Crystal structure of the Clostridium limosum C3 exoenzyme

C3-like toxins ADP-ribosylate and inactivate Rho GTPases. Seven C3-like ADP-ribosyltransferases produced by Clostridium botulinum, Clostridium limosum, Bacillus cereus and Staphylococcus aureus were identified and two representatives - C3bot from C. botulinum and C3stau2 from S. aureus - were crystallized. Here we present the 1.8 Å structure of C. limosum C3 transferase C3lim and compare it to the structures of other family members. In contrast to the structure of apo-C3bot, the canonical ADP-ribosylating turn turn motif is observed in a primed conformation, ready for NAD binding. This suggests an impact on the binding mode of NAD and on the transferase reaction. The crystal structure explains why auto-ADP-ribosylation of C3lim at Arg41 interferes with the ADP-ribosyltransferase activity of the toxin.     

Structure-function analysis of the Rho-ADP-ribosylating exoenzyme C3stau2 from Staphylococcus aureus

Exoenzyme C3stau2 from Staphylococcus aureus is a new member of the family of C3-like ADP-ribosyltransferases that ADP-ribosylates RhoA, -B, and -C. Additionally, it modifies RhoE and Rnd3. Here we report on studies of the structure-function relationship of recombinant C3stau2 by site-directed mutagenesis. Exchange of Glu(180) with leucine caused a complete loss of both ADP-ribosyltransferase and NAD glycohydrolase activity. By contrast, exchange of the glutamine residue two positions upstream (Gln(178)) with lysine blocked ADP-ribosyltransferase activity without major changes in NAD glycohydrolase activity. NAD and substrate binding of this mutant protein was comparable to that of the recombinant wild type. Exchange of amino acid Tyr(175), which is part of the recently described "ADP-ribosylating toxin turn-turn" (ARTT) motif [Han, S., Arvai, A. S., Clancy, S. B., and Tainer, J. A. (2001) J. Mol.Biol. 305, 95-107], with alanine, lysine, or threonine caused a loss of or a decrease in ADP-ribosyltransferase activity but an increase in NAD glycohydrolase activity. Recombinant C3stau2 Tyr175Ala and Tyr175Lys were not precipitated by matrix-bound Rho, supporting a role of Tyr(175) in protein substrate recognition. Exchange of Arg(48) and/or Arg(85) resulted in a 100-fold reduced transferase activity, while the recombinant C3stau2 double mutant R48K/R85K was totally inactive. The data indicate that amino acid residues Arg(48), Arg(85), Tyr(175), Gln(178), and Glu(180) are essential for ADP-ribosyltransferase activity of recombinant C3stau2 and support the role of the ARTT motif in substrate recognition of RhoA by C3-like ADP-ribosyltransferases.     

Interaction of the Rho-ADP-ribosylating C3 exoenzyme with RalA

RhoA, -B, and -C are ADP-ribosylated and biologically inactivated by Clostridium botulinum C3 exoenzyme and related C3-like transferases. We report that RalA GTPase, which is not ADP-ribosylated by C3, inhibits ADP-ribosylation of RhoA by C3 from C. botulinum (C3bot), Clostridium limosum (C3lim), and Bacillus cereus (C3cer) but not from Staphylococcus aureus (C3stau) in human platelet membranes and rat brain lysate. Inhibition by RalA occurs with the GDP- and guanosine 5'-3-O-(thio)triphosphate-bound forms of RalA and is overcome by increasing concentrations of C3. A direct interaction of RalA with C3 was verified by precipitation of the transferase with GST-RalA-Sepharose. The affinity constant (K(d)) of the binding of RalA to C3lim was 12 nm as determined by fluorescence titration. RalA increased the NAD glycohydrolase activity of C3bot by about 5-fold. Although RalA had no effect on glucosylation of Rho GTPases by Clostridium difficile toxin B, C3bot and C3lim inhibited glucosylation of RalA by Clostridium sordellii lethal toxin. Furthermore, C3bot decreased activation of phospholipase D by RalA. The data indicate that several C3 exoenzymes directly interact with RalA without ADP-ribosylating the GTPase. The interaction is of high affinity and interferes with essential functions of C3 and RalA.     

