[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The sterile alpha motif and HD domain-containing protein 1 (SAMHD1), a dNTPase, prevents the infection of nondividing cells by retroviruses, including HIV, by depleting the cellular dNTP pool available for viral reverse transcription. SAMHD1 is a major regulator of cellular dNTP levels in mammalian cells. Mutations in SAMHD1 are associated with chronic lymphocytic leukemia (CLL) and the autoimmune condition Aicardi Goutières syndrome (AGS). The dNTPase activity of SAMHD1 can be regulated by dGTP, with which SAMHD1 assembles into catalytically active tetramers. Here we present extensive biochemical and structural data that reveal an exquisite activation mechanism of SAMHD1 via combined action of both GTP and dNTPs. We obtained 26 crystal structures of SAMHD1 in complex with different combinations of GTP and dNTP mixtures, which depict the full spectrum of GTP/dNTP binding at the eight allosteric and four catalytic sites of the SAMHD1 tetramer. Our data demonstrate how SAMHD1 is activated by binding of GTP or dGTP at allosteric site 1 and a dNTP of any type at allosteric site 2. Our enzymatic assays further reveal a robust regulatory mechanism of SAMHD1 activity, which bares resemblance to that of the ribonuclease reductase responsible for cellular dNTP production. These results establish a complete framework for a mechanistic understanding of the important functions of SAMHD1 in the regulation of cellular dNTP levels, as well as in HIV restriction and the pathogenesis of CLL and AGS.
Proceedings of the National Academy of Sciences of the United States of America. 09/2014;
[Show abstract][Hide abstract] ABSTRACT: Fanconi anemia (FA) represents a paradigm of rare genetic diseases where the quest for cause and cure has led to seminal discoveries in cancer biology. While a total of 16 FA genes have been identified thus far, the biochemical function of many of the FA proteins remains to be elucidated. FA is rare, yet the fact that 5 FA genes are in fact familial breast cancer genes and FA gene mutations are found frequently in sporadic cancers suggest wider applicability in hematopoiesis and oncology. Establishing the interaction network involving the FA proteins and their associated partners has revealed an intersection of FA with several DNA repair pathways, including homologous recombination, DNA mismatch repair, nucleotide excision repair, and translesion DNA synthesis. Importantly, recent studies have shown a major involvement of the FA pathway in the tolerance of reactive aldehydes. Moreover, despite improved outcomes in stem cell transplantation in the treatment of FA, many challenges remain in patient care.
[Show abstract][Hide abstract] ABSTRACT: Focal adhesions (FAs) are macromolecular complexes that connect the actin cytoskeleton to the extracellular matrix. Dynamic turnover of FAs is critical for cell migration. Paxillin is a multi-adaptor protein that plays an important role in regulating FA dynamics. Here, we identify TRIM15, a member of the TRIpartite Motif protein family, as a paxillin-interacting factor and a component of FAs. TRIM15 localizes to focal contacts in a myosin II-independent manner by an interaction between its coiled coil domain and the LD2 motif of paxillin. Unlike other FA proteins, TRIM15 is a stable FA component with restricted mobility due to its ability to form oligomers. TRIM15-depleted cells display impaired cell migration and FA disassembly rates in addition to enlarged FAs. Thus, our studies demonstrate a cellular function for TRIM15 as a regulatory component of FA turnover and cell migration.
[Show abstract][Hide abstract] ABSTRACT: Neu-Laxova syndrome (NLS) is a rare autosomal-recessive disorder characterized by severe fetal growth restriction, microcephaly, a distinct facial appearance, ichthyosis, skeletal anomalies, and perinatal lethality. The pathogenesis of NLS remains unclear despite extensive clinical and pathological phenotyping of the >70 affected individuals reported to date, emphasizing the need to identify the underlying genetic etiology, which remains unknown. In order to identify the cause of NLS, we conducted a positional-mapping study combining autozygosity mapping and whole-exome sequencing in three consanguineous families affected by NLS. Surprisingly, the NLS-associated locus identified in this study was solved at the gene level to reveal mutations in PHGDH, which is known to be mutated in individuals with microcephaly and developmental delay. PHGDH encodes the first enzyme in the phosphorylated pathway of de novo serine synthesis, and complete deficiency of its mouse ortholog recapitulates many of the key features of NLS. This study shows that NLS represents the extreme end of a known inborn error of serine metabolism and highlights the power of genomic sequencing in revealing the unsuspected allelic nature of apparently distinct clinical entities.
