[Show abstract][Hide abstract] ABSTRACT: ABSTRACT Clostridium perfringens is an anaerobic bacterium that causes numerous important human and animal diseases, primarily as a result of its ability to produce many different protein toxins. In chickens, C. perfringens causes necrotic enteritis, a disease of economic importance to the worldwide poultry industry. The secreted pore-forming toxin NetB is a key virulence factor in the pathogenesis of avian necrotic enteritis and is similar to alpha-hemolysin, a β-barrel pore-forming toxin from Staphylococcus aureus. To address the molecular mechanisms underlying NetB-mediated tissue damage, we determined the crystal structure of the monomeric form of NetB to 1.8 Å. Structural comparisons with other members of the alpha-hemolysin family revealed significant differences in the conformation of the membrane binding domain. These data suggested that NetB may recognize different membrane receptors or use a different mechanism for membrane-protein interactions. Consistent with this idea, electrophysiological experiments with planar lipid bilayers revealed that NetB formed pores with much larger single-channel conductance than alpha-hemolysin. Channel conductance varied with phospholipid net charge. Furthermore, NetB differed in its ion selectivity, preferring cations over anions. Using hemolysis as a screen, we carried out a random-mutagenesis study that identified several residues that are critical for NetB-induced cell lysis. Mapping of these residues onto the crystal structure revealed that they were clustered in regions predicted to be required for oligomerization or membrane binding. Together these data provide an insight into the mechanism of NetB-mediated pore formation and will contribute to our understanding of the mode of action of this important toxin. IMPORTANCE Necrotic enteritis is an economically important disease of the worldwide poultry industry and is mediated by Clostridium perfringens strains that produce NetB, a β-pore-forming toxin. We carried out structural and functional studies of NetB to provide a mechanistic insight into its mode of action and to assist in the development of a necrotic enteritis vaccine. We determined the structure of the monomeric form of NetB to 1.8 Å, used both site-directed and random mutagenesis to identify key residues that are required for its biological activity, and analyzed pore formation by NetB and its substitution-containing derivatives in planar lipid bilayers.
[Show abstract][Hide abstract] ABSTRACT: The stability of RNAs bearing AU-rich elements in their 3'-UTRs, and thus the level of expression of their protein products, is regulated by interactions with cytoplasmic RNA-binding proteins. Binding by HuR generally leads to mRNA stabilization and increased protein production, whereas binding by AUF1 isoforms generally lead to rapid degradation of the mRNA and reduced protein production. The exact nature of the interplay between these and other RNA-binding proteins remains unclear, although recent studies have shown close interactions between them and even suggested competition between the two for binding to their cognate recognition sequences. Other recent reports have suggested that the sequences recognized by the two proteins are different. We therefore performed a detailed in vitro analysis of the binding site(s) for HuR and AUF1 present in androgen receptor mRNA to define their exact target sequences, and show that the same sequence is contacted by both proteins. Furthermore, we analysed a proposed HuR target within the 3'-UTR of MTA1 mRNA, and show that the contacted bases lie outside of the postulated motif and are a better match to a classical ARE than the postulated motif. The defining features of these HuR binding sites are their U-richness and single strandedness.
Journal of Biochemistry 02/2012; 151(4):423-37. · 3.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Peptidoglycan hydrolases that are specifically associated with bacterial conjugation systems are postulated to facilitate the assembly of the transfer apparatus by creating a temporally and spatially controlled local opening in the peptidoglycan layer. To date little is known about the role of such enzymes in conjugation systems from Gram-positive bacteria. Conjugative plasmids from the Gram-positive pathogen Clostridium perfringens all encode two putative peptidoglycan hydrolases, TcpG and TcpI, within the conserved tcp transfer locus. Mutation and complementation analysis was used to demonstrate that a functional tcpG gene, but not the tcpI gene, was required for efficient conjugative transfer of pCW3. Furthermore, it was also shown that each of the two predicted catalytic domains of TcpG was functional in C. perfringens and that the predicted catalytic site residues, E-111, D-136, and C-238, present within these functional domains were required for optimal TcpG function. Escherichia coli cells producing TcpG demonstrated a distinctive autoagglutination phenotype and partially purified recombinant TcpG protein was shown to have peptidoglycan hydrolase-like activity on cognate peptidoglycan from C. perfringens. Based on these results it is suggested that TcpG is a functional peptidoglycan hydrolase that is required for efficient conjugative transfer of pCW3, presumably by facilitating the penetration of the pCW3 translocation complex through the cell wall.