Cosubstrate-induced dynamics of D-3-hydroxybutyrate dehydrogenase from Pseudomonas putida

D-3-Hydroxybutyrate dehydrogenase from Pseudomonas putida belongs to the family of short-chain dehydrogenases/reductases. We have determined X-ray structures of the D-3-hydroxybutyrate dehydrogenase from Pseudomonas putida, which was recombinantly expressed in Escherichia coli, in three different crystal forms to resolutions between 1.9 and 2.1 A. The so-called substrate-binding loop (residues 187-210) was partially disordered in several subunits, in both the presence and absence of NAD(+). However, in two subunits, this loop was completely defined in an open conformation in the apoenzyme and in a closed conformation in the complex structure with NAD(+). Structural comparisons indicated that the loop moves as a rigid body by about 46 degrees . However, the two small alpha-helices (alphaFG1 and alphaFG2) of the loop also re-orientated slightly during the conformational change. Probably, the interactions of Val185, Thr187 and Leu189 with the cosubstrate induced the conformational change. A model of the binding mode of the substrate D-3-hydroxybutyrate indicated that the loop in the closed conformation, as a result of NAD(+) binding, is positioned competent for catalysis. Gln193 is the only residue of the substrate-binding loop that interacts directly with the substrate. A translation, libration and screw (TLS) analysis of the rigid body movement of the loop in the crystal showed significant librational displacements, describing the coordinated movement of the substrate-binding loop in the crystal. NAD(+) binding increased the flexibility of the substrate-binding loop and shifted the equilibrium between the open and closed forms towards the closed form. The finding that all NAD(+) -bound subunits are present in the closed form and all NAD(+) -free subunits in the open form indicates that the loop closure is induced by cosubstrate binding alone. This mechanism may contribute to the sequential binding of cosubstrate followed by substrate.     

Crystal structure and novel recognition motif of rho ADP-ribosylating C3 exoenzyme from Clostridium botulinum: structural insights for recognition specificity and catalysis

Clostridium botulinum C3 exoenzyme inactivates the small GTP-binding protein family Rho by ADP-ribosylating asparagine 41, which depolymerizes the actin cytoskeleton. C3 thus represents a major family of the bacterial toxins that transfer the ADP-ribose moiety of NAD to specific amino acids in acceptor proteins to modify key biological activities in eukaryotic cells, including protein synthesis, differentiation, transformation, and intracellular signaling. The 1.7 A resolution C3 exoenzyme structure establishes the conserved features of the core NAD-binding beta-sandwich fold with other ADP-ribosylating toxins despite little sequence conservation. Importantly, the central core of the C3 exoenzyme structure is distinguished by the absence of an active site loop observed in many other ADP-ribosylating toxins. Unlike the ADP-ribosylating toxins that possess the active site loop near the central core, the C3 exoenzyme replaces the active site loop with an alpha-helix, alpha3. Moreover, structural and sequence similarities with the catalytic domain of vegetative insecticidal protein 2 (VIP2), an actin ADP-ribosyltransferase, unexpectedly implicates two adjacent, protruding turns, which join beta5 and beta6 of the toxin core fold, as a novel recognition specificity motif for this newly defined toxin family. Turn 1 evidently positions the solvent-exposed, aromatic side-chain of Phe209 to interact with the hydrophobic region of Rho adjacent to its GTP-binding site. Turn 2 evidently both places the Gln212 side-chain for hydrogen bonding to recognize Rho Asn41 for nucleophilic attack on the anomeric carbon of NAD ribose and holds the key Glu214 catalytic side-chain in the adjacent catalytic pocket. This proposed bipartite ADP-ribosylating toxin turn-turn (ARTT) motif places the VIP2 and C3 toxin classes into a single ARTT family characterized by analogous target protein recognition via turn 1 aromatic and turn 2 hydrogen-bonding side-chain moieties. Turn 2 centrally anchors the catalytic Glu214 within the ARTT motif, and furthermore distinguishes the C3 toxin class by a conserved turn 2 Gln and the VIP2 binary toxin class by a conserved turn 2 Glu for appropriate target side-chain hydrogen-bonding recognition. Taken together, these structural results provide a molecular basis for understanding the coupled activity and recognition specificity for C3 and for the newly defined ARTT toxin family, which acts in the depolymerization of the actin cytoskeleton. This beta5 to beta6 region of the toxin fold represents an experimentally testable and potentially general recognition motif region for other ADP-ribosylating toxins that have a similar beta-structure framework.     