The American Journal of Human Genetics 05/2014; · 11.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: BST2/tetherin, an antiviral restriction factor, inhibits the release of enveloped viruses from the cell surface. Human immunodeficiency virus-1 (HIV-1) antagonizes BST2 through viral protein u (Vpu), which downregulates BST2 from the cell surface. We report the crystal structure of a protein complex containing Vpu and BST2 cytoplasmic domains and the core of the clathrin adaptor protein complex 1 (AP1). This, together with our biochemical and functional validations, reveals how Vpu hijacks the AP1-dependent membrane trafficking pathways to mistraffick BST2. Vpu mimics a canonical acidic dileucine-sorting motif to bind AP1 in the cytosol, while simultaneously interacting with BST2 in the membrane. These interactions enable Vpu to build on an intrinsic interaction between BST2 and AP1, presumably causing the observed retention of BST2 in juxtanuclear endosomes and stimulating its degradation in lysosomes. The ability of Vpu to hijack AP-dependent trafficking pathways suggests a potential common theme for Vpu-mediated downregulation of host proteins.DOI: http://dx.doi.org/10.7554/eLife.02362.001.
[Show abstract][Hide abstract] ABSTRACT: We have previously described a syndrome characterized by facial dysmorphism, lens dislocation, anterior-segment abnormalities, and spontaneous filtering blebs (FDLAB, or Traboulsi syndrome). In view of the consanguineous nature of the affected families and the likely autosomal-recessive inheritance pattern of this syndrome, we undertook autozygosity mapping and whole-exome sequencing to identify ASPH as the disease locus, in which we identified two homozygous mutations. ASPH encodes aspartyl/asparaginyl β-hydroxylase (ASPH), which has been found to hydroxylate aspartic acid and asparagine residues on epidermal growth factor (EGF)-domain-containing proteins. The truncating and missense mutations we identified are predicted to severely impair the enzymatic function of ASPH, which suggests a possible link to other forms of ectopia lentis given that many of the genes implicated in this phenotype encode proteins that harbor EGF domains. Developmental analysis of Asph revealed an expression pattern consistent with the proposed link to the human syndrome. Indeed, Asph-knockout mice had a foreshortened snout, which corresponds to the facial abnormalities in individuals with Traboulsi syndrome. These data support a genetic basis for a syndromic form of ectopia lentis and the role of aspartyl hydroxylation in human development.
The American Journal of Human Genetics 04/2014; · 11.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy with multisystem involvement. So far, 18 BBS genes have been identified and the majority of them are essential for the function of BBSome, a protein complex involved in transporting membrane proteins into and from cilia. Yet defects in the identified genes cannot account for all the BBS cases. The genetic heterogeneity of this disease pose significant challenge to the identification of additional BBS genes. In this study, we coupled human genetics with functional validation in zebrafish and identified IFT27 as a novel BBS gene (BBS19). This is the first time an IFT (intraflagellar transport) gene is implicated in the pathogenesis of BBS, highlighting the genetic complexity of this disease.
Human Molecular Genetics 01/2014; · 7.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The conserved MHF1-MHF2 (MHF) complex functions in the activation of the Fanconi anaemia pathway of the DNA damage response, in regulating homologous recombination, and in DNA replication fork maintenance. MHF facilitates the processing of multiple types of branched DNAs by the DNA translocase FANCM. Here we report the crystal structure of a human MHF-DNA complex that reveals the DNA-binding mode of MHF. The structure suggests that MHF prefers branched DNA over double-stranded DNA because it engages two duplex arms. Biochemical analyses verify that MHF preferentially engages DNA forks or various four-way junctions independent of the junction-site structure. Furthermore, genetic experiments provide evidence that the observed DNA-binding interface of MHF is important for cellular resistance to DNA damage. These results offer insights into how the MHF complex recognizes branched DNA and stimulates FANCM activity at such a structure to promote genome maintenance.
[Show abstract][Hide abstract] ABSTRACT: The HIV-1 virion infectivity factor (Vif) targets the cellular cytidine deaminases Apobec3G (A3G) and Apobec3F (A3F) for degradation via the host ubiquitin-proteasome pathway. Vif recruits a cellular E3 ubiquitin ligase to polyubiquitinate A3G/F. The activity of Vif critically depends on the cellular core binding factor-β (CBFβ). In this study we investigate the Vif-CBFβ interaction and the role of CBFβ in E3 ligase assembly. Vif-CBFβ interaction requires an extensive region of Vif spanning most of its amino terminus and zinc finger region, and Cul5 binding enhances the stability of the Vif-CBFβ interaction. Our results further demonstrate that CBFβ plays a critical role in facilitating Cul5 binding to the Vif-EloBC complex. Vif, with or without bound substrate, is unable to bind Cul5 in the absence of CBFβ. These studies support the notion that CBFβ serves as a molecular chaperone to facilitate Vif-E3 ligase assembly.
