Widespread macromolecular interaction perturbations in human genetic disorders.

How disease-associated mutations impair protein activities in the context of biological networks remains mostly undetermined. Although a few renowned alleles are well characterized, functional information is missing for over 100,000 disease-associated variants. Here we functionally profile several thousand missense mutations across a spectrum of Mendelian disorders using various interaction assays. The majority of disease-associated alleles exhibit wild-type chaperone binding profiles, suggesting they preserve protein folding or stability. While common variants from healthy individuals rarely affect interactions, two-thirds of disease-associated alleles perturb protein-protein interactions, with half corresponding to "edgetic" alleles affecting only a subset of interactions while leaving most other interactions unperturbed. With transcription factors, many alleles that leave protein-protein interactions intact affect DNA binding. Different mutations in the same gene leading to different interaction profiles often result in distinct disease phenotypes. Thus disease-associated alleles that perturb distinct protein activities rather than grossly affecting folding and stability are relatively widespread.

A reference map of the human binary protein interactome.

Global insights into cellular organization and genome function require comprehensive understanding of the interactome networks that mediate genotype-phenotype relationships1,2. Here we present a human 'all-by-all' reference interactome map of human binary protein interactions, or 'HuRI'. With approximately 53,000 protein-protein interactions, HuRI has approximately four times as many such interactions as there are high-quality curated interactions from small-scale studies. The integration of HuRI with genome3, transcriptome4 and proteome5 data enables cellular function to be studied within most physiological or pathological cellular contexts. We demonstrate the utility of HuRI in identifying the specific subcellular roles of protein-protein interactions. Inferred tissue-specific networks reveal general principles for the formation of cellular context-specific functions and elucidate potential molecular mechanisms that might underlie tissue-specific phenotypes of Mendelian diseases. HuRI is a systematic proteome-wide reference that links genomic variation to phenotypic outcomes.

Profiling Cullin4-E3 ligases interactomes and their rewiring in influenza A virus infection.

Understanding the integrated regulation of cellular processes during viral infection is crucial for developing host-targeted approaches. We have previously reported that an optimal in vitro infection by influenza A (IAV) requires three components of Cullin 4-RING E3 ubiquitin ligases (CRL4) complexes, namely the DDB1 adaptor and two Substrate Recognition Factors (SRF), DCAF11 and DCAF12L1, which mediate non-degradative poly-ubiquitination of the PB2 subunit of the viral polymerase. However, the impact of IAV infection on the CRL4 interactome remains elusive. Here, using Affinity Purification coupled with Mass Spectrometry (AP-MS) approaches, we identified cellular proteins interacting with these CRL4 components in IAV-infected and non-infected contexts. IAV infection induces significant modulations in protein interactions, resulting in a global loss of DDB1 and DCAF11 interactions, and an increase in DCAF12L1-associated proteins. The distinct rewiring of CRL4's associations upon infection impacted cellular proteins involved in protein folding, ubiquitination, translation, splicing, and stress responses. Using a split-nanoluciferase-based assay, we identified direct partners of CRL4 components and via siRNA-mediated silencing validated their role in IAV infection, representing potential substrates or regulators of CRL4 complexes. Our findings unravel the dynamic remodeling of the proteomic landscape of CRL4's E3 ubiquitin ligases during IAV infection, likely involved in shaping a cellular environment conducive to viral replication and offer potential for the exploration of future host-targeted antiviral therapeutic strategies.

Sodium-dependent phosphate transporter PiT1/SLC20A1 as the receptor for the endogenous retroviral envelope syncytin-B involved in mouse placenta formation.