NAD binding induces conformational changes in Rho ADP-ribosylating clostridium botulinum C3 exoenzyme

We have solved the crystal structures of Clostridium botulinum C3 exoenzyme free and complexed to NAD in the same crystal form, at 2.7 and 1.95 A, respectively. The asymmetric unit contains four molecules, which, in the free form, share the same conformation. Upon NAD binding, C3 underwent various conformational changes, whose amplitudes were differentially limited in the four molecules of the crystal unit. A major rearrangement concerns the loop that contains the functionally important ARTT motif (ADP-ribosyltransferase toxin turn-turn). The ARTT loop undergoes an ample swinging motion to adopt a conformation that covers the nicotinamide moiety of NAD. In particular, Gln-212, which belongs to the ARTT motif, flips over from a solvent-exposed environment to a buried conformation in the NAD binding pocket. Mutational experiments showed that Gln-212 is neither involved in NAD binding nor in the NAD-glycohydrolase activity of C3, whereas it plays a critical role in the ADP-ribosyl transfer to the substrate Rho. We observed additional NAD-induced movements, including a crab-claw motion of a subdomain that closes the NAD binding pocket. The data emphasized a remarkable NAD-induced plasticity of the C3 binding pocket and suggest that the NAD-induced ARTT loop conformation may be favored by the C3-NAD complex to bind to the substrate Rho. Our structural observations, together with a number of mutational experiments suggest that the mechanisms of Rho ADP-ribosylation by C3-NAD may be more complex than initially anticipated.     

Crystal structure of the Mycobacterium tuberculosis enoyl-ACP reductase, InhA, in complex with NAD+ and a C16 fatty acyl substrate.

Enoyl-ACP reductases participate in fatty acid biosynthesis by utilizing NADH to reduce the trans double bond between positions C2 and C3 of a fatty acyl chain linked to the acyl carrier protein. The enoyl-ACP reductase from Mycobacterium tuberculosis, known as InhA, is a member of an unusual FAS-II system that prefers longer chain fatty acyl substrates for the purpose of synthesizing mycolic acids, a major component of mycobacterial cell walls. The crystal structure of InhA in complex with NAD+ and a C16 fatty acyl substrate, trans-2-hexadecenoyl-(N-acetylcysteamine)-thioester, reveals that the substrate binds in a general "U-shaped" conformation, with the trans double bond positioned directly adjacent to the nicotinamide ring of NAD+. The side chain of Tyr158 directly interacts with the thioester carbonyl oxygen of the C16 fatty acyl substrate and therefore could help stabilize the enolate intermediate, proposed to form during substrate catalysis. Hydrophobic residues, primarily from the substrate binding loop (residues 196-219), engulf the fatty acyl chain portion of the substrate. The substrate binding loop of InhA is longer than that of other enoyl-ACP reductases and creates a deeper substrate binding crevice, consistent with the ability of InhA to recognize longer chain fatty acyl substrates.     

