Mutants of human ACE2 differentially promote SARS-CoV and SARS-CoV-2 spike mediated infection.
PLoS Pathogens. 2021 July 16
SARS-CoV and SARS-CoV-2 encode spike proteins that bind human ACE2 on the cell surface to enter target cells during infection. A small fraction of humans encode variants of ACE2, thus altering the biochemical properties at the protein interaction interface. These and other ACE2 coding mutants can reveal how the spike proteins of each virus may differentially engage the ACE2 protein surface during infection. We created an engineered HEK 293T cell line for facile stable transgenic modification, and expressed the major human ACE2 allele or 28 of its missense mutants, 24 of which are possible through single nucleotide changes from the human reference sequence. Infection with SARS-CoV or SARS-CoV-2 spike pseudotyped lentiviruses revealed that high ACE2 cell-surface expression could mask the effects of impaired binding during infection. Drastically reducing ACE2 cell surface expression revealed a range of infection efficiencies across the panel of mutants. Our infection results revealed a non-linear relationship between soluble SARS-CoV-2 RBD binding to ACE2 and pseudovirus infection, supporting a major role for binding avidity during entry. While ACE2 mutants D355N, R357A, and R357T abrogated entry by both SARS-CoV and SARS-CoV-2 spike proteins, the Y41A mutant inhibited SARS-CoV entry much more than SARS-CoV-2, suggesting differential utilization of the ACE2 side-chains within the largely overlapping interaction surfaces utilized by the two CoV spike proteins. These effects correlated well with cytopathic effects observed during SARS-CoV-2 replication in ACE2-mutant cells. The panel of ACE2 mutants also revealed altered ACE2 surface dependencies by the N501Y spike variant, including a near-complete utilization of the K353D ACE2 variant, despite decreased infection mediated by the parental SARS-CoV-2 spike. Our results clarify the relationship between ACE2 abundance, binding, and infection, for various SARS-like coronavirus spike proteins and their mutants, and inform our understanding for how changes to ACE2 sequence may correspond with different susceptibilities to infection.
Extracellular vesicles carry SARS-CoV-2 spike protein and serve as decoys for neutralizing antibodies.
Journal of Extracellular Vesicles. 2021 June 18
In late 2019, a novel coronavirus named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in Wuhan, China. SARS-CoV-2 and the disease it causes, coronavirus disease 2019 (COVID-19), spread rapidly and became a global pandemic in early 2020. SARS-CoV-2 spike protein is responsible for viral entry and binds to angiotensin converting enzyme 2 (ACE2) on host cells, making it a major target of the immune system – particularly neutralizing antibodies (nAbs) that are induced by infection or vaccines. Extracellular vesicles (EVs) are small membraned particles constitutively released by cells, including virally-infected cells. EVs and viruses enclosed within lipid membranes share some characteristics: they are small, sub-micron particles and they overlap in cellular biogenesis and egress routes. Given their shared characteristics, we hypothesized that EVs released from spike-expressing cells could carry spike and serve as decoys for anti-spike nAbs, promoting viral infection. Here, using mass spectrometry and nanoscale flow cytometry (NFC) approaches, we demonstrate that SARS-CoV-2 spike protein can be incorporated into EVs. Furthermore, we show that spike-carrying EVs act as decoy targets for convalescent patient serum-derived nAbs, reducing their effectiveness in blocking viral entry. These findings have important implications for the pathogenesis of SARS-CoV-2 infection in vivo and highlight the complex interplay between viruses, extracellular vesicles, and the immune system that occurs during viral infections.
Multiplexed measurement of variant abundance and activity reveals VKOR topology, active site and human variant impact.
Elife. 2020 Sep 1
Vitamin K epoxide reductase (VKOR) drives the vitamin K cycle, activating vitamin K-dependent blood clotting factors. VKOR is also the target of the widely used anticoagulant drug, warfarin. Despite VKOR’s pivotal role in coagulation, its structure and active site remain poorly understood. In addition, VKOR variants can cause vitamin K-dependent clotting factor deficiency or alter warfarin response. Here, we used multiplexed, sequencing-based assays to measure the effects of 2,695 VKOR missense variants on abundance and 697 variants on activity in cultured human cells. The large-scale functional data, along with an evolutionary coupling analysis, supports a four transmembrane domain topology, with variants in transmembrane domains exhibiting strongly deleterious effects on abundance and activity. Functionally constrained regions of the protein define the active site, and we find that, of four conserved cysteines putatively critical for function, only three are absolutely required. Finally, 25% of human VKOR missense variants show reduced abundance or activity, possibly conferring warfarin sensitivity or causing disease.