[Show abstract][Hide abstract] ABSTRACT: Bacterial conjugation is important for the acquisition of virulence and antibiotic resistance genes. We investigated the mechanism of conjugation in Gram-positive pathogens using a model plasmid pCW3 from Clostridium perfringens. pCW3 encodes tetracycline resistance and contains the tcp locus, which is essential for conjugation. We showed that the unique TcpC protein (359 amino acids, 41 kDa) was required for efficient conjugative transfer, localized to the cell membrane independently of other conjugation proteins, and that membrane localization was important for its function, oligomerization and interaction with the conjugation proteins TcpA, TcpH and TcpG. The crystal structure of the C-terminal component of TcpC (TcpC(99-359)) was determined to 1.8-Å resolution. TcpC(99-359) contained two NTF2-like domains separated by a short linker. Unexpectedly, comparative structural analysis showed that each of these domains was structurally homologous to the periplasmic region of VirB8, a component of the type IV secretion system from Agrobacterium tumefaciens. Bacterial two-hybrid studies revealed that the C-terminal domain was critical for interactions with other conjugation proteins. The N-terminal region of TcpC was required for efficient conjugation, oligomerization and protein-protein interactions. We conclude that by forming oligomeric complexes, TcpC contributes to the stability and integrity of the conjugation apparatus, facilitating efficient pCW3 transfer.
[Show abstract][Hide abstract] ABSTRACT: The ovine footrot pathogen, Dichelobacter nodosus, secretes three subtilisin-like proteases that play an important role in the pathogenesis of footrot through their ability to mediate tissue destruction. Virulent and benign strains of D. nodosus secrete the basic proteases BprV and BprB, respectively, with the catalytic domain of these enzymes having 96% sequence identity. At present, it is not known how sequence variation between these two putative virulence factors influences their respective biological activity. We have determined the high resolution crystal structures of BprV and BprB. These data reveal that that the S1 pocket of BprV is more hydrophobic but smaller than that of BprB. We show that BprV is more effective than BprB in degrading extracellular matrix components of the host tissue. Mutation of two residues around the S1 pocket of BprB to the equivalent residues in BprV dramatically enhanced its proteolytic activity against elastin substrates. Application of a novel approach for profiling substrate specificity, the Rapid Endopeptidase Profiling Library (REPLi) method, revealed that both enzymes prefer cleaving after hydrophobic residues (and in particular P1 leucine) but that BprV has more restricted primary substrate specificity than BprB. Furthermore, for P1 Leu-containing substrates we found that BprV is a significantly more efficient enzyme than BprB. Collectively, these data illuminate how subtle changes in D. nodosus proteases may significantly influence tissue destruction as part of the ovine footrot pathogenesis process.
Journal of Biological Chemistry 12/2011; 286(49):42180-7. · 4.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ovine footrot is a contagious and debilitating disease that is of major economic significance to the sheep meat and wool industries. The causative bacterium is the gram negative anaerobe Dichelobacter nodosus. Research that has used a classical molecular genetics approach has led to major advances in our understanding of the role of the key virulence factors of D. nodosus in the disease process. D. nodosus strains produce polar type IV fimbriae and extracellular serine proteases. Mutagenesis of the fimbrial subunit gene fimA and the pilT gene, which is required for fimbrial retraction, and subsequent testing of these mutants in sheep virulence trials has shown that type IV fimbriae-mediated twitching motility is essential for virulence. The extracellular protease genes aprV2, aprV5 and bprV have also been mutated. Analysis of these mutants has shown that ArpV5 is the major extracellular protease and that AprV2 is the thermostable protease that is responsible for the extracellular elastase activity. Structural analysis of AprV2 has revealed that it contains several novel loops, one of which appears to act as an exosite that may modulate substrate accessibility. Finally, virulence experiments in sheep have shown that the AprV2 protease is required for virulence.