Substrate binding stabilizes S-adenosylhomocysteine hydrolase in a closed conformation

Comparison of crystal structures of S-adenosylhomocysteine (AdoHcy) hydrolase in the substrate-free, NAD(+) form [Hu, Y., Komoto, J., Huang, Y., Gomi, T., Ogawa, H., Takata, Y., Fujioka, M., and Takusagawa, F. (1999) Biochemistry 38, 8323-8333] and a substrate-bound, NADH form [Turner, M. A., Yuan, C.-S., Borchardt, R. T., Hershfield, M. S., Smith, G. D., and Howell, P. L. (1998) Nat. Struct. Biol. 5, 369-376] indicates large differences in the spatial arrangement of the catalytic and NAD(+) binding domains. The substrate-free, NAD(+) form exists in an "open" form with respect to catalytic and NAD(+) binding domains, whereas the substrate-bound, NADH form exists in a closed form with respect to those domains. To address whether domain closure is induced by substrate binding or its subsequent oxidation, we have measured the rotational dynamics of spectroscopic probes covalently bound to Cys(113) and Cys(421) within the catalytic and carboxyl-terminal domains. An independent domain motion is associated with the catalytic domain prior to substrate binding, suggesting the presence of a flexible hinge element between the catalytic and NAD(+) binding domains. Following binding of substrates (i.e., adenosine or neplanocin A) or a nonsubstrate (i.e., 3'-deoxyadenosine), the independent domain motion associated with the catalytic domain is essentially abolished. Likewise, there is a substantial decrease in the average hydrodynamic volume of the protein that is consistent with a reduction in the overall dimensions of the homotetrameric enzyme following substrate binding and oxidation observed in earlier crystallographic studies. Thus, the catalytic and NAD(+) binding domains are stabilized to form a closed active site through interactions with the substrate prior to substrate oxidation.     

Basis for half-site ligand binding in yeast NAD(+)-specific isocitrate dehydrogenase

Yeast NAD(+)-specific isocitrate dehydrogenase is an allosterically regulated octameric enzyme composed of four heterodimers of a catalytic IDH2 subunit and a regulatory IDH1 subunit. Despite structural predictions that the enzyme would contain eight isocitrate binding sites, four NAD(+) binding sites, and four AMP binding sites, only half of the sites for each ligand can be measured in binding assays. On the basis of a potential interaction between side chains of Cys-150 residues in IDH2 subunits in each tetramer of the enzyme, ligand binding assays of wild-type (IDH1/IDH2) and IDH1/IDH2(C150S) octameric enzymes were conducted in the presence of dithiothreitol. These assays demonstrated the presence of eight isocitrate and four AMP binding sites for the wild-type enzyme in the presence of dithiothreitol and for the IDH1/IDH2(C150S) enzyme in the absence or presence of this reagent, suggesting that interactions between sulfhydryl side chains of IDH2 Cys-150 residues limit access to these sites. However, only two NAD(+) sites could be measured for either enzyme. A tetrameric form of IDH (an IDH1(G15D)/IDH2 mutant enzyme) demonstrated half-site binding for isocitrate (two sites) in the absence of dithiothreitol and full-site binding (four sites) in the presence of dithiothreitol. Only one NAD(+) site could be measured for the tetramer under both conditions. In the context of the structure of the enzyme, these results suggest that an observed asymmetry between heterotetramers in the holoenzyme contributes to interactions between IDH2 Cys-150 residues and to half-site binding of isocitrate, but that a form of negative cooperativity may limit access to apparently equivalent NAD(+) binding sites.     