MHC class II transactivator CIITA induces cell resistance to Ebola virus and SARS-like coronaviruses.
Science. 2020 Aug 27
Recent outbreaks of Ebola virus (EBOV) and SARS-CoV-2 have exposed our limited therapeutic options and poor understanding of cellular mechanisms that block viral infections. Using a transposon-mediated gene-activation screen in human cells, we identify that the MHC class II transactivator (CIITA) has antiviral activity against EBOV. CIITA induces resistance by activating expression of the p41 isoform of invariant chain CD74, which inhibits viral entry by blocking cathepsin-mediated processing of the Ebola glycoprotein (EboGP). We further show that CD74 p41 can block the endosomal entry pathway of coronaviruses, including SARS-CoV-2. These data therefore implicate CIITA and CD74 in host defense against a range of viruses, and identify an additional function of these proteins beyond their canonical roles in antigen presentation.
High-throughput discovery of trafficking-deficient variants in the cardiac potassium channel KV11.1.
Heart Rhythm. 2020 June 6.
KCHN2 encodes the KV11.1 potassium channel responsible for IKr, a major repolarization current during the cardiomyocyte action potential. Variants in KCNH2 that lead to decreased IKr have been associated with long QT syndrome type 2 (LQT2). The mechanism of LQT2 is most often induced loss of KV11.1 trafficking to the cell surface. Accurately discriminating between variants with normal and abnormal trafficking would aid in understanding the deleterious nature of these variants; however, the volume of reported nonsynonymous KCNH2 variants precludes the use of conventional methods for functional study.
The purpose of this study was to report a high-throughput, multiplexed screening method for KCNH2 genetic variants capable of measuring the cell surface abundance of hundreds of missense variants in the resulting KV11.1 channel. We developed a method to quantitate KV11.1 variant trafficking on a pilot region of 11 residues in the S5 helix. We generated trafficking scores for 220 of 231 missense variants in the pilot region. For 5 of 5 variants, high-throughput trafficking scores validated when tested in single variant flow cytometry and confocal microscopy experiments. We further explored these results with planar patch electrophysiology and found that loss-of-trafficking variants do not produce IKr. Conversely, but expectedly, some variants that traffic normally were still functionally compromised. We describe a new method for detecting KV11.1 trafficking-deficient variants in a multiplexed assay. This new method accurately generated trafficking data for variants in KV11.1 and is extendable both to all residues in KV11.1 and to other cell surface proteins.
A Premalignant Cell-Based Model for Functionalization and Classification of PTEN Variants.
Cancer Research. 2020 May 4
As sequencing becomes more economical, we are identifying sequence variations in the population faster than ever. For disease-associated genes, it is imperative that we differentiate a sequence variant as either benign or pathogenic such that the appropriate therapeutic interventions or surveillance can be implemented. PTEN (Phosphatase and TENsin homolog) is a frequently mutated tumor suppressor that has been linked to the PTEN hamartoma tumor syndrome. While the domain structure of PTEN and the functional impact of a number of its most common tumor-linked mutations have been characterized, there is a lack of information about many recently identified clinical variants. To address this challenge, we developed a cell-based assay that utilizes a premalignant phenotype of normal mammary epithelial cells lacking PTEN. We measured the ability of PTEN variants to rescue the spheroid formation phenotype of PTEN-/- MCF10A cells maintained in suspension. As proof of concept, we functionalized 47 missense variants using this assay, only 19 of which have clear classifications in ClinVar. We utilized a machine learning model trained with annotated genotypic data to classify variants as benign or pathogenic based on our functional scores. Our model predicted with high accuracy that loss of PTEN function was indicative of pathogenicity. We also determined that the pathogenicity of certain variants may have arisen from reduced stability of the protein product. Overall, this assay outperformed computational predictions, was scalable, and had a short run time, serving as an ideal alternative for annotating the clinical significance of cancer-associated PTEN variants.