Syncytins are envelope genes of retroviral origin that play a critical role in the formation of a syncytial structure at the fetomaternal interface via their fusogenic activity. The mouse placenta is unique among placental mammals since the fetomaternal interface comprises two syncytiotrophoblast layers (ST-I and ST-II) instead of one observed in all other hemochorial placentae. Each layer specifically expresses a distinct mouse syncytin, namely syncytin-A (SynA) for ST-I and syncytin-B (SynB) for ST-II, which have been shown to be essential to placentogenesis and embryonic development. The cellular receptor for SynA has been identified as the membrane protein LY6E and is not the receptor for SynB. Here, by combining a cell-cell fusion assay with the screening of a human ORFeome-derived expression library, we identified the transmembrane multipass sodium-dependent phosphate transporter 1 PiT1/SLC20A1 as the receptor for SynB. Transfection of cells with the cloned receptor, but not the closely related PiT2/SLC20A2, leads to their fusion with cells expressing SynB, with no cross-reactive fusion activity with SynA. The interaction between the two partners was further demonstrated by immunoprecipitation. PiT1/PiT2 chimera and truncation experiments identified the PiT1 N-terminus as the major determinant for SynB-mediated fusion. RT-qPCR analysis of PiT1 expression on a panel of mouse adult and fetal tissues revealed a concomitant increase of PiT1 and SynB specifically in the developing placenta. Finally, electron microscopy analysis of the placenta of PiT1 null embryo before they die (E11.5) disclosed default of ST-II formation with lack of syncytialization, as previously observed in cognate SynB null placenta, and consistent with the present identification of PiT1 as the SynB partner.IMPORTANCESyncytins are envelope genes of endogenous retroviruses, coopted for a physiological function in placentation. They are fusogenic proteins that mediate cell-cell fusion by interacting with receptors present on the partner cells. Here, by devising an in vitro fusion assay that enables the screening of an ORFeome-derived expression library, we identified the long-sought receptor for syncytin-B (SynB), a mouse syncytin responsible for syncytiotrophoblast formation at the fetomaternal interface of the mouse placenta. This protein - PiT1/SLC20A1 - is a multipass transmembrane protein, also known as the receptor for a series of infectious retroviruses. Its profile of expression is consistent with a role in both ancestral endogenization of a SynB founder retrovirus and present-day mouse placenta formation, with evidence-in PiT1 knockout mice-of unfused cells at the level of the cognate placental syncytiotrophoblast layer.

SKAP2 Modular Organization Differently Recognizes SRC Kinases Depending on Their Activation Status and Localization.

Dimerization of SRC kinase adaptor phosphoprotein 2 (SKAP2) induces an increase of binding for most SRC kinases suggesting a fine-tuning with transphosphorylation for kinase activation. This work addresses the molecular basis of SKAP2-mediated SRC kinase regulation through the lens of their interaction capacities. By combining a luciferase complementation assay and extensive site-directed mutagenesis, we demonstrated that SKAP2 interacts with SRC kinases through a modular organization depending both on their phosphorylation-dependent activation and subcellular localization. SKAP2 contains three interacting modules consisting in the dimerization domain, the SRC homology 3 (SH3) domain, and the second interdomain located between the Pleckstrin homology and the SH3 domains. Functionally, the dimerization domain is necessary and sufficient to bind to most activated and myristyl SRC kinases. In contrast, the three modules are necessary to bind SRC kinases at their steady state. The Pleckstrin homology and SH3 domains of SKAP2 as well as tyrosines located in the interdomains modulate these interactions. Analysis of mutants of the SRC kinase family member hematopoietic cell kinase supports this model and shows the role of two residues, Y390 and K7, on its degradation following activation. In this article, we show that a modular architecture of SKAP2 drives its interaction with SRC kinases, with the binding capacity of each module depending on both their localization and phosphorylation state activation. This work opens new perspectives on the molecular mechanisms of SRC kinases activation, which could have significant therapeutic impact.

Measuring the subcellular compartmentalization of viral infections by protein complementation assay.

The recent emergence and reemergence of viruses in the human population has highlighted the need to develop broader panels of therapeutic molecules. High-throughput screening assays opening access to untargeted steps of the viral replication cycle will provide powerful leverage to identify innovative antiviral molecules. We report here the development of an innovative protein complementation assay, termed αCentauri, to measure viral translocation between subcellular compartments. As a proof of concept, the Centauri fragment was either tethered to the nuclear pore complex or sequestered in the nucleus, while the complementary α fragment (<16 amino acids) was attached to the integrase proteins of infectious HIV-1. The translocation of viral ribonucleoproteins from the cytoplasm to the nuclear envelope or to the nucleoplasm efficiently reconstituted superfolder green fluorescent protein or NanoLuc αCentauri reporters. These fluorescence- or bioluminescence-based assays offer a robust readout of specific steps of viral infection in a multiwell format that is compatible for high-throughput screening and is validated by a short hairpin RNA-based prototype screen.

