Virus-human cell interactomes

Near-Infrared Luciferase Complementation Assay with Enhanced Bioluminescence for Studying Protein-Protein Interactions and Drug Evaluation Under Physiological Conditions

Identification of protein-protein interactions (PPIs) that occur in various cellular processes helps to reveal their potential molecular mechanisms, and there is still an urgent need to develop the assays to explore PPIs in living subjects. Here, we reported a near-infrared split luciferase complementation assay (SLCA) with enhanced bioluminescence produced by cleaving a luciferase, Akaluc, for exploring and visualizing PPIs in living cells and live mice. Compared with the previously developed and widely used red SLCA based on split firefly luciferase (Fluc-SLCA), the signal intensities for PPI recognition in living cells and live mice of the Akaluc-SLCA increased by ∼3.79-fold and ∼18.06-fold in the measured condition, respectively. Additionally, the interactions between the nucleocapsid protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and cellular RNA processing proteins were identified, and the drug evaluation assays were also performed in living cells using Akaluc-SLCA. This study provides a new tool in the near-infrared region for the identification of PPIs in living cells and in vivo and new information for the understanding and treatment of SARS-CoV-2.

Influenza virus protein PB1-F2 interacts with CALCOCO2 (NDP52) to modulate innate immune response

PB1-F2 is a viral protein encoded by influenza A viruses (IAVs). PB1-F2 is implicated in virulence by triggering immune cell apoptosis and enhancing inflammation. To obtain an insight into the molecular mechanisms of PB1-F2-mediated virulence, we used the yeast two-hybrid approach to find new PB1-F2 cellular interactors. This allowed us to identify calcium-binding and coiled-coil domain 2 (CALCOCO2, also known as NDP52) as a binding partner of PB1-F2. Binding of PB1-F2 to CALCOCO2 was confirmed by pull-down. Surface plasmon resonance binding experiments enabled us to estimate the dissociation constant (Kd) of the two partners to be around 20 nM. Using bioinformatics tools, we designed a CALCOCO2 interaction map based on previous knowledge and showed a strong connection between this protein and the type I interferon production pathways and the I-κB kinase/NF-κB signalling pathway. NF-κB reporter assays in which CALCOCO2, MAVS and PB1-F2 were co-expressed showed a cooperation of these three proteins to increase the inflammatory response. By contrast, PB1-F2 inhibits the TBK1-dependent activation of an ISRE reporter plasmid. We also demonstrated that the signal transducer TRAF6 is implicated in the enhancement of NF-κB activity mediated by PB1-F2/CALCOCO2 binding. Altogether, this report provides evidence of an interaction link between PB1-F2 and human proteins, and allows a better understanding of the involvement of PB1-F2 in the pathologic process mediated by IAV.

Discovering Protein-Protein Interactions Using Nucleic Acid Programmable Protein Arrays

We have developed a protocol enabling the study of protein-protein interactions (PPIs) at the proteome level using in vitro-synthesized proteins. Assay preparation requires molecular cloning of the query gene into a vector that supports in vitro transcription/translation (IVTT) and appends a HaloTag to the query protein of interest. In parallel, protein microarrays are prepared by printing plasmids encoding glutathione S-transferase (GST)-tagged target proteins onto a carrier matrix/glass slide coated with antibody directed against GST. At the time of the experiment, the query protein and the target protein are produced separately through IVTT. The query protein is then applied to nucleic acid programmable protein arrays (NAPPA) that display thousands of freshly produced target proteins captured by anti-GST antibody. Interactions between the query and immobilized target proteins are detected through addition of a fluorophore-labeled HaloTag ligand. Our protocol allows the elucidation of PPIs in a high-throughput fashion using proteins produced in vitro, obviating the scientific challenges, high cost, and laborious work, as well as concerns about protein stability, which are usually present in protocols using conventional protein arrays. © 2017 by John Wiley & Sons, Inc.