Massively parallel variant characterization identifies NUDT15 alleles associated with thiopurine toxicity.
Proc. Natl. Acad. Sci. U.S.A. 2020 Feb 24
As a prototype of genomics-guided precision medicine, individualized thiopurine dosing based on pharmacogenetics is a highly effective way to mitigate hematopoietic toxicity of this class of drugs. Recently, NUDT15 deficiency was identified as a genetic cause of thiopurine toxicity, and NUDT15-informed preemptive dose reduction was quickly adopted in clinical settings. To exhaustively identify pharmacogenetic variants in this gene, we developed massively parallel NUDT15 function assays to determine the variants’ effect on protein abundance and thiopurine cytotoxicity. Of the 3,097 possible missense variants, we characterized the abundance of 2,922 variants and found 54 hotspot residues at which variants resulted in complete loss of protein stability. Analyzing 2,935 variants in the thiopurine cytotoxicity-based assay, we identified 17 additional residues where variants altered NUDT15 activity without affecting protein stability. We identified structural elements key to NUDT15 stability and/or catalytical activity with single amino acid resolution. Functional effects for NUDT15 variants accurately predicted toxicity risk alleles in patients treated with thiopurines with far superior sensitivity and specificity compared to bioinformatic prediction algorithms. In conclusion, our massively parallel variant function assays identified 1,152 deleterious NUDT15 variants, providing a comprehensive reference of variant function and vastly improving the ability to implement pharmacogenetics-guided thiopurine treatment individualization.
Deep Mutational Scan of an SCN5A Voltage Sensor.
Circ Genom Precis Med. 2020 Jan 12
Background – Variants in ion channel genes have classically been studied in low-throughput by patch clamping. Deep Mutational Scanning (DMS) is a complementary approach that can simultaneously assess function of thousands of variants. Methods – We have developed and validated a method to perform a DMS of variants in SCN5A, which encodes the major voltage-gated sodium channel in the heart. We created a library of nearly all possible variants in a 36 base region of SCN5A in the S4 voltage sensor of domain IV and stably integrated the library into HEK293T cells. Results – In preliminary experiments, challenge with three drugs (veratridine, brevetoxin, and ouabain) could discriminate wildtype channels from gain and loss of function pathogenic variants. High-throughput sequencing of the pre- and post-drug challenge pools was used to count the prevalence of each variant and identify variants with abnormal function. The DMS scores identified 40 putative gain of function and 33 putative loss of function variants. For 8/9 variants, patch clamping data was consistent with the scores. Conclusions – These experiments demonstrate the accuracy of a high-throughput in vitro scan of SCN5A variant function, which can be used to identify deleterious variants in SCN5A and other ion channel genes.
An improved platform for functional assessment of large protein libraries in mammalian cells.
Nucleic Acids Res. 2019 Oct 15.
Determining the pathogenicity of genetic variants is a critical challenge, and functional assessment is often the only option. Experimentally characterizing millions of possible missense variants in thousands of clinically important genes requires generalizable, scalable assays. We describe variant abundance by massively parallel sequencing (VAMP-seq), which measures the effects of thousands of missense variants of a protein on intracellular abundance simultaneously. We apply VAMP-seq to quantify the abundance of 7,801 single-amino-acid variants of PTEN and TPMT, proteins in which functional variants are clinically actionable. We identify 1,138 PTEN and 777 TPMT variants that result in low protein abundance, and may be pathogenic or alter drug metabolism, respectively. We observe selection for low-abundance PTEN variants in cancer, and show that p.Pro38Ser, which accounts for ~10% of PTEN missense variants in melanoma, functions via a dominant-negative mechanism. Finally, we demonstrate that VAMP-seq is applicable to other genes, highlighting its generalizability.
Multiplex assessment of protein variant abundance by massively parallel sequencing.