[Show abstract][Hide abstract] ABSTRACT: Many bacterial pathogens produce extracellular proteases that degrade the extracellular matrix of the host and therefore are involved in disease pathogenesis. Dichelobacter nodosus is the causative agent of ovine footrot, a highly contagious disease that is characterized by the separation of the hoof from the underlying tissue. D. nodosus secretes three subtilisin-like proteases whose analysis forms the basis of diagnostic tests that differentiate between virulent and benign strains and have been postulated to play a role in virulence. We have constructed protease mutants of D. nodosus; their analysis in a sheep virulence model revealed that one of these enzymes, AprV2, was required for virulence. These studies challenge the previous hypothesis that the elastase activity of AprV2 is important for disease progression, since aprV2 mutants were virulent when complemented with aprB2, which encodes a variant that has impaired elastase activity. We have determined the crystal structures of both AprV2 and AprB2 and characterized the biological activity of these enzymes. These data reveal that an unusual extended disulphide-tethered loop functions as an exosite, mediating effective enzyme-substrate interactions. The disulphide bond and Tyr92, which was located at the exposed end of the loop, were functionally important. Bioinformatic analyses suggested that other pathogenic bacteria may have proteases that utilize a similar mechanism. In conclusion, we have used an integrated multidisciplinary combination of bacterial genetics, whole animal virulence trials in the original host, biochemical studies, and comprehensive analysis of crystal structures to provide the first definitive evidence that the extracellular secreted proteases produced by D. nodosus are required for virulence and to elucidate the molecular mechanism by which these proteases bind to their natural substrates. We postulate that this exosite mechanism may be used by proteases produced by other bacterial pathogens of both humans and animals.
[Show abstract][Hide abstract] ABSTRACT: Dichelobacter nodosus is the principal causative agent of ovine footrot, a disease of significant economic importance to the sheep industry. D. nodosus secretes a number of subtilisin-like serine proteases which mediate tissue damage and presumably contribute to the pathogenesis of footrot. Strains causing virulent footrot secrete the proteases AprV2, AprV5 and BprV and strains causing benign footrot secrete the closely related proteases AprB2, AprB5 and BprB. Here, the cloning, purification and crystallization of AprV2, AprB2, BprV and BprB are reported. Crystals of AprV2 and AprB2 diffracted to 2.0 and 1.7 A resolution, respectively. The crystals of both proteases belonged to space group P1, with unit-cell parameters a = 43.1, b = 46.0, c = 47.2 A, alpha = 97.8, beta = 115.2, gamma = 115.2 degrees for AprV2 and a = 42.7, b = 45.8, c = 45.7 A, alpha = 98.4, beta = 114.0, gamma = 114.6 degrees for AprB2. Crystals of BprV and BprB diffracted to 2.0 and 1.8 A resolution, respectively. The crystals of both proteases belonged to space group P2(1), with unit-cell parameters a = 38.5, b = 89.6, c = 47.7 A, beta = 113.6 degrees for BprV and a = 38.5, b = 90.5, c = 44.1 A, beta = 109.9 degrees for BprB. The crystals of all four proteases contained one molecule in the asymmetric unit, with a solvent content ranging from 36 to 40%.
Acta Crystallographica Section F Structural Biology and Crystallization Communications 03/2010; 66(Pt 3):289-93. · 0.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Plasmodium falciparum parasites are responsible for the major global disease malaria, which results in >2 million deaths each year. With the rise of drug-resistant malarial parasites, novel drug targets and lead compounds are urgently required for the development of new therapeutic strategies. Here, we address this important problem by targeting the malarial neutral aminopeptidases that are involved in the terminal stages of hemoglobin digestion and essential for the provision of amino acids used for parasite growth and development within the erythrocyte. We characterize the structure and substrate specificity of one such aminopeptidase, PfA-M1, a validated drug target. The X-ray crystal structure of PfA-M1 alone and in complex with the generic inhibitor, bestatin, and a phosphinate dipeptide analogue with potent in vitro and in vivo antimalarial activity, hPheP[CH(2)]Phe, reveals features within the protease active site that are critical to its function as an aminopeptidase and can be exploited for drug development. These results set the groundwork for the development of antimalarial therapeutics that target the neutral aminopeptidases of the parasite.