The cargo adapter protein CLINT1 is phosphorylated by the Numb-associated kinase BIKE and mediates dengue virus infection.

The signaling pathways and cellular functions regulated by the four Numb-associated kinases are largely unknown. We reported that AAK1 and GAK control intracellular trafficking of RNA viruses and revealed a requirement for BIKE in early and late stages of dengue virus (DENV) infection. However, the downstream targets phosphorylated by BIKE have not yet been identified. Here, to identify BIKE substrates, we conducted a barcode fusion genetics-yeast two-hybrid screen and retrieved publicly available data generated via affinity-purification mass spectrometry. We subsequently validated 19 of 47 putative BIKE interactors using mammalian cell-based protein-protein interaction assays. We found that CLINT1, a cargo-specific adapter implicated in bidirectional Golgi-to-endosome trafficking, emerged as a predominant hit in both screens. Our experiments indicated that BIKE catalyzes phosphorylation of a threonine 294 CLINT1 residue both in vitro and in cell culture. Our findings revealed that CLINT1 phosphorylation mediates its binding to the DENV nonstructural 3 protein and subsequently promotes DENV assembly and egress. Additionally, using live-cell imaging we revealed that CLINT1 cotraffics with DENV particles and is involved in mediating BIKE's role in DENV infection. Finally, our data suggest that additional cellular BIKE interactors implicated in the host immune and stress responses and the ubiquitin proteasome system might also be candidate phosphorylation substrates of BIKE. In conclusion, these findings reveal cellular substrates and pathways regulated by the understudied Numb-associated kinase enzyme BIKE, a mechanism for CLINT1 regulation, and control of DENV infection via BIKE signaling, with potential implications for cell biology, virology, and host-targeted antiviral design.

Proteomic Analysis Uncovers Measles Virus Protein C Interaction With p65-iASPP Protein Complex.

Viruses manipulate the central machineries of host cells to their advantage. They prevent host cell antiviral responses to create a favorable environment for their survival and propagation. Measles virus (MV) encodes two nonstructural proteins MV-V and MV-C known to counteract the host interferon response and to regulate cell death pathways. Several molecular mechanisms underlining MV-V regulation of innate immunity and cell death pathways have been proposed, whereas MV-C host-interacting proteins are less studied. We suggest that some cellular factors that are controlled by MV-C protein during viral replication could be components of innate immunity and the cell death pathways. To determine which host factors are targeted by MV-C, we captured both direct and indirect host-interacting proteins of MV-C protein. For this, we used a strategy based on recombinant viruses expressing tagged viral proteins followed by affinity purification and a bottom-up mass spectrometry analysis. From the list of host proteins specifically interacting with MV-C protein in different cell lines, we selected the host targets that belong to immunity and cell death pathways for further validation. Direct protein interaction partners of MV-C were determined by applying protein complementation assay and the bioluminescence resonance energy transfer approach. As a result, we found that MV-C protein specifically interacts with p65-iASPP protein complex that controls both cell death and innate immunity pathways and evaluated the significance of these host factors on virus replication.

Novel Antiviral Molecules against Ebola Virus Infection.

Infection with Ebola virus (EBOV) is responsible for hemorrhagic fever in humans with a high mortality rate. Combined efforts of prevention and therapeutic intervention are required to tackle highly variable RNA viruses, whose infections often lead to outbreaks. Here, we have screened the 2P2I3D chemical library using a nanoluciferase-based protein complementation assay (NPCA) and isolated two compounds that disrupt the interaction of the EBOV protein fragment VP35IID with the N-terminus of the dsRNA-binding proteins PKR and PACT, involved in IFN response and/or intrinsic immunity, respectively. The two compounds inhibited EBOV infection in cell culture as well as infection by measles virus (MV) independently of IFN induction. Consequently, we propose that the compounds are antiviral by restoring intrinsic immunity driven by PACT. Given that PACT is highly conserved across mammals, our data support further testing of the compounds in other species, as well as against other negative-sense RNA viruses.

Influenza A virus co-opts ERI1 exonuclease bound to histone mRNA to promote viral transcription.