Structure of the yeast Hst2 protein deacetylase in ternary complex with 2'-O-acetyl ADP ribose and histone peptide

Sir2 proteins are NAD(+)-dependant protein deactylases that have been implicated in playing roles in gene silencing, DNA repair, genome stability, longevity, metabolism, and cell physiology. To define the mechanism of Sir2 activity, we report the 1.5 A crystal structure of the yeast Hst2 (yHst2) Sir2 protein in ternary complex with 2'-O-acetyl ADP ribose and an acetylated histone H4 peptide. The structure captures both ligands meeting within an enclosed tunnel between the small and large domains of the catalytic protein core and permits the assignment of a detailed catalytic mechanism for the Sir2 proteins that is consistent with solution and enzymatic studies. Comparison of the ternary complex with the yHst2/NAD(+) complex, also reported here, and nascent yHst2 structure also reveals that NAD(+) binding accompanies intramolecular loop rearrangement for more stable NAD(+) and acetyl-lysine binding, and that acetyl-lysine peptide binding induces a trimer-monomer protein transition involving nonconserved Sir2 residues.     

From cosubstrate similarity to inhibitor diversity--inhibitors of ADP-ribosyltransferases from kinase inhibitor screening

ADP-ribosyltransferases (ADP-RTs) use NAD(+) to transfer an ADP-ribosyl group to target proteins. Although some ADP-RTs are bacterial toxins only few inhibitors are known. Here we present the development of fluorescence-based assays and a focussed library screening using kinase inhibitors as a new approach towards inhibitors of ADP-RTs. Different screening setups were established using surrogate small molecule substrates or the quantitation of the cofactor NAD(+). Proof-of-principle screening experiments were performed using a kinase inhibitor library in order to target the NAD(+) binding pockets. This led to the discovery of structurally different lead inhibitors for the mono-ADP-ribosyltransferases Mosquitocidal toxin (MTX) from Bacillus sphaericus SSII-1, C3bot toxin from Clostridium botulinum and CDTa from Clostridium difficile. The interaction of the inhibitors with the toxin proteins was analyzed by means of docking and binding free energy calculations. Binding at the nicotinamide subpocket, which shows a significant difference in the three enzymes, is used to explain the selectivity of the identified inhibitors and offers an opportunity for further development of potent and selective inhibitors.     

Functional comparison of the NAD binding cleft of ADP-ribosylating toxins

Although a common core structure forms the active site of ADP-ribosylating (ADPRT) toxins, the limited-sequence homology within this region suggests that different mechanisms are being used by toxins to perform their shared function. To explain differences in their mechanisms of NAD binding and hydrolysis, the functional interrelationship of residues predicted to perform similar functions in the beta3-strand of the NAD binding cleft of different ADPRT toxins was compared. Replacing Tyr54 in the A-subunit of diphtheria toxin (DTA) with a serine, its functional homologue in cholera toxin (CT), resulted in the loss of catalytic function but not NAD binding. The catalytic role of the aromatic portion of Tyr54 in the ADPRT reaction was confirmed by the ability of a Tyr54-to-phenylalanine DTA mutant to retain ADPRT activity. In reciprocal studies, positioning a tyrosine in the beta3-strand of the A1-subunit of CT (CTA1) caused both loss of function and altered structure. The restricted flexibility of the CTA1 active site relative to function became evident upon the loss of ADPRT activity when a conservative Val60-to-leucine mutation was performed. We conclude from our studies that DT and CT maintain a similar mechanism of NAD binding but differ in their mechanisms of NAD hydrolysis. The aromatic moiety at position 54 in DT is integral to NAD hydrolysis, while NAD hydrolysis in CT appears highly dependent on the precise positioning of specific residues within the beta3-strand of the active-site cleft.     