Proceedings of the National Academy of Sciences 02/2009; 106(8):2537-42. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: There is a pressing need for the archiving and curation of raw X-ray diffraction data. This information is critical for validation, methods development and improvement of archived structures. However, the relatively large size of these data sets has presented challenges for storage in a single worldwide repository such as the Protein Data Bank archive. This problem can be avoided by using a federated approach, where each institution utilizes its institutional repository for storage, with a discovery service overlaid. Institutional repositories are relatively stable and adequately funded, ensuring persistence. Here, a simple repository solution is described, utilizing Fedora open-source database software and data-annotation and deposition tools that can be deployed at any site cheaply and easily. Data sets and associated metadata from federated repositories are given a unique and persistent handle, providing a simple mechanism for search and retrieval via web interfaces. In addition to ensuring that valuable data is not lost, the provision of raw data has several uses for the crystallographic community. Most importantly, structure determination can only be truly repeated or verified when the raw data are available. Moreover, the availability of raw data is extremely useful for the development of improved methods of image analysis and data processing.
[Show abstract][Hide abstract] ABSTRACT: Proteins containing membrane attack complex/perforin (MACPF) domains play important roles in vertebrate immunity, embryonic
development, and neural-cell migration. In vertebrates, the ninth component of complement and perforin form oligomeric pores
that lyse bacteria and kill virus-infected cells, respectively. However, the mechanism of MACPF function is unknown. We determined
the crystal structure of a bacterial MACPF protein, Plu-MACPF from Photorhabdus luminescens, to 2.0 angstrom resolution. The MACPF domain reveals structural similarity with poreforming cholesterol-dependent cytolysins
(CDCs) from Gram-positive bacteria. This suggests that lytic MACPF proteins may use a CDC-like mechanism to form pores and
disrupt cell membranes. Sequence similarity between bacterial and vertebrate MACPF domains suggests that the fold of the CDCs,
a family of proteins important for bacterial pathogenesis, is probably used by vertebrates for defense against infection.
[Show abstract][Hide abstract] ABSTRACT: In Bacillus subtilis, the termination of DNA replication via polar fork arrest is effected by a specific protein:DNA complex formed between the replication terminator protein (RTP) and DNA terminator sites. We report the crystal structure of a replication terminator protein homologue (RTP.C110S) of B. subtilis in complex with the high affinity component of one of its cognate DNA termination sites, known as the TerI B-site, refined at 2.5 A resolution. The 21 bp RTP:DNA complex displays marked structural asymmetry in both the homodimeric protein and the DNA. This is in contrast to the previously reported complex formed with a symmetrical TerI B-site homologue. The induced asymmetry is consistent with the complex's solution properties as determined using NMR spectroscopy. Concomitant with this asymmetry is variation in the protein:DNA binding pattern for each of the subunits of the RTP homodimer. It is proposed that the asymmetric "wing" positions, as well as other asymmetrical features of the RTP:DNA complex, are critical for the cooperative binding that underlies the mechanism of polar fork arrest at the complete terminator site.
Journal of Molecular Biology 08/2007; 370(3):481-91. · 3.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Lysins are peptidoglycan hydrolases that are produced by bacteriophage and act to lyse the bacterial host cell wall during progeny phage release. Here, we describe the structure and function of a novel bacteriophage-derived lysin, PlyB, which displays potent lytic activity against the Bacillus anthracis-like strain ATCC 4342. This molecule comprises an N-terminal catalytic domain (PlyB(cat)) and a C-terminal bacterial SH3-like domain, SH3b. It is shown that both domains are required for effective catalytic activity against ATCC 4342. Further, PlyB has specific activity comparable to the phage lysin PlyG, an amidase being developed as a therapeutic against anthrax. In contrast to PlyG, however, the 1.6 A X-ray crystal structure of PlyB(cat) reveals that the catalytic domain adopts the glycosyl hydrolase (GH)-25, rather than phage T7 lysozyme-like fold. PlyB therefore represents a new class of anthrax lysin and a new defensive tool in the armament against anthrax-mediated bioterrorism.