Cellular exonucleases involved in the processes that regulate RNA stability and quality control have been shown to restrict or to promote the multiplication cycle of numerous RNA viruses. Influenza A viruses are major human pathogens that are responsible for seasonal epidemics, but the interplay between viral proteins and cellular exonucleases has never been specifically studied. Here, using a stringent interactomics screening strategy and an siRNA-silencing approach, we identified eight cellular factors among a set of 75 cellular proteins carrying exo(ribo)nuclease activities or involved in RNA decay processes that support influenza A virus multiplication. We show that the exoribonuclease ERI1 interacts with the PB2, PB1 and NP components of the viral ribonucleoproteins and is required for viral mRNA transcription. More specifically, we demonstrate that the protein-protein interaction is RNA dependent and that both the RNA binding and exonuclease activities of ERI1 are required to promote influenza A virus transcription. Finally, we provide evidence that during infection, the SLBP protein and histone mRNAs co-purify with vRNPs alongside ERI1, indicating that ERI1 is most probably recruited when it is present in the histone pre-mRNA processing complex in the nucleus.

Destabilization of the human RED-SMU1 splicing complex as a basis for host-directed antiinfluenza strategy.

New therapeutic strategies targeting influenza are actively sought due to limitations in current drugs available. Host-directed therapy is an emerging concept to target host functions involved in pathogen life cycles and/or pathogenesis, rather than pathogen components themselves. From this perspective, we focused on an essential host partner of influenza viruses, the RED-SMU1 splicing complex. Here, we identified two synthetic molecules targeting an α-helix/groove interface essential for RED-SMU1 complex assembly. We solved the structure of the SMU1 N-terminal domain in complex with RED or bound to one of the molecules identified to disrupt this complex. We show that these compounds inhibiting RED-SMU1 interaction also decrease endogenous RED-SMU1 levels and inhibit viral mRNA splicing and viral multiplication, while preserving cell viability. Overall, our data demonstrate the potential of RED-SMU1 destabilizing molecules as an antiviral therapy that could be active against a wide range of influenza viruses and be less prone to drug resistance.

Systematic screening reveals a role for BRCA1 in the response to transcription-associated DNA damage.

BRCA1 is a breast and ovarian tumor suppressor. Given its numerous incompletely understood functions and the possibility that more exist, we performed complementary systematic screens in search of new BRCA1 protein-interacting partners. New BRCA1 functions and/or a better understanding of existing ones were sought. Among the new interacting proteins identified, genetic interactions were detected between BRCA1 and four of the interactors: TONSL, SETX, TCEANC, and TCEA2. Genetic interactions were also detected between BRCA1 and certain interactors of TONSL, including both members of the FACT complex. From these results, a new BRCA1 function in the response to transcription-associated DNA damage was detected. Specifically, new roles for BRCA1 in the restart of transcription after UV damage and in preventing or repairing damage caused by stabilized R loops were identified. These roles are likely carried out together with some of the newly identified interactors. This new function may be important in BRCA1 tumor suppression, since the expression of several interactors, including some of the above-noted transcription proteins, is repeatedly aberrant in both breast and ovarian cancers.

Core-Modified Coelenterazine Luciferin Analogues: Synthesis and Chemiluminescence Properties.

In this work on the design and studies of luciferins related to the blue-hued coelenterazine, the synthesis of heterocyclic analogues susceptible to produce a photon, possibly at a different wavelength, is undertaken. Here, the synthesis of O-acetylated derivatives of imidazo[1,2-b]pyridazin-3(5 H)-one, imidazo[2,1-f][1,2,4]triazin-7(1 H)-one, imidazo[1,2-a]pyridin-3-ol, imidazo[1,2-a]quinoxalin-1(5 H)-one, benzo[f]imidazo[1,2-a]quinoxalin-3(11 H)-one, imidazo[1',2':1,6]pyrazino[2,3-c]quinolin-3(11 H)-one, and 5,11-dihydro-3 H-chromeno[4,3-e]imidazo[1,2-a]pyrazin-3-one is described thanks to extensive use of the Buchwald-Hartwig N-arylation reaction. The acidic hydrolysis of these derivatives then gave solutions of the corresponding luciferin analogues, which were studied. Not too unexpectedly, even if these were "dressed" with substituents found in actual substrates of the nanoKAZ/NanoLuc luciferase, no bioluminescence was observed with these compounds. However, in a phosphate buffer, all produced a light signal, by chemiluminescence, with extensive variations in their respective intensity and this could be increased by adding a quaternary ammonium salt in the buffer. This aspect was actually instrumental to determine the emission spectra of many of these luciferin analogues.