Epstein-Barr virus nuclear antigen 3A protein regulates CDKN2B transcription via interaction with MIZ-1

The Epstein-Barr virus (EBV) nuclear antigen 3 family of protein is critical for the EBV-induced primary B-cell growth transformation process. Using a yeast two-hybrid screen we identified 22 novel cellular partners of the EBNA3s. Most importantly, among the newly identified partners, five are known to play direct and important roles in transcriptional regulation. Of these, the Myc-interacting zinc finger protein-1 (MIZ-1) is a transcription factor initially characterized as a binding partner of MYC. MIZ-1 activates the transcription of a number of target genes including the cell cycle inhibitor CDKN2B. Focusing on the EBNA3A/MIZ-1 interaction we demonstrate that binding occurs in EBV-infected cells expressing both proteins at endogenous physiological levels and that in the presence of EBNA3A, a significant fraction of MIZ-1 translocates from the cytoplasm to the nucleus. Moreover, we show that a trimeric complex composed of a MIZ-1 recognition DNA element, MIZ-1 and EBNA3A can be formed, and that interaction of MIZ-1 with nucleophosmin (NPM), one of its coactivator, is prevented by EBNA3A. Finally, we show that, in the presence of EBNA3A, expression of the MIZ-1 target gene, CDKN2B, is downregulated and repressive H3K27 marks are established on its promoter region suggesting that EBNA3A directly counteracts the growth inhibitory action of MIZ-1.

The interactomes of influenza virus NS1 and NS2 proteins identify new host factors and provide insights for ADAR1 playing a supportive role in virus replication

Influenza A NS1 and NS2 proteins are encoded by the RNA segment 8 of the viral genome. NS1 is a multifunctional protein and a virulence factor while NS2 is involved in nuclear export of viral ribonucleoprotein complexes. A yeast two-hybrid screening strategy was used to identify host factors supporting NS1 and NS2 functions. More than 560 interactions between 79 cellular proteins and NS1 and NS2 proteins from 9 different influenza virus strains have been identified. These interacting proteins are potentially involved in each step of the infectious process and their contribution to viral replication was tested by RNA interference. Validation of the relevance of these host cell proteins for the viral replication cycle revealed that 7 of the 79 NS1 and/or NS2-interacting proteins positively or negatively controlled virus replication. One of the main factors targeted by NS1 of all virus strains was double-stranded RNA binding domain protein family. In particular, adenosine deaminase acting on RNA 1 (ADAR1) appeared as a pro-viral host factor whose expression is necessary for optimal viral protein synthesis and replication. Surprisingly, ADAR1 also appeared as a pro-viral host factor for dengue virus replication and directly interacted with the viral NS3 protein. ADAR1 editing activity was enhanced by both viruses through dengue virus NS3 and influenza virus NS1 proteins, suggesting a similar virus-host co-evolution.

Viruses are obligate intracellular parasites that rely on cellular functions for efficient replication. As most biological processes are sustained by protein-protein interactions, the identification of interactions between viral and host proteins can provide a global overview about the cellular functions engaged during viral replication. Influenza viruses express 13 viral proteins, including NS1 and NS2, which are translated from an alternatively spliced RNA derived from the same genome segment. We present here a comprehensive overview of possible interactions of cellular proteins with NS1 and NS2 from 9 viral strains. Seventy nine cellular proteins were identified to interact with NS1, NS2 or both NS1 and NS2. These interacting host cell proteins are potentially involved in many steps of the virus life cycle and 7 can directly control the viral replication. Most of the cellular targets are shared by the majority of the virus strains, especially the double-stranded RNA binding domain protein family that is strikingly targeted by NS1. One of its members, ADAR1, is essential for influenza virus replication. ADAR1 colocalizes with NS1 in nuclear structures and its editing activity is enhanced by NS1 expressed on its own and during virus infection. A similar phenomenon is observed for dengue virus whose NS3 protein also interacts with ADAR1, suggesting a parallel virus-host co-evolution.