Journal of Molecular Biology 03/2007; 366(2):540-50. · 3.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Human growth factor receptor bound protein 7 (Grb7) is an adapter protein that mediates the coupling of tyrosine kinases with their downstream signaling pathways. Grb7 is frequently overexpressed in invasive and metastatic human cancers and is implicated in cancer progression via its interaction with the ErbB2 receptor and focal adhesion kinase (FAK) that play critical roles in cell proliferation and migration. It is thus a prime target for the development of novel anti-cancer therapies. Recently, an inhibitory peptide (G7-18NATE) has been developed which binds specifically to the Grb7 SH2 domain and is able to attenuate cancer cell proliferation and migration in various cancer cell lines.
As a first step towards understanding how Grb7 may be inhibited by G7-18NATE, we solved the crystal structure of the Grb7 SH2 domain to 2.1 A resolution. We describe the details of the peptide binding site underlying target specificity, as well as the dimer interface of Grb 7 SH2. Dimer formation of Grb7 was determined to be in the muM range using analytical ultracentrifugation for both full-length Grb7 and the SH2 domain alone, suggesting the SH2 domain forms the basis of a physiological dimer. ITC measurements of the interaction of the G7-18NATE peptide with the Grb7 SH2 domain revealed that it binds with a binding affinity of Kd = approximately 35.7 microM and NMR spectroscopy titration experiments revealed that peptide binding causes perturbations to both the ligand binding surface of the Grb7 SH2 domain as well as to the dimer interface, suggesting that dimerisation of Grb7 is impacted on by peptide binding.
Together the data allow us to propose a model of the Grb7 SH2 domain/G7-18NATE interaction and to rationalize the basis for the observed binding specificity and affinity. We propose that the current study will assist with the development of second generation Grb7 SH2 domain inhibitors, potentially leading to novel inhibitors of cancer cell migration and invasion.
[Show abstract][Hide abstract] ABSTRACT: G7-18NATE is a nonphosphorylated, cyclic peptide that specifically inhibits the Grb7 adapter protein implicated in several pathways critical to cell proliferation and migration. It has been shown that G7-18NATE is able to compete with natural ligands for the Grb7 SH2 phosphotyrosine binding site, and to attenuate cell migration in a pancreatic cancer cell line. It is thus an important lead in the development of a selective inhibitor of Grb7 and potential novel anticancer therapeutics. The current study reports the solution properties of G7- 18NATE determined using NMR spectroscopy, in both water (pH 2-3) and phosphate buffer (pH 6.0), with 100 mM NaCl. The spectra reveal that G7-18NATE exists in two distinguishable conformational states on the NMR timescale, most likely due to cis-trans proline isomerization. In addition, the chemical shift data are consistent with a tendency of G7-18NATE to form a turn about the YDN motif, known to be important for binding, and suggest that this turn is stabilized in low salt and low pH conditions. Low NH temperature coefficients of Tyr-5 and Asn-7 amide protons may reflect their involvement in the formation of hydrogen bonds that stabilize such a turn. Overall, however, the peptide does not form a rigid structure, but exists in a highly flexible state in solution. Averaged 3JNH-H coupling constants and a lack of interresidue NOEs are characteristic of such peptide solution behavior. This suggests that there is scope for increasing the rigidity of the peptide that may enhance its binding affinity and specificity for Grb7.
[Show abstract][Hide abstract] ABSTRACT: The replication terminator protein (RTP) of Bacillus subtilis binds to specific DNA sequences that halt the progression of the replisome in a polar manner. These terminator complexes flank a defined region of the chromosome into which they allow replication forks to enter but not exit. Forcing the fusion of replication forks in a specific zone is thought to allow the coordination of post-replicative processes. The functional terminator complex comprises two homodimers each of 29 kDa bound to overlapping binding sites. A preparation of RTP and a 37-base-pair TerI sequence (comprising two binding sites for RTP) has been purified and crystallized. A data set to 3.9 A resolution with 97.0% completeness and an R(sym) of 12% was collected from a single flash-cooled crystal using synchrotron radiation. The diffraction data are consistent with space group P622, with unit-cell parameters a = b = 118.8, c = 142.6 A.
