a novel arrayed CRISPR screen based on a plasmid library expressing a single guide RNA (sgRNA) and disrupted 1514 genes, encoding kinases, proteins related to endocytosis, and Golgi-localized proteins, individually using 4542 sgRNAs (three sgRNAs per gene). This screen revealed host factors required for infection by coxsackievirus B3 (CVB3) from Picornaviridae, which includes human pathogens causing diverse diseases. Many host factors that had been overlooked in a conventional pooled screen were identified for CVB3 infection, including entry-related factors, translational initiation factors, and several replication factors with different functions, demonstrating the advantage of the arrayed screen. This screen was quite reliable and reproducible, as most genes identified in the primary screen were confirmed in secondary screens. Moreover, ACBD3, whose phenotype was not affected by siRNA-mediated knockdown, was reliably identified. We propose that arrayed CRISPR screens based on sgRNA plasmid libraries are powerful tools for arrayed genetic screening and applicable to larger-scale screens.

Model of OSBP-Mediated Cholesterol Supply to Aichi Virus RNA Replication Sites Involving Protein-Protein Interactions among Viral Proteins, ACBD3, OSBP, VAP-A/B, and SAC1

Positive-strand RNA viruses, including picornaviruses, utilize cellular machinery for genome replication. Previously, we reported that each of the 2B, 2BC, 2C, 3A, and 3AB proteins of Aichi virus (AiV), a picornavirus, forms a complex with the Golgi apparatus protein ACBD3 and phosphatidylinositol 4-kinase IIIβ (PI4KB) at viral RNA replication sites (replication organelles [ROs]), enhancing PI4KB-dependent phosphatidylinositol 4-phosphate (PI4P) production. Here, we demonstrate AiV hijacking of the cellular cholesterol transport system involving oxysterol-binding protein (OSBP), a PI4P-binding cholesterol transfer protein. AiV RNA replication was inhibited by silencing cellular proteins known to be components of this pathway, OSBP, the ER membrane proteins VAPA and VAPB (VAP-A/B), the PI4P-phosphatase SAC1, and PI-transfer protein β. OSBP, VAP-A/B, and SAC1 were present at RNA replication sites. We also found various previously unknown interactions among the AiV proteins (2B, 2BC, 2C, 3A, and 3AB), ACBD3, OSBP, VAP-A/B, and SAC1, and the interactions were suggested to be involved in recruiting the component proteins to AiV ROs. Importantly, the OSBP-2B interaction enabled PI4P-independent recruitment of OSBP to AiV ROs, indicating preferential recruitment of OSBP among PI4P-binding proteins. Protein-protein interaction-based OSBP recruitment has not been reported for other picornaviruses. Cholesterol was accumulated at AiV ROs, and inhibition of OSBP-mediated cholesterol transfer impaired cholesterol accumulation and AiV RNA replication. Electron microscopy showed that AiV-induced vesicle-like structures were close to ER membranes. Altogether, we conclude that AiV directly recruits the cholesterol transport machinery through protein-protein interactions, resulting in formation of membrane contact sites between the ER and AiV ROs and cholesterol supply to the ROs.IMPORTANCE Positive-strand RNA viruses utilize host pathways to modulate the lipid composition of viral RNA replication sites for replication. Previously, we demonstrated that Aichi virus (AiV), a picornavirus, forms a complex comprising certain proteins of AiV, the Golgi apparatus protein ACBD3, and the lipid kinase PI4KB to synthesize PI4P lipid at the sites for AiV RNA replication. Here, we confirmed cholesterol accumulation at the AiV RNA replication sites, which are established by hijacking the host cholesterol transfer machinery mediated by a PI4P-binding cholesterol transfer protein, OSBP. We showed that the component proteins of the machinery, OSBP, VAP, SAC1, and PITPNB, are all essential host factors for AiV replication. Importantly, the machinery is directly recruited to the RNA replication sites through previously unknown interactions of VAP/OSBP/SAC1 with the AiV proteins and with ACBD3. Consequently, we propose a specific strategy employed by AiV to efficiently accumulate cholesterol at the RNA replication sites via protein-protein interactions.