Crystal structure of the RNA 2'-phosphotransferase from Aeropyrum pernix K1

In the final step of tRNA splicing, the 2'-phosphotransferase catalyzes the transfer of the extra 2'-phosphate from the precursor-ligated tRNA to NAD. We have determined the crystal structure of the 2'-phosphotransferase protein from Aeropyrum pernix K1 at 2.8 Angstroms resolution. The structure of the 2'-phosphotransferase contains two globular domains (N and C-domains), which form a cleft in the center. The N-domain has the winged helix motif, a subfamily of the helix-turn-helix family, which is shared by many DNA-binding proteins. The C-domain of the 2'-phosphotransferase superimposes well on the NAD-binding fold of bacterial (diphtheria) toxins, which catalyze the transfer of ADP ribose from NAD to target proteins, indicating that the mode of NAD binding by the 2'-phosphotransferase could be similar to that of the bacterial toxins. The conserved basic residues are assembled at the periphery of the cleft and could participate in the enzyme contact with the sugar-phosphate backbones of tRNA. The modes by which the two functional domains recognize the two different substrates are clarified by the present crystal structure of the 2'-phosphotransferase.     

Computer modelling of the NAD binding site of ADP-ribosylating toxins: active-site structure and mechanism of NAD binding

Five ADP-ribosylating bacterial toxins, pertussis toxin, cholera toxin, diphtheria toxin, Escherichia LT toxin and Pseudomonas exotoxin A, show significant homology in selected segments of their sequence. Site-directed mutagenesis and chemical modification of residues within these regions cause loss of catalytic activity and of NAD binding. On the basis of these results and of molecular modelling based on the three-dimensional structure of exotoxin A, the geometry of an NAD binding site common to all the toxins is deduced and described in the paper. For diphtheria toxin, sequence similarity with exotoxin A is such that its preliminary structure can be computed by molecular modelling, whereas for the other toxins similarity appears to be restricted to the NAD binding site. Moreover, an analysis of molecular fitting of the NAD molecule into its binding cavity suggests a new model for the conformation of the bound NAD that better accounts for all available experimental information.     

The catalytic reaction and inhibition mechanism of Drosophila alcohol dehydrogenase: observation of an enzyme-bound NAD-ketone adduct at 1.4 A resolution by X-ray crystallography.

Drosophila alcohol dehydrogenase (DADH) is an NAD+-dependent enzyme that catalyzes the oxidation of alcohols to aldehydes/ketones. DADH is the member of the short-chain dehydrogenases/reductases family (SDR) for which the largest amount of biochemical data has been gathered during the last three decades. The crystal structures of one binary form (NAD+) and three ternary complexes with NAD+.acetone, NAD+.3-pentanone and NAD+.cyclohexanone were solved at 2.4, 2.2, 1. 4 and 1.6 A resolution, respectively. From the molecular interactions observed, the reaction mechanism could be inferred. The structure of DADH undergoes a conformational change in order to bind the coenzyme. Furthermore, upon binding of the ketone, a region that was disordered in the apo form (186-191) gets stabilized and closes the active site cavity by creating either a small helix (NAD+. acetone, NAD+.3-pentanone) or an ordered loop (NAD+.cyclohexanone). The active site pocket comprises a hydrophobic bifurcated cavity which explains why the enzyme is more efficient in oxidizing secondary aliphatic alcohols (preferably R form) than primary ones. Difference Fourier maps showed that the ketone inhibitor molecule has undergone a covalent reaction with the coenzyme in all three ternary complexes. Due to the presence of the positively charged ring of the coenzyme (NAD+) and the residue Lys155, the amino acid Tyr151 is in its deprotonated (tyrosinate) state at physiological pH. Tyr151 can subtract a proton from the enolic form of the ketone and catalyze a nucleophilic attack of the Calphaatom to the C4 position of the coenzyme creating an NAD-ketone adduct. The binding of these NAD-ketone adducts to DADH accounts for the inactivation of the enzyme. The catalytic reaction proceeds in a similar way, involving the same amino acids as in the formation of the NAD-ketone adduct. The p Kavalue of 9-9.5 obtained by kinetic measurements on apo DADH can be assigned to a protonated Tyr151 which is converted to an unprotonated tyrosinate (p Ka7.6) by the influence of the positively charged nicotinamide ring in the binary enzyme-NAD+form. pH independence during the release of NADH from the binary complex enzyme-NADH can be explained by either a lack of electrostatic interaction between the coenzyme and Tyr151 or an apparent p Kavalue for this residue higher than 10.0.     