Acta Crystallographica Section F Structural Biology and Crystallization Communications 12/2006; 62(Pt 11):1104-7. · 0.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The alphaCP family of proteins [also known as poly(C)-binding or heterogeneous nuclear ribonucleoprotein E proteins] are involved in the regulation of messenger RNA (mRNA) stability and translational efficiency. They bind via their triple heterologous nuclear ribonucleoprotein K homology (KH) domain structures to C-rich mRNA, and are thought to interact with other mRNA-binding proteins as well as provide direct nuclease protection. In particular, alphaCP1 and alphaCP2 have been shown to bind to a specific region of androgen receptor (AR) mRNA, resulting in its increased stability. The roles of each of the KH motifs in the binding affinity and the specificity is not yet understood. We report the beginning of a systematic study of each of the alphaCP KH domains, with the cloning and expression of alphaCP1-KH2 and alphaCP1-KH3. We report the ability of alphaCP1-KH3, but not alphaCP1-KH2, to bind the target AR mRNA sequence using an RNA electrophoretic mobility gel shift assay. We also report the preparation of an alphaCP1-KH3/AR mRNA complex for structural studies. (1)H-(15)N heteronuclear single quantum correlation NMR spectra of (15)N-labelled alphaCP1-KH3 verified the integrity and good solution behaviour of the purified domain. The titration of the 11-nucleotide RNA target sequence from AR mRNA resulted in a rearrangement of the (1)H-(15)N correlations, demonstrating the complete binding of the protein to form a homogeneous protein/RNA complex suitable for future structural studies.
European Biophysics Journal 08/2005; 34(5):423-9. · 2.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Growth factor receptor bound protein 7 (Grb7) is an adaptor protein that is co-overexpressed and forms a tight complex with the ErbB2 receptor in a number of breast tumours and breast cancer cell lines. The interaction of Grb7 with the ErbB2 receptor is mediated via its Src homology 2 (SH2) domain. Whilst most SH2 domains exist as monomers, recently reported studies have suggested that the Grb7-SH2 domain exists as a homodimer. The self-association properties of the Grb7-SH2 domain were therefore studied using sedimentation equilibrium ultracentrifugation. Analysis of the data demonstrated that the Grb7-SH2 domain is dimeric with a dissociation constant of approximately 11 muM. We also demonstrate, using size-exclusion chromatography, that mutation of phenylalanine 511 to an arginine produces a monomeric form of the Grb7-SH2 domain. This mutation represents the first step in the engineering of a Grb7-SH2 domain with good solution properties for further biophysical and structural investigation.
European Biophysics Journal 08/2005; 34(5):454-60. · 2.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Poly(C)-binding proteins (CPs) are important regulators of mRNA stability and translational regulation. They recognize C-rich RNA through their triple KH (hn RNP K homology) domain structures and are thought to carry out their function though direct protection of mRNA sites as well as through interactions with other RNA-binding proteins. We report the crystallographically derived structure of the third domain of alphaCP1 to 2.1 A resolution. alphaCP1-KH3 assumes a classical type I KH domain fold with a triple-stranded beta-sheet held against a three-helix cluster in a betaalphaalphabetabetaalpha configuration. Its binding affinity to an RNA sequence from the 3'-untranslated region (3'-UTR) of androgen receptor mRNA was determined using surface plasmon resonance, giving a K(d) of 4.37 microM, which is indicative of intermediate binding. A model of alphaCP1-KH3 with poly(C)-RNA was generated by homology to a recently reported RNA-bound KH domain structure and suggests the molecular basis for oligonucleotide binding and poly(C)-RNA specificity.
Nucleic Acids Research 02/2005; 33(4):1213-21. · 8.81 Impact Factor