Crystal structure of TRAIL-DR5 complex identifies a critical role of the unique frame insertion in conferring recognition specificity

TRAIL is a cytokine that induces apoptosis in a wide variety of tumor cells but rarely in normal cells. It contains an extraordinarily elongated loop because of an unique insertion of 12-16 amino acids compared with the other members of tumor necrosis factor family. Biological implication of the frame insertion has not been clarified. We have determined the crystal structure of TRAIL in a complex with the extracellular domain of death receptor DR5 at 2.2 A resolution. The structure reveals extensive contacts between the elongated loop and DR5 in an interaction mode that would not be allowed without the frame insertion. These interactions are missing in the structures of the complex determined by others recently. This observation, along with structure-inspired deletion analysis, identifies the critical role of the frame insertion as a molecular strategy conferring specificity upon the recognition of cognate receptors. The structure also suggests that a built-in flexibility of the tumor necrosis factor receptor family members is likely to play a general and important role in the binding and recognition of tumor necrosis factor family members.     

Structure of human tryptophanyl-tRNA synthetase in complex with tRNATrp reveals the molecular basis of tRNA recognition and specificity

Aminoacyl-tRNA synthetases (aaRSs) are a family of enzymes responsible for the covalent link of amino acids to their cognate tRNAs. The selectivity and species-specificity in the recognitions of both amino acid and tRNA by aaRSs play a vital role in maintaining the fidelity of protein synthesis. We report here the first crystal structure of human tryptophanyl-tRNA synthetase (hTrpRS) in complex with tRNA^Trp and Trp which, together with biochemical data, reveals the molecular basis of a novel tRNA binding and recognition mechanism. hTrpRS recognizes the tRNA acceptor arm from the major groove; however, the 3′ end CCA of the tRNA makes a sharp turn to bind at the active site with a deformed conformation. The discriminator base A73 is specifically recognized by an α-helix of the unique N-terminal domain and the anticodon loop by an α-helix insertion of the C-terminal domain. The N-terminal domain appears to be involved in Trp activation, but not essential for tRNA binding and acylation. Structural and sequence comparisons suggest that this novel tRNA binding and recognition mechanism is very likely shared by other archaeal and eukaryotic TrpRSs, but not by bacterial TrpRSs. Our findings provide insights into the molecular basis of tRNA specificity and species-specificity.     

The ternary complex of antithrombin-anhydrothrombin-heparin reveals the basis of inhibitor specificity

Antithrombin, the principal physiological inhibitor of the blood coagulation proteinase thrombin, requires heparin as a cofactor. We report the crystal structure of the rate-determining encounter complex formed between antithrombin, anhydrothrombin and an optimal synthetic 16-mer oligosaccharide. The antithrombin reactive center loop projects from the serpin body and adopts a canonical conformation that makes extensive backbone and side chain contacts from P5 to P6' with thrombin's restrictive specificity pockets, including residues in the 60-loop. These contacts rationalize many earlier mutagenesis studies on thrombin specificity. The 16-mer oligosaccharide is just long enough to form the predicted bridge between the high-affinity pentasaccharide-binding site on antithrombin and the highly basic exosite 2 on thrombin, validating the design strategy for this synthetic heparin. The protein-protein and protein-oligosaccharide interactions together explain the basis for heparin activation of antithrombin as a thrombin inhibitor.     

Structure of the eukaryotic initiation factor (eIF) 5 reveals a fold common to several translation factors

Eukaryotic initiation factor 5 (eIF5) plays multiple roles in translation initiation. Its N-terminal domain functions as a GTPase-activator protein (GAP) for GTP bound to eIF2, while its C-terminal region nucleates the interactions between multiple translation factors, including eIF1, which acts to inhibit GTP hydrolysis or P(i) release, and the beta subunit of eIF2. These proteins and the events in which they participate are critical for the accurate recognition of the correct start codon during translation initiation. Here, we report the three-dimensional solution structure of the N-terminal domain of human eIF5, comprising two subdomains, both reminiscent of nucleic-acid-binding modules. The N-terminal subdomain contains the "arginine finger" motif that is essential for GAP function but which, unusually, resides in a partially disordered region of the molecule. This implies that a conformational reordering of this portion of eIF5 is likely to occur upon formation of a competent complex for GTP hydrolysis, following the appropriate activation signal. Interestingly, the N-terminal subdomain of eIF5 reveals an alpha/beta fold structurally similar to both the archaeal orthologue of the beta subunit of eIF2 and, unexpectedly, to eIF1. These results reveal a novel protein fold common to several factors involved in related steps of translation initiation. The implications of these observations are discussed in terms of the mechanism of translation initiation.     

Function of eukaryotic initiation factor 5 in the formation of an 80 S ribosomal polypeptide chain initiation complex.