Nat Genet. 2018 Jun; 50(6):874-882
Multiplex genetic assays can simultaneously test thousands of genetic variants for a property of interest. However, limitations of existing multiplex assay methods in cultured mammalian cells hinder the breadth, speed and scale of these experiments. Here, we describe a series of improvements that greatly enhance the capabilities of a Bxb1 recombinase-based landing pad system for conducting different types of multiplex genetic assays in various mammalian cell lines. We incorporate the landing pad into a lentiviral vector, easing the process of generating new landing pad cell lines. We also develop several new landing pad versions, including one where the Bxb1 recombinase is expressed from the landing pad itself, improving recombination efficiency more than 2-fold and permitting rapid prototyping of transgenic constructs. Other versions incorporate positive and negative selection markers that enable drug-based enrichment of recombinant cells, enabling the use of larger libraries and reducing costs. A version with dual convergent promoters allows enrichment of recombinant cells independent of transgene expression, permitting the assessment of libraries of transgenes that perturb cell growth and survival. Lastly, we demonstrate these improvements by assessing the effects of a combinatorial library of oncogenes and tumor suppressors on cell growth. Collectively, these advancements make multiplex genetic assays in diverse cultured cell lines easier, cheaper and more effective, facilitating future studies probing how proteins impact cell function, using transgenic variant libraries tested individually or in combination.
On the design of CRISPR-based single cell molecular screens.
Nat Methods. 2018 Apr; 15(4):271-274.
Several groups recently coupled CRISPR perturbations and single-cell RNA-seq for pooled genetic screens. We demonstrate that vector designs of these studies are susceptible to ∼50% swapping of guide RNA-barcode associations because of lentiviral template switching. We optimized a published alternative, CROP-seq, in which the guide RNA also serves as the barcode, and here confirm that this strategy performs robustly and doubled the rate at which guides are assigned to cells to 94%.
A platform for functional assessment of large variant libraries in mammalian cells.
Nucleic Acids Res. 2017 Jun;15(4):e102.
Sequencing-based, massively parallel genetic assays have revolutionized our ability to quantify the relationship between many genotypes and a phenotype of interest. Unfortunately, variant library expression platforms in mammalian cells are far from ideal, hindering the study of human gene variants in their physiologically relevant cellular contexts. Here, we describe a platform for phenotyping variant libraries in transfectable mammalian cell lines in two steps. First, a landing pad cell line with a genomically integrated, Tet-inducible cassette containing a Bxb1 recombination site is created. Second, a single variant from a library of transfected, promoter-less plasmids is recombined into the landing pad in each cell. Thus, every cell in the recombined pool expresses a single variant, allowing for parallel, sequencing-based assessment of variant effect. We describe a method for incorporating a single landing pad into a defined site of a cell line of interest, and show that our approach can be used generate more than 20 000 recombinant cells in a single experiment. Finally, we use our platform in combination with a sequencing-based assay to explore the N-end rule by simultaneously measuring the effects of all possible N-terminal amino acids on protein expression.
Host and viral determinants for MxB restriction of HIV-1 infection.
Retrovirology. 2014 Oct; 11:90.
BACKGROUND: Interferon-induced cellular proteins play important roles in the host response against viral infection. The Mx family of dynamin-like GTPases, which include MxA and MxB, target a wide variety of viruses. Despite considerable evidence demonstrating the breadth of antiviral activity of MxA, human MxB was only recently discovered to specifically inhibit lentiviruses. Here we assess both host and viral determinants that underlie MxB restriction of HIV-1 infection.
RESULTS: Heterologous expression of MxB in human osteosarcoma cells potently inhibited HIV-1 infection (~12-fold), yet had little to no effect on divergent retroviruses. The anti-HIV effect manifested as a partial block in the formation of 2-long terminal repeat circle DNA and hence nuclear import, and we accordingly found evidence for an additional post-nuclear entry block. A large number of previously characterized capsid mutations, as well as mutations that abrogated integrase activity, counteracted MxB restriction. MxB expression suppressed integration into gene-enriched regions of chromosomes, similar to affects observed previously when cells were depleted for nuclear transport factors such as transportin 3. MxB activity did not require predicted GTPase active site residues or a series of unstructured loops within the stalk domain that confer functional oligomerization to related dynamin family proteins. In contrast, we observed an N-terminal stretch of residues in MxB to harbor key determinants. Protein localization conferred by a nuclear localization signal (NLS) within the N-terminal 25 residues, which was critical, was fully rescuable by a heterologous NLS. Consistent with this observation, a heterologous nuclear export sequence (NES) abolished full-length MxB activity. We additionally mapped sub-regions within amino acids 26-90 that contribute to MxB activity, finding sequences present within residues 27-50 particularly important.