[Show abstract][Hide abstract] ABSTRACT: Sharpening is a powerful method to restore the details from blurred electron density in crystals with high overall temperature factors (B-factors). This valuable technique is currently not optimally used because of the uncertainty in the scope of its application and ambiguities in practice. We performed an analysis of ~2,000 crystal data sets deposited in the PDB and show that sharpening improves the electron density map in many cases across all resolution ranges, often with dramatic enhancement for mid- and low-resolution structures. It is effective when used with either experimental or model phases without introducing additional bias. Our tests also provide a practical guide for optimal sharpening. We further show anisotropic diffraction correction improves electron density in many cases but should be used with caution. Our study demonstrates that a routine practice of electron density sharpening may have a broad impact on the outcomes of structural biology studies.
Journal of Molecular Biology 11/2013; · 3.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: SAMHD1, a dNTP triphosphohydrolase (dNTPase), has a key role in human innate immunity. It inhibits infection of blood cells by retroviruses, including HIV, and prevents the development of the autoinflammatory Aicardi-Goutières syndrome (AGS). The inactive apo-SAMHD1 interconverts between monomers and dimers, and in the presence of dGTP the protein assembles into catalytically active tetramers. Here, we present the crystal structure of the human tetrameric SAMHD1-dGTP complex. The structure reveals an elegant allosteric mechanism of activation through dGTP-induced tetramerization of two inactive dimers. Binding of dGTP to four allosteric sites promotes tetramerization and induces a conformational change in the substrate-binding pocket to yield the catalytically active enzyme. Structure-based biochemical and cell-based biological assays confirmed the proposed mechanism. The SAMHD1 tetramer structure provides the basis for a mechanistic understanding of its function in HIV restriction and the pathogenesis of AGS.
[Show abstract][Hide abstract] ABSTRACT: Tripartite motif protein isoform 5 alpha (TRIM5α) is a potent antiviral protein that restricts infection by HIV-1 and other retroviruses. TRIM5α recognizes the lattice of the retrovirus capsid through its B30.2 (PRY/SPRY) domain in a species-specific manner. Upon binding, TRIM5α induces premature disassembly of the viral capsid and activates the downstream innate immune response. We have determined the crystal structure of the rhesus TRIM5α PRY/SPRY domain that reveals essential features for capsid binding. Combined cryo-electron microscopy and biochemical data show that the monomeric rhesus TRIM5α PRY/SPRY, but not the human TRIM5α PRY/SPRY, can bind to HIV-1 capsid protein assemblies without causing disruption of the capsid. This suggests that the PRY/SPRY domain alone constitutes an important pattern-sensing component of TRIM5α that is capable of interacting with viral capsids of different curvatures. Our results provide molecular insights into the mechanisms of TRIM5α-mediated retroviral restriction.
Proceedings of the National Academy of Sciences 10/2012; · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The HIV-1 protein Nef inhibits antigen presentation by class I major histocompatibility complex (MHC-I). We determined the mechanism of this activity by solving the crystal structure of a protein complex comprising Nef, the MHC-I cytoplasmic domain (MHC-I CD) and the μ1 subunit of the clathrin adaptor protein complex 1. A ternary, cooperative interaction clamps the MHC-I CD into a narrow binding groove at the Nef-μ1 interface, which encompasses the cargo-recognition site of μ1 and the proline-rich strand of Nef. The Nef C terminus induces a previously unobserved conformational change in μ1, whereas the N terminus binds the Nef core to position it optimally for complex formation. Positively charged patches on μ1 recognize acidic clusters in Nef and MHC-I. The structure shows how Nef functions as a clathrin-associated sorting protein to alter the specificity of host membrane trafficking and enable viral evasion of adaptive immunity.
[Show abstract][Hide abstract] ABSTRACT: APOBEC3H (A3H) is a member of the APOBEC3 family of proteins with varying activities against retroviruses and retrotransposons. The A3H gene contains several single nucleotide polymorphisms and up to seven haplotypes have been detected in humans. Although variations in anti-viral function among A3H haplotypes are not fully understood, only 15N105R-containing A3H variants are known to have potent activities against Vif-deficient HIV-1. Unique motif RLYY(F/Y)W of APOBEC3G (A3G) and APOBEC3F (A3F) required for 7SL RNA binding and HIV-1 incorporation is also conserved in all A3H variants. Like A3G, A3H HapII also demonstrated high binding affinity to host small RNAs such as 7SL and Y RNAs. Mutation of a critical amino acid, W115A resulted in reduced expression level, decreased affinity for 7SL RNA, impairment of virion packaging and reduced anti-viral activity. By comparison, A3H HapI had lower binding affinities to host small RNAs and reduced efficiency of virion incorporation, resulting in significantly reduced anti-viral activity. The SNP ΔN15 commonly found in A3H HapIII and HapIV abolished their abilities to associate with RNAs, and A3H HapIIΔ15N failed to package into HIV-1 virions or exhibited any anti-viral activity. Finally, we showed that A3H variants had distinct cellular localization patterns, which correlated with their different RNA binding affinities. Thus, Pol-III RNA such as 7SL RNA binding is a conserved feature of potent anti-HIV human APOBEC3 cytidine deaminases.