Bioluminescence Profiling of NanoKAZ/NanoLuc Luciferase Using a Chemical Library of Coelenterazine Analogues.

We describe here an extensive structure-bioluminescence relationship study of a chemical library of analogues of coelenterazine, using nanoKAZ/NanoLuc, a mutated luciferase originated from the catalytic subunit of the deep-sea shrimp Oplophorus gracilirostris. Out of the 135 O-acetylated precursors that were prepared by using our recently reported synthesis and following their hydrolysis to give solutions of the corresponding luciferins, notable bioluminescence improvements were achieved in comparison with furimazine, which is currently amongst the best substrates of nanoKAZ/NanoLuc. For instance, the rather more lipophilic analogue 8-(2,3-difluorobenzyl)-2-((5-methylfuran-2-yl)methyl)-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one provided a 1.5-fold improvement of the total light output over a 2 h period, a close to threefold increase of the initial signal intensity and a signal-to-background ratio five times greater than furimazine. The kinetic parameters for the enzymatic reaction were obtained for a selection of luciferin analogues and provided unexpected insights into the luciferase activity. Most prominently, along with a general substrate-dependent and irreversible inactivation of this enzyme, in the case of the optimized luciferin mentioned above, the consumption of 2664 molecules was found to be required for the detection of a single Relative Light Unit (RLU; a luminometer-dependent fraction of a photon).

Gram-scale synthesis of luciferins derived from coelenterazine and original insights into their bioluminescence properties.

An original gram-scale synthesis of O-acetylated forms of coelenterazine, furimazine or hydroxy-bearing analogues of luciferins is described. The comparison over two hours of their bioluminescence, using the nanoKAZ/NanoLuc luciferase, provides remarkable insights useful for the selection of a substrate adapted for a given application.

Quantifying domain-ligand affinities and specificities by high-throughput holdup assay.

Many protein interactions are mediated by small linear motifs interacting specifically with defined families of globular domains. Quantifying the specificity of a motif requires measuring and comparing its binding affinities to all its putative target domains. To this end, we developed the high-throughput holdup assay, a chromatographic approach that can measure up to 1,000 domain-motif equilibrium binding affinities per day. After benchmarking the approach on 210 PDZ-peptide pairs with known affinities, we determined the affinities of two viral PDZ-binding motifs derived from human papillomavirus E6 oncoproteins for 209 PDZ domains covering 79% of the human 'PDZome'. We obtained sharply sequence-dependent binding profiles that quantitatively describe the PDZome recognition specificity of each motif. This approach, applicable to many categories of domain-ligand interactions, has wide potential for quantifying the specificities of interactomes.

Pooled-matrix protein interaction screens using Barcode Fusion Genetics.

High-throughput binary protein interaction mapping is continuing to extend our understanding of cellular function and disease mechanisms. However, we remain one or two orders of magnitude away from a complete interaction map for humans and other major model organisms. Completion will require screening at substantially larger scales with many complementary assays, requiring further efficiency gains in proteome-scale interaction mapping. Here, we report Barcode Fusion Genetics-Yeast Two-Hybrid (BFG-Y2H), by which a full matrix of protein pairs can be screened in a single multiplexed strain pool. BFG-Y2H uses Cre recombination to fuse DNA barcodes from distinct plasmids, generating chimeric protein-pair barcodes that can be quantified via next-generation sequencing. We applied BFG-Y2H to four different matrices ranging in scale from ~25 K to 2.5 M protein pairs. The results show that BFG-Y2H increases the efficiency of protein matrix screening, with quality that is on par with state-of-the-art Y2H methods.

Microtubule-associated proteins 1 (MAP1) promote human immunodeficiency virus type I (HIV-1) intracytoplasmic routing to the nucleus.