CONCLUSIONS: MxB inhibits HIV-1 by interfering with minimally two steps of infection, nuclear entry and post-nuclear trafficking and/or integration, without destabilizing the inherent catalytic activity of viral preintegration complexes. Putative MxB GTPase active site residues and stalk domain Loop 4 — both previously shown to be necessary for MxA function — were dispensable for MxB antiviral activity. Instead, we highlight subcellular localization and a yet-determined function(s) present in the unique MxB N-terminal region to be required for HIV-1 restriction.
Structural insight into HIV-1 restriction by MxB.
Cell Host Microbe. 2014 Nov; 16:1-12.
* contributed equally
The myxovirus resistance (Mx) proteins are interferon-induced dynamin GTPases that can inhibit a variety of viruses. Recently, MxB, but not MxA, was shown to restrict HIV-1 by an unknown mechanism that likely occurs in close proximity to the host cell nucleus and involves the viral capsid. Here, we present the crystal structure of MxB and reveal determinants involved in HIV-1 restriction. MxB adopts an extended antiparallel dimer and dimerization, but not higher-ordered oligomerization, is critical for restriction. Although MxB is structurally similar to MxA, the orientation of individual domains differs between MxA and MxB, and their antiviral functions rely on separate determinants, indicating distinct mechanisms for virus inhibition. Additionally, MxB directly binds the HIV-1 capsid, and this interaction depends on dimerization and the N terminus of MxB as well as the assembled capsid lattice. These insights establish a framework for understanding the mechanism by which MxB restricts HIV-1.
Nucleoporin NUP153 phenylalanine-glycine motifs engage a common binding pocket within the HIV-1 capsid protein to mediate lentiviral infectivity.
PLoS Pathog. 2013 Oct;9(10):e1003693.
Lentiviruses can infect non-dividing cells, and various cellular transport proteins provide crucial functions for lentiviral nuclear entry and integration. We previously showed that the viral capsid (CA) protein mediated the dependency on cellular nucleoporin (NUP) 153 during HIV-1 infection, and now demonstrate a direct interaction between the CA N-terminal domain and the phenylalanine-glycine (FG)-repeat enriched NUP153 C-terminal domain (NUP153(C)). NUP153(C) fused to the effector domains of the rhesus Trim5α restriction factor (Trim-NUP153(C)) potently restricted HIV-1, providing an intracellular readout for the NUP153(C)-CA interaction during retroviral infection. Primate lentiviruses and equine infectious anemia virus (EIAV) bound NUP153(C) under these conditions, results that correlated with direct binding between purified proteins in vitro. These binding phenotypes moreover correlated with the requirement for endogenous NUP153 protein during virus infection. Mutagenesis experiments concordantly identified NUP153(C) and CA residues important for binding and lentiviral infectivity. Different FG motifs within NUP153(C) mediated binding to HIV-1 versus EIAV capsids. HIV-1 CA binding mapped to residues that line the common alpha helix 3/4 hydrophobic pocket that also mediates binding to the small molecule PF-3450074 (PF74) inhibitor and cleavage and polyadenylation specific factor 6 (CPSF6) protein, with Asn57 (Asp58 in EIAV) playing a particularly important role. PF74 and CPSF6 accordingly each competed with NUP153(C) for binding to the HIV-1 CA pocket, and significantly higher concentrations of PF74 were needed to inhibit HIV-1 infection in the face of Trim-NUP153(C) expression or NUP153 knockdown. Correlation between CA mutant viral cell cycle and NUP153 dependencies moreover indicates that the NUP153(C)-CA interaction underlies the ability of HIV-1 to infect non-dividing cells. Our results highlight similar mechanisms of binding for disparate host factors to the same region of HIV-1 CA during viral ingress. We conclude that a subset of lentiviral CA proteins directly engage FG-motifs present on NUP153 to affect viral nuclear import.
Differential effects of human immunodeficiency virus type 1 capsid and cellular factors nucleoporin 153 and LEDGF/p75 on the efficiency and specificity of viral DNA integration.
J Virol. 2013 Jan;87(1):648-58.