Development of a robust cell-based high-throughput screening assay to identify targets of HIV-1 viral protein R dimerization

Targeting protein-protein interactions (PPI) is an emerging field in drug discovery. Dimerization and PPI are essential properties of human immunodeficiency virus (HIV)-1 proteins, their mediated functions, and virus biology. Additionally, dimerization is required for the functional interaction of HIV-1 proteins with many host cellular components. In this study, a bimolecular fluorescence complementation (BiFC)-based screening assay was developed that can quantify changes in dimerization, using HIV-1 viral protein R (Vpr) dimerization as a "proof of concept." Results demonstrated that Venus Vpr (generated by BiFC Vpr constructs) could be competed off in a dose-dependent manner using untagged, full-length Vpr as a competitor molecule. The change in signal intensity was measured quantitatively through flow cytometry and fluorescence microscopy in a high content screening assay. High content imaging was used to screen a library of small molecules for an effect on Vpr dimerization. Among the tested molecules, a few of the small molecules demonstrate an effect on Vpr dimerization in a dose-dependent manner.

Host cell factors as antiviral targets in arenavirus infection

Among the members of the Arenaviridae family, Lassa virus and Junin virus generate periodic annual outbreaks of severe human hemorrhagic fever (HF) in endemic areas of West Africa and Argentina, respectively. Given the human health threat that arenaviruses represent and the lack of a specific and safe chemotherapy, the search for effective antiviral compounds is a continuous demanding effort. Since diverse host cell pathways and enzymes are used by RNA viruses to fulfill their replicative cycle, the targeting of a host process has turned an attractive antiviral approach in the last years for many unrelated virus types. This strategy has the additional benefit to reduce the serious challenge for therapy of RNA viruses to escape from drug effects through selection of resistant variants triggered by their high mutation rate. This article focuses on novel strategies to identify inhibitors for arenavirus therapy, analyzing the potential for antiviral developments of diverse host factors essential for virus infection.

Epstein-Barr virus protein EB2 stimulates cytoplasmic mRNA accumulation by counteracting the deleterious effects of SRp20 on viral mRNAs

The Epstein-Barr Virus (EBV) protein EB2 (also called Mta, SM and BMLF1), is an essential nuclear protein produced during the replicative cycle of EBV. EB2 is required for the efficient cytoplasmic accumulation of viral mRNAs derived from intronless genes. EB2 is an RNA-binding protein whose expression has been shown to influence RNA stability, splicing, nuclear export and translation. Using a yeast two-hybrid screen, we have identified three SR proteins, SF2/ASF, 9G8 and SRp20, as cellular partners of EB2. Then, by using siRNA to deplete cells of specific SR proteins, we found that SRp20 plays an essential role in the processing of several model mRNAs: the Renilla luciferase reporter mRNA, the human β-globin cDNA transcript and two EBV late mRNAs. These four mRNAs were previously found to be highly dependent on EB2 for their efficient cytoplasmic accumulation. Here, we show that SRp20 depletion results in an increase in the accumulation of these mRNAs, which correlates with an absence of additive effect of EB2, suggesting that EB2 functions by antagonizing SRp20. Moreover, by using RNA-immunoprecipitation assays we found that EB2 enhances the association of SRp20 with the β-globin transcript suggesting that EB2 acts by stabilizing SRp20's labile interactions with the RNA.

Generation and comprehensive analysis of an influenza virus polymerase cellular interaction network

The influenza virus transcribes and replicates its genome inside the nucleus of infected cells. Both activities are performed by the viral RNA-dependent RNA polymerase that is composed of the three subunits PA, PB1, and PB2, and recent studies have shown that it requires host cell factors to transcribe and replicate the viral genome. To identify these cellular partners, we generated a comprehensive physical interaction map between each polymerase subunit and the host cellular proteome. A total of 109 human interactors were identified by yeast two-hybrid screens, whereas 90 were retrieved by literature mining. We built the FluPol interactome network composed of the influenza virus polymerase (PA, PB1, and PB2) and the nucleoprotein NP and 234 human proteins that are connected through 279 viral-cellular protein interactions. Analysis of this interactome map revealed enriched cellular functions associated with the influenza virus polymerase, including host factors involved in RNA polymerase II-dependent transcription and mRNA processing. We confirmed that eight influenza virus polymerase-interacting proteins are required for virus replication and transcriptional activity of the viral polymerase. These are involved in cellular transcription (C14orf166, COPS5, MNAT1, NMI, and POLR2A), translation (EIF3S6IP), nuclear transport (NUP54), and DNA repair (FANCG). Conversely, we identified PRKRA, which acts as an inhibitor of the viral polymerase transcriptional activity and thus is required for the cellular antiviral response.