PLoS ONE 01/2012; 7(7):e38771. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Human immunodeficiency virus type 1 (HIV-1) requires reverse transcriptase (RT) and HIV-1 nucleocapsid protein (NCp7) for proper viral replication. HIV-1 NCp7 has been shown to enhance various steps in reverse transcription including tRNA initiation and strand transfer, which may be mediated through interactions with RT as well as RNA and DNA oligonucleotides. With the use of DNA oligonucleotides, we have examined the interaction of NCp7 with RT and the kinetics of reverse transcription during (+)-strand synthesis with an NCp7-facilitated annealed primer-template. Through the use of a pre-steady-state kinetics approach, the NCp7-annealed primer-template has a substantial increase (3- to 7-fold) in the rate of incorporation (k(pol)) by RT as compared to heat-annealed primer-template with single-nucleotide incorporation. There was also a 2-fold increase in the binding affinity constant (K(d)) of the nucleotide. These differences in k(pol) and K(d) were not through direct interactions between HIV-1 RT and NCp7. When extension by RT was examined, the data suggest that the NCp7-annealed primer-template facilitates the formation of a longer product more quickly compared to the heat-annealed primer-template. This enhancement in rate is mediated through interactions with NCp7's zinc fingers and N-terminal domain and nucleic acids. The NCp7-annealed primer-template also enhances the fidelity of RT (3-fold) by slowing the rate of incorporation of an incorrect nucleotide. Taken together, this study elucidates a new role of NCp7 by facilitating DNA-directed DNA synthesis during reverse transcription by HIV-1 RT that may translate into enhanced viral fitness and offers an avenue to exploit for targeted therapeutic intervention against HIV.
Journal of Molecular Biology 12/2011; 415(5):866-80. · 3.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The anaphase promoting complex (APC) is a ubiquitin ligase that promotes the degradation of cell-cycle regulators by the 26S proteasome. Cdc20 and Cdh1 are WD40-containing APC co-activators that bind destruction boxes (DB) and KEN boxes within substrates to recruit them to the APC for ubiquitination. Acm1 is an APC(Cdh1) inhibitor that utilizes a DB and a KEN box to bind Cdh1 and prevent substrate binding, although Acm1 itself is not a substrate. We investigated what differentiates an APC substrate from an inhibitor. We identified the Acm1 A-motif that interacts with Cdh1 and together with the DB and KEN box is required for APC(Cdh1) inhibition. A genetic screen identified Cdh1 WD40 domain residues important for Acm1 A-motif interaction and inhibition that appears to reside near Cdh1 residues important for DB recognition. Specific lysine insertion mutations within Acm1 promoted its ubiquitination by APC(Cdh1) whereas lysine removal from the APC substrate Hsl1 converted it into a potent APC(Cdh1) inhibitor. These findings suggest that tight Cdh1 binding combined with the inaccessibility of ubiquitinatable lysines contributes to pseudosubstrate inhibition of APC(Cdh1).
The EMBO Journal 04/2011; 30(9):1818-29. · 9.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The glycine riboswitch regulates gene expression through the cooperative recognition of its amino acid ligand by a tandem pair of aptamers. A 3.6 Å crystal structure of the tandem riboswitch from the glycine permease operon of Fusobacterium nucleatum reveals the glycine binding sites and an extensive network of interactions, largely mediated by asymmetric A-minor contacts, that serve to communicate ligand binding status between the aptamers. These interactions provide a structural basis for how the glycine riboswitch cooperatively regulates gene expression.
[Show abstract][Hide abstract] ABSTRACT: CCA-adding enzymes [ATP(CTP):tRNA nucleotidyltransferases] add CCA onto the 3' end of transfer RNA (tRNA) precursors without using a nucleic acid template. Although the mechanism by which cytosine (C) is selected at position 75 of tRNA has been established, the mechanism by which adenine (A) is selected at position 76 remains elusive. Here, we report five cocrystal structures of the enzyme complexed with both a tRNA mimic and nucleoside triphosphates under catalytically active conditions. These structures suggest that adenosine 5'-monophosphate is incorporated onto the A76 position of the tRNA via a carboxylate-assisted, one-metal-ion mechanism with aspartate 110 functioning as a general base. The discrimination against incorporation of cytidine 5'-triphosphate (CTP) at position 76 arises from improper placement of the α phosphate of the incoming CTP, which results from the interaction of C with arginine 224 and prevents the nucleophilic attack by the 3' hydroxyl group of cytidine75.