Retroviruses integrate into cellular DNA nonrandomly. Lentiviruses such as human immunodeficiency virus type 1 (HIV-1) favor the bodies of active genes and gene-enriched transcriptionally active regions of chromosomes. The interaction between lentiviral integrase and the cellular protein lens epithelium-derived growth factor (LEDGF)/p75 underlies the targeting of gene bodies, whereas recent research has highlighted roles for the HIV-1 capsid (CA) protein and cellular factors implicated in viral nuclear import, including transportin 3 (TNPO3) and nucleoporin 358 (NUP358), in the targeting of gene-dense regions of chromosomes. Here, we show that CA mutations, which include the substitution of Asp for Asn74 (N74D), significantly reduce the dependency of HIV-1 on LEDGF/p75 during infection and that this difference correlates with the efficiency of viral DNA integration. The distribution of integration sites mapped by Illumina sequencing confirms that the N74D mutation reduces integration into gene-rich regions of chromosomes and gene bodies and reveals previously unrecognized roles for NUP153 (another HIV-1 cofactor implicated in viral nuclear import) and LEDGF/p75 in the targeting of the viral preintegration complex to gene-dense regions of chromatin. A role for the CA protein in determining the dependency of HIV-1 on LEDGF/p75 during infection highlights a connection between the viral capsid and chromosomal DNA integration.
The requirement for nucleoporin NUP153 during human immunodeficiency virus type 1 infection is determined by the viral capsid.
J Virol. 2011 Aug;85(15):7818-27.
Lentiviruses likely infect nondividing cells by commandeering host nuclear transport factors to facilitate the passage of their preintegration complexes (PICs) through nuclear pore complexes (NPCs) within nuclear envelopes. Genome-wide small interfering RNA screens previously identified karyopherin β transportin-3 (TNPO3) and NPC component nucleoporin 153 (NUP153) as being important for infection by human immunodeficiency virus type 1 (HIV-1). The knockdown of either protein significantly inhibited HIV-1 infectivity, while infection by the gammaretrovirus Moloney murine leukemia virus (MLV) was unaffected. Here, we establish that primate lentiviruses are particularly sensitive to NUP153 knockdown and investigate HIV-1-encoded elements that contribute to this dependency. Mutants lacking functional Vpr or the central DNA flap remained sensitive to NUP153 depletion, while MLV/HIV-1 chimera viruses carrying MLV matrix, capsid, or integrase became less sensitive when the latter two elements were substituted. Two capsid missense mutant viruses, N74D and P90A, were largely insensitive to NUP153 depletion, as was wild-type HIV-1 when cyclophilin A was depleted simultaneously or when infection was conducted in the presence of cyclosporine A. The codepletion of NUP153 and TNPO3 yielded synergistic effects that outweighed those calculated based on individual knockdowns, indicating potential interdependent roles for these factors during HIV-1 infection. Quantitative PCR revealed normal levels of late reverse transcripts, a moderate reduction of 2-long terminal repeat (2-LTR) circles, and a relatively large reduction in integrated proviruses upon NUP153 knockdown. These results suggest that capsid, likely by the qualities of its uncoating, determines whether HIV-1 requires cellular NUP153 for PIC nuclear import.
Differential sensitivities of retroviruses to integrase strand transfer inhibitors.
J Virol. 2011 Apr;85(7):3677-82.
Integrase inhibitors are emerging anti-human immunodeficiency virus (HIV) drugs, and multiple retroviruses and transposable elements were evaluated here for susceptibilities to raltegravir (RAL) and elvitegravir (EVG). All viruses, including primate and nonprimate lentiviruses, a Betaretrovirus, a Gammaretrovirus, and the Alpharetrovirus Rous sarcoma virus (RSV), were susceptible to inhibition by RAL. EVG potently inhibited all lentiviruses and intermediately inhibited Betaretrovirus and Gammaretrovirus infections yet was basically ineffective against RSV. Substitutions based on HIV type 1 (HIV-1) resistance changes revealed that integrase residue Ser150 contributed significantly to the resistance of RSV. The drugs intermediately inhibited intracisternal A-particle retrotransposition but were inactive against Sleeping Beauty transposition and long interspersed nucleotide element 1 (LINE-1) retrotransposition.