Enterovirus A71 Proteins: Structure and Function

Enterovirus A71 (EV-A71) infection has grown to become a serious threat to global public health. It is one of the major causes of hand, foot, and mouth disease (HFMD) in infants and young children. EV-A71 can also infect the central nervous system (CNS) and induce diverse neurological complications, such as brainstem encephalitis, aseptic meningitis, and acute flaccid paralysis, or even death. Viral proteins play a crucial role in EV-A71 infection. Many recent studies have discussed the structure and function of EV-A71 proteins, and the findings reported will definitely aid the development of vaccines and therapeutic approaches. This article reviews the progress in the research on the structure and function of EV-A71 proteins. Available literature can provide a basis for studying the pathogenesis of EV-A71 infection in detail.

Enterovirus 3A Facilitates Viral Replication by Promoting Phosphatidylinositol 4-Kinase IIIβ-ACBD3 Interaction

Like other enteroviruses, enterovirus 71 (EV71) relies on phosphatidylinositol 4-kinase IIIβ (PI4KB) for genome RNA replication. However, how PI4KB is recruited to the genome replication sites of EV71 remains elusive. Recently, we reported that a host factor, ACBD3, is needed for EV71 replication by interacting with viral 3A protein. Here, we show that ACBD3 is required for the recruitment of PI4KB to RNA replication sites. Overexpression of viral 3A or EV71 infection stimulates the interaction of PI4KB and ACBD3. Consistently, EV71 infection induces the production of phosphatidylinositol-4-phosphate (PI4P). Furthermore, PI4KB, ACBD3, and 3A are all localized to the viral-RNA replication sites. Accordingly, PI4KB or ACBD3 depletion by small interfering RNA (siRNA) leads to a reduction in PI4P production after EV71 infection. I44A or H54Y substitution in 3A interrupts the stimulation of PI4KB and ACBD3. Further analysis suggests that stimulation of ACBD3-PI4KB interaction is also important for the replication of enterovirus 68 but disadvantageous to human rhinovirus 16. These results reveal a mechanism of enterovirus replication that involves a selective strategy for recruitment of PI4KB to the RNA replication sites.IMPORTANCE Enterovirus 71, like other human enteroviruses, replicates its genome within host cells, where viral proteins efficiently utilize cellular machineries. While multiple factors are involved, it is largely unclear how viral replication is controlled. We show that the 3A protein of enterovirus 71 recruits an enzyme, phosphatidylinositol 4-kinase IIIβ, by interacting with ACBD3, which alters cellular membranes through the production of a lipid, PI4P. Consequently, the viral and host proteins form a large complex that is necessary for RNA synthesis at replication sites. Notably, PI4KB-ACBD3 interaction also differentially mediates the replication of enterovirus 68 and rhinovirus 16. These results provide new insight into the molecular network of enterovirus replication.

ACBD3 functions as a scaffold to organize the Golgi stacking proteins and a Rab33b-GAP

Golgin45 plays important roles in Golgi stack assembly and is known to bind both the Golgi stacking protein GRASP55 and Rab2 in the medial-Golgi cisternae. In this study, we sought to further characterize the cisternal adhesion complex using a proteomics approach. We report here that Acyl-CoA binding domain containing 3 (ACBD3) is likely to be a novel binding partner of Golgin45. ACBD3 interacts with Golgin45 via its GOLD domain, while its co-expression significantly increases Golgin45 targeting to the Golgi. Furthermore, ACBD3 recruits TBC1D22, a Rab33b GTPase activating protein (GAP), to a large multi-protein complex containing Golgin45 and GRASP55. These results suggest that ACBD3 may provide a scaffolding to organize the Golgi stacking proteins and a Rab33b-GAP at the medial-Golgi.