Structure of variant 2 scorpion toxin from Centruroides sculpturatus Ewing

Centruroides sculpturatus Ewing variant 2 toxin (CsE-v2) is a neurotoxin isolated from the venom of a scorpion native to the Arizona desert. The structure of CsE-v2 was solved in two different crystal forms using a combination of molecular replacement and multiple isomorphous replacement techniques. Crystals of CsE-v2 display a temperature-dependent, reversible-phase transition near room temperature. At lower temperature the space group changes from P3(2)21 to P3(1)21 with an approximate doubling of the C-axis. The small-cell structure, which has one molecule per asymmetric unit, has an R factor of 0.229 at 2.8 A resolution. The large-cell structure has two molecules per asymmetric unit and was refined at 2.2 A resolution to an R factor of 0.255. CsE-v2 is a rigid, compact structure with four intrachain disulfide bonds. The structure is similar to other long-chain beta neurotoxins, and the largest differences occur in the last six residues. The high-resolution structure of CsE-v2 corrects an error in the reported C-terminal sequence; the terminal tripeptide sequence is Ser 64-Cys 65-Ser 66 rather than Ser 64-Ser 65-Cys 66. Comparison of CsE-v2 with long-chain alpha toxins reveals four insertions and one deletion, as well as additional residues at the N and C termini. Structural alignment of alpha and beta toxins suggests that the primary distinguishing feature between the two classes is the length of the loop between the second and third strands in a three-strand beta sheet. The shorter loop in alpha toxins exposes a critical lysine side chain, whereas the longer loop in beta toxins buries the corresponding basic residue (either arginine or lysine).     

A Recombinant Fusion Toxin Based on Enzymatic Inactive C3bot1 Selectively Targets Macrophages

Background

The C3bot1 protein (∼23 kDa) from Clostridium botulinum ADP-ribosylates and thereby inactivates Rho. C3bot1 is selectively taken up into the cytosol of monocytes/macrophages but not of other cell types such as epithelial cells or fibroblasts. Most likely, the internalization occurs by a specific endocytotic pathway via acidified endosomes.

Methodology/Principal Findings

Here, we tested whether enzymatic inactive C3bot1E174Q serves as a macrophage-selective transport system for delivery of enzymatic active proteins into the cytosol of such cells. Having confirmed that C3bot1E174Q does not induce macrophage activation, we used the actin ADP-ribosylating C2I (∼50 kDa) from Clostridium botulinum as a reporter enzyme for C3bot1E174Q-mediated delivery into macrophages. The recombinant C3bot1E174Q-C2I fusion toxin was cloned and expressed as GST-protein in Escherichia coli. Purified C3bot1E174Q-C2I was recognized by antibodies against C2I and C3bot and showed C2I-specific enzyme activity in vitro. When applied to cultured cells C3bot1E174Q-C2I ADP-ribosylated actin in the cytosol of macrophages including J774A.1 and RAW264.7 cell lines as well as primary cultured human macrophages but not of epithelial cells. Together with confocal fluorescence microscopy experiments, the biochemical data indicate the selective uptake of a recombinant C3-fusion toxin into the cytosol of macrophages.

Conclusions/Significance

In summary, we demonstrated that C3bot1E174Q can be used as a delivery system for fast, selective and specific transport of enzymes into the cytosol of living macrophages. Therefore, C3-based fusion toxins can represent valuable molecular tools in experimental macrophage pharmacology and cell biology as well as attractive candidates to develop new therapeutic approaches against macrophage-associated diseases.     