The requirement for cellular transportin 3 (TNPO3 or TRN-SR2) during infection maps to human immunodeficiency virus type 1 capsid and not integrase.
J Virol. 2010 Jan;84(1):397-406.
* contributed equally
Recent genome-wide screens have highlighted an important role for transportin 3 in human immunodeficiency virus type 1 (HIV-1) infection and preintegration complex (PIC) nuclear import. Moreover, HIV-1 integrase interacted with recombinant transportin 3 protein under conditions whereby Moloney murine leukemia virus (MLV) integrase failed to do so, suggesting that integrase-transportin 3 interactions might underscore active retroviral PIC nuclear import. Here we correlate infectivity defects in transportin 3 knockdown cells with in vitro protein binding affinities for an expanded set of retroviruses that include simian immunodeficiency virus (SIV), bovine immunodeficiency virus (BIV), equine infectious anemia virus (EIAV), feline immunodeficiency virus (FIV), and Rous sarcoma virus (RSV) to critically address the role of integrase-transportin 3 interactions in viral infection. Lentiviruses, with the exception of FIV, display a requirement for transportin 3 in comparison to MLV and RSV, yielding an infection-based dependency ranking of SIV > HIV-1 > BIV and EIAV > MLV, RSV, and FIV. In vitro pulldown and surface plasmon resonance assays, in contrast, define a notably different integrase-transportin 3 binding hierarchy: FIV, HIV-1, and BIV > SIV and MLV > EIAV. Our results therefore fail to support a critical role for integrase binding in dictating transportin 3 dependency during retrovirus infection. In addition to integrase, capsid has been highlighted as a retroviral nuclear import determinant. Accordingly, MLV/HIV-1 chimera viruses pinpoint the genetic determinant of sensitization to transportin 3 knockdown to the HIV-1 capsid protein. We therefore conclude that capsid, not integrase, is the dominant viral factor that dictates transportin 3 dependency during HIV-1 infection.
Polybasic KKR motif in the cytoplasmic tail of Nipah virus fusion protein modulates membrane fusion by inside-out signaling.
J Virol. 2007 May;81(9):4520-32.
The cytoplasmic tails of the envelope proteins from multiple viruses are known to contain determinants that affect their fusogenic capacities. Here we report that specific residues in the cytoplasmic tail of the Nipah virus fusion protein (NiV-F) modulate its fusogenic activity. Truncation of the cytoplasmic tail of NiV-F greatly inhibited cell-cell fusion. Deletion and alanine scan analysis identified a tribasic KKR motif in the membrane-adjacent region as important for modulating cell-cell fusion. The K1A mutation increased fusion 5.5-fold, while the K2A and R3A mutations decreased fusion 3- to 5-fold. These results were corroborated in a reverse-pseudotyped viral entry assay, where receptor-pseudotyped reporter virus was used to infect cells expressing wild-type or mutant NiV envelope glycoproteins. Differential monoclonal antibody binding data indicated that hyper- or hypofusogenic mutations in the KKR motif affected the ectodomain conformation of NiV-F, which in turn resulted in faster or slower six-helix bundle formation, respectively. However, we also present evidence that the hypofusogenic phenotypes of the K2A and R3A mutants were effected via distinct mechanisms. Interestingly, the K2A mutant was also markedly excluded from lipid rafts, where approximately 20% of wild-type F and the other mutants can be found. Finally, we found a strong negative correlation between the relative fusogenic capacities of these cytoplasmic-tail mutants and the avidities of NiV-F and NiV-G interactions (P = 0.007, r(2) = 0.82). In toto, our data suggest that inside-out signaling by specific residues in the cytoplasmic tail of NiV-F can modulate its fusogenicity by multiple distinct mechanisms.
N-glycans on Nipah virus fusion protein protect against neutralization but reduce membrane fusion and viral entry.
J Virol. 2006 May;80(10):4878-89.