After cell entry, HIV undergoes rapid transport toward the nucleus using microtubules and microfilaments. Neither the cellular cytoplasmic components nor the viral proteins that interact to mediate transport have yet been identified. Using a yeast two-hybrid screen, we identified four cytoskeletal components as putative interaction partners for HIV-1 p24 capsid protein: MAP1A, MAP1S, CKAP1, and WIRE. Depletion of MAP1A/MAP1S in indicator cell lines and primary human macrophages led to a profound reduction in HIV-1 infectivity as a result of impaired retrograde trafficking, demonstrated by a characteristic accumulation of capsids away from the nuclear membrane, and an overall defect in nuclear import. MAP1A/MAP1S did not impact microtubule network integrity or cell morphology but contributed to microtubule stabilization, which was shown previously to facilitate infection. In addition, we found that MAP1 proteins interact with HIV-1 cores both in vitro and in infected cells and that interaction involves MAP1 light chain LC2. Depletion of MAP1 proteins reduced the association of HIV-1 capsids with both dynamic and stable microtubules, suggesting that MAP1 proteins help tether incoming viral capsids to the microtubular network, thus promoting cytoplasmic trafficking. This work shows for the first time that following entry into target cells, HIV-1 interacts with the cytoskeleton via its p24 capsid protein. Moreover, our results support a role for MAP1 proteins in promoting efficient retrograde trafficking of HIV-1 by stimulating the formation of stable microtubules and mediating the association of HIV-1 cores with microtubules.

Amyloid Precursor-like Protein 2 and Sortilin Do Not Regulate the PCSK9 Convertase-mediated Low Density Lipoprotein Receptor Degradation but Interact with Each Other.

Amyloid precursor-like protein 2 (APLP2) and sortilin were reported to individually bind the proprotein convertase subtilisin/kexin type 9 (PCSK9) and regulate its activity on the low-density lipoprotein receptor (LDLR). The data presented herein demonstrate that mRNA knockdowns of APLP2, sortilin, or both in the human hepatocyte cell lines HepG2 and Huh7 do not affect the ability of extracellular PCSK9 to enhance the degradation of the LDLR. Furthermore, mice deficient in APLP2 or sortilin do not exhibit significant changes in liver LDLR or plasma total cholesterol levels. Moreover, cellular overexpression of one or both proteins does not alter PCSK9 secretion, or its activity on the LDLR. We conclude that PCSK9 enhances the degradation of the LDLR independently of either APLP2 or sortilin both ex vivo and in mice. Interestingly, when co-expressed with PCSK9, both APLP2 and sortilin were targeted for lysosomal degradation. Using chemiluminescence proximity and co-immunoprecipitation assays, as well as biosynthetic analysis, we discovered that sortilin binds and stabilizes APLP2, and hence could regulate its intracellular functions on other targets.

Recruitment of RED-SMU1 complex by Influenza A Virus RNA polymerase to control Viral mRNA splicing.

Influenza A viruses are major pathogens in humans and in animals, whose genome consists of eight single-stranded RNA segments of negative polarity. Viral mRNAs are synthesized by the viral RNA-dependent RNA polymerase in the nucleus of infected cells, in close association with the cellular transcriptional machinery. Two proteins essential for viral multiplication, the exportin NS2/NEP and the ion channel protein M2, are produced by splicing of the NS1 and M1 mRNAs, respectively. Here we identify two human spliceosomal factors, RED and SMU1, that control the expression of NS2/NEP and are required for efficient viral multiplication. We provide several lines of evidence that in infected cells, the hetero-trimeric viral polymerase recruits a complex formed by RED and SMU1 through interaction with its PB2 and PB1 subunits. We demonstrate that the splicing of the NS1 viral mRNA is specifically affected in cells depleted of RED or SMU1, leading to a decreased production of the spliced mRNA species NS2, and to a reduced NS2/NS1 protein ratio. In agreement with the exportin function of NS2, these defects impair the transport of newly synthesized viral ribonucleoproteins from the nucleus to the cytoplasm, and strongly reduce the production of infectious influenza virions. Overall, our results unravel a new mechanism of viral subversion of the cellular splicing machinery, by establishing that the human splicing factors RED and SMU1 act jointly as key regulators of influenza virus gene expression. In addition, our data point to a central role of the viral RNA polymerase in coupling transcription and alternative splicing of the viral mRNAs.