Structure of Toxoplasma gondii LDH1: active-site differences from human lactate dehydrogenases and the structural basis for efficient APAD+ use

While within a human host the opportunistic pathogen Toxoplasma gondii relies heavily on glycolysis for its energy needs. Lactate dehydrogenase (LDH), the terminal enzyme in anaerobic glycolysis necessary for NAD(+) regeneration, therefore represents an attractive therapeutic target. The tachyzoite stage lactate dehydrogenase (LDH1) from the parasite T. gondii has been crystallized in apo form and in ternary complexes containing NAD(+) or the NAD(+)-analogue 3-acetylpyridine adenine dinucleotide (APAD(+)) and sulfate or the inhibitor oxalate. Comparison of the apo and ternary models shows an active-site loop that becomes ordered upon substrate binding. This active-site loop is five residues longer than in most LDHs and necessarily adopts a different conformation. While loop isomerization is fully rate-limiting in prototypical LDHs, kinetic data suggest that LDH1's rate is limited by chemical steps. The importance of charge neutralization in ligand binding is supported by the complexes that have been crystallized as well as fluorescence quenching experiments performed with ligands at low and high pH. A methionine that replaces a serine residue and displaces an ordered water molecule often seen in LDH structures provides a structural explanation for reduced substrate inhibition. Superimposition of LDH1 with human muscle- and heart-specific LDH isoforms reveals differences in residues that line the active site that increase LDH1's hydrophobicity. These differences will aid in designing inhibitors specific for LDH1 that may be useful in treating toxoplasmic encephalitis and other complications that arise in immune-compromised individuals.     

Structural basis for the NAD-hydrolysis mechanism and the ARTT-loop plasticity of C3 exoenzymes

C3-like exoenzymes are ADP-ribosyltransferases that specifically modify some Rho GTPase proteins, leading to their sequestration in the cytoplasm, and thus inhibiting their regulatory activity on the actin cytoskeleton. This modification process goes through three sequential steps involving NAD-hydrolysis, Rho recognition, and binding, leading to Rho ADP-ribosylation. Independently, three distinct residues within the ARTT loop of the C3 exoenzymes are critical for each of these steps. Supporting the critical role of the ARTT loop, we have shown previously that it adopts a distinct conformation upon NAD binding. Here, we present seven wild-type and ARTT loop-mutant structures of C3 exoenzyme of Clostridium botulinum free and bound to its true substrate, NAD, and to its NAD-hydrolysis product, nicotinamide. Altogether, these structures expand our understanding of the conformational diversity of the C3 exoenzyme, mainly within the ARTT loop.     

Structure of the extracellular portion of CD46 provides insights into its interactions with complement proteins and pathogens

The human membrane cofactor protein (MCP, CD46) is a central component of the innate immune system. CD46 protects autologous cells from complement attack by binding to complement proteins C3b and C4b and serving as a cofactor for their cleavage. Recent data show that CD46 also plays a role in mediating acquired immune responses, and in triggering autophagy. In addition to these physiologic functions, a significant number of pathogens, including select adenoviruses, measles virus, human herpes virus 6 (HHV-6), Streptococci, and Neisseria, use CD46 as a cell attachment receptor. We have determined the crystal structure of the extracellular region of CD46 in complex with the human adenovirus type 11 fiber knob. Extracellular CD46 comprises four short consensus repeats (SCR1-SCR4) that form an elongated structure resembling a hockey stick, with a long shaft and a short blade. Domains SCR1, SCR2 and SCR3 are arranged in a nearly linear fashion. Unexpectedly, however, the structure reveals a profound bend between domains SCR3 and SCR4, which has implications for the interactions with ligands as well as the orientation of the protein at the cell surface. This bend can be attributed to an insertion of five hydrophobic residues in a SCR3 surface loop. Residues in this loop have been implicated in interactions with complement, indicating that the bend participates in binding to C3b and C4b. The structure provides an accurate framework for mapping all known ligand binding sites onto the surface of CD46, thereby advancing an understanding of how CD46 acts as a receptor for pathogens and physiologic ligands of the immune system.