Nipah virus (NiV) is a deadly emerging paramyxovirus. The NiV attachment (NiV-G) and fusion (NiV-F) envelope glycoproteins mediate both syncytium formation and viral entry. Specific N-glycans on paramyxovirus fusion proteins are generally required for proper conformational integrity and biological function. However, removal of individual N-glycans on NiV-F had little negative effect on processing or fusogenicity and has even resulted in slightly increased fusogenicity. Here, we report that in both syncytium formation and viral entry assays, removal of multiple N-glycans on NiV-F resulted in marked increases in fusogenicity (>5-fold) but also resulted in increased sensitivity to neutralization by NiV-F-specific antisera. The mechanism underlying the hyperfusogenicity of these NiV-F N-glycan mutants is likely due to more-robust six-helix bundle formation, as these mutants showed increased fusion kinetics and were more resistant to neutralization by a fusion-inhibitory reagent based on the C-terminal heptad repeat region of NiV-F. Finally, we demonstrate that the fusogenicities of the NiV-F N-glycan mutants were inversely correlated with the relative avidities of NiV-F’s interactions with NiV-G, providing support for the attachment protein “displacement” model of paramyxovirus fusion. Our results indicate that N-glycans on NiV-F protect NiV from antibody neutralization, suggest that this “shielding” role comes together with limiting cell-cell fusion and viral entry efficiencies, and point to the mechanisms underlying the hyperfusogenicity of these N-glycan mutants. These features underscore the varied roles that N-glycans on NiV-F play in the pathobiology of NiV entry but also shed light on the general mechanisms of paramyxovirus fusion with host cells.
The Impact of Genetic Variants on PTEN Molecular Functions and Cellular Phenotypes.
Cold Spring Harb Perspect Med. 2019 Aug 26. pii: a036228. doi: 10.1101/cshperspect.a036228.
Phosphatase and tensin homolog (PTEN) is a tumor suppressor that directly regulates a diverse array of cellular phenotypes, including growth, migration, morphology, and genome stability. How a single protein impacts so many important cellular processes remains a fascinating question. This question has been partially resolved by the characterization of a slew of missense variants that alter or eliminate PTEN’s various molecular functions, including its enzymatic activity, subcellular localization, and posttranslational modifications. Here, we review what is known about how PTEN variants impact molecular function and, consequently, cellular phenotype. In particular, we highlight eight informative “sentinel variants” that abrogate distinct molecular functions of PTEN. We consider two published massively parallel assays of variant effect that measured the effect of thousands of PTEN variants on protein abundance and enzymatic activity. Finally, we discuss how characterization of clinically ascertained variants, establishment of clinical sequencing databases, and massively parallel assays of variant effect yield complementary datasets for dissecting PTEN‘s role in disease.
Viral and cellular requirements for the nuclear entry of retroviral preintegration nucleoprotein complexes.
Viruses. 2013 Oct 7;5(10):2483-511. doi: 10.3390/v5102483.
Retroviruses integrate their reverse transcribed genomes into host cell chromosomes as an obligate step in virus replication. The nuclear envelope separates the chromosomes from the cell cytoplasm during interphase, and different retroviral groups deal with this physical barrier in different ways. Gammaretroviruses are dependent on the passage of target cells through mitosis, where they are believed to access chromosomes when the nuclear envelope dissolves for cell division. Contrastingly, lentiviruses such as HIV-1 infect non-dividing cells, and are believed to enter the nucleus by passing through the nuclear pore complex. While numerous virally encoded elements have been proposed to be involved in HIV-1 nuclear import, recent evidence has highlighted the importance of HIV-1 capsid. Furthermore, capsid was found to be responsible for the viral requirement of various nuclear transport proteins, including transportin 3 and nucleoporins NUP153 and NUP358, during infection. In this review, we describe our current understanding of retroviral nuclear import, with emphasis on recent developments on the role of the HIV-1 capsid protein.
Viral latency and potential eradication of HIV-1.
Expert Rev Anti Infect Ther. 2012 Aug;10(8):855-7.
* contributed equally
Although HAART can suppress plasma viral loads to undetectable levels, individuals infected with HIV-1 harbor latent reservoirs of integrated proviruses that re-emerge upon the cessation of drug treatment. The 2012 Keystone Symposium on Frontiers in HIV Pathogenesis, Therapy and Eradication highlighted the current understanding of latent infection and new methods to activate and target these reservoirs for eradication. This report focuses on a select few aspects of the discussion, including the extent that ongoing replication might contribute to the persistent viral reservoir, recent advances in activating the expression of latent proviruses, progress in developing effective animal models and potential avenues to eradicate the cells that constitute the latent reservoir.