Eukaryotic initiation factor 5 (eIF-5), isolated from rabbit reticulocyte lysates, is a monomeric protein of 58-62 kDa. The function of eIF-5 in the formation of an 80 S polypeptide chain initiation complex from a 40 S initiation complex has been investigated. Incubation of the isolated 40 S initiation complex (40 S.AUG.Met.tRNAf.eIF-2 GTP) with eIF-5 resulted in the rapid and quantitative hydrolysis of GTP bound to the 40 S initiation complex. The rate of this reaction was unaffected by the presence of 60 S ribosomal subunits. Analysis of eIF-5-catalyzed reaction products by gel filtration indicated that both eIF-2.GDP binary complex and Pi formed were released from the ribosomal complex whereas Met-tRNAf remained bound to 40 S ribosomes as a Met-tRNAf.40 S.AUG complex. Reactions carried out with biologically active 32P-labeled eIF-5 indicated that this protein was not associated with the 40 S.AUG.Met-tRNAf complex; similar results were obtained by immunological methods using monospecific anti-eIF-5 antibodies. The isolated 40 S.AUG.Met-RNAf complex, free of eIF-2.GDP binary complex and eIF-5, readily interacted with 60 S ribosomal subunits in the absence of exogenously added eIF-5 to form the 80 S initiation complex capable of transferring Met-tRNAf into peptide linkages. These results indicate that the sole function of eIF-5 in the initiation of protein synthesis is to mediate hydrolysis of GTP bound to the 40 S initiation complex in the absence of 60 S ribosomal subunits. This leads to formation of the intermediate 40 S.AUG.Met-tRNAf and dissociation of the eIF-2.GDP binary complex. Subsequent joining of 60 S ribosomal subunits to the intermediate 40 S.AUG.Met-tRNAf complex does not require participation of eIF-5. Thus, the formation of an 80 S ribosomal polypeptide chain initiation complex from a 40 S ribosomal initiation complex can be summarized by the following sequence of partial reactions. (40 S.AUG.Met-tRNAf.eIF-2.GTP) eIF-5----(40 S.AUG.Met-tRNAf) + (eIF-2.GDP) + Pi (1) (40 S.AUG.Met-tRNAf) + 60 S----(80 S.AUG.Met-tRNAf) (2) 80 S initiation complex.     

Ubiquinone-binding site mutagenesis reveals the role of mitochondrial complex II in cell death initiation

Respiratory complex II (CII, succinate dehydrogenase, SDH) inhibition can induce cell death, but the mechanistic details need clarification. To elucidate the role of reactive oxygen species (ROS) formation upon the ubiquinone-binding (Q_p) site blockade, we substituted CII subunit C (SDHC) residues lining the Q_p site by site-directed mutagenesis. Cell lines carrying these mutations were characterized on the bases of CII activity and exposed to Q_p site inhibitors MitoVES, thenoyltrifluoroacetone (TTFA) and Atpenin A5. We found that I56F and S68A SDHC variants, which support succinate-mediated respiration and maintain low intracellular succinate, were less efficiently inhibited by MitoVES than the wild-type (WT) variant. Importantly, associated ROS generation and cell death induction was also impaired, and cell death in the WT cells was malonate and catalase sensitive. In contrast, the S68A variant was much more susceptible to TTFA inhibition than the I56F variant or the WT CII, which was again reflected by enhanced ROS formation and increased malonate- and catalase-sensitive cell death induction. The R72C variant that accumulates intracellular succinate due to compromised CII activity was resistant to MitoVES and TTFA treatment and did not increase ROS, even though TTFA efficiently generated ROS at low succinate in mitochondria isolated from R72C cells. Similarly, the high-affinity Q_p site inhibitor Atpenin A5 rapidly increased intracellular succinate in WT cells but did not induce ROS or cell death, unlike MitoVES and TTFA that upregulated succinate only moderately. These results demonstrate that cell death initiation upon CII inhibition depends on ROS and that the extent of cell death correlates with the potency of inhibition at the Q_p site unless intracellular succinate is high. In addition, this validates the Q_p site of CII as a target for cell death induction with relevance to cancer therapy.Mitochondrial respiratory complex II (CII), aka succinate dehydrogenase (SDH), directly links the tricarboxylic acid (TCA) cycle to the electron transport chain (ETC) by mediating electron transfer from the TCA cycle metabolite succinate to ubiquinone (UbQ).¹ For this reason, CII is subjected to a high electron flux between the succinate-binding dicarboxylate site in the matrix-exposed subunit A and the proximal UbQ-binding (Q_p) site, formed by the subunits C (SDHC) and D embedded in the mitochondrial inner membrane (Figure 1b).^{2, 3, 4, 5} Disruption of electron transfer to UbQ, for example by Q_p site inhibition, leads to reactive oxygen species (ROS) generation from CII due to the leakage of ‘stalled'' electrons to molecular oxygen at the reduced flavin adenine dinucleotide (FAD) prosthetic group. However, ROS production from reduced FAD is only possible when the adjacent dicarboxylate site is neither occupied by its substrate succinate, typically at low succinate conditions, nor inhibited by other dicarboxylates, for example by malonate.^{6, 7, 8, 9, 10}Open in a separate windowFigure 1Amino-acid substitutions in the Q_p site of CII. (a) Multiple species alignment of the SDHC region bordering the Q_p site shows a high level of conservation. Amino-acid substitutions prepared for this study are indicated in human SDHC. (b) Three dimensional representation of CII and the topology of the Q_p site. SDHC residues mutated in this study are indicated by arrows. Displayed is the humanized crystal structure of porcine CII.³ (c) A snapshot from molecular dynamics simulation of MitoVES interaction with the Q_p site of CII in the presence of phospholipid bilayer.¹⁶ One of the possible conformations of MitoVES is shown in orange, substituted SDHC residues are depicted in magentaBeyond bioenergetics, CII has emerged as an important factor in cell death induction.^{11, 12} On one hand, it has been proposed that increased ROS production from CII, resulting from changes in matrix pH and calcium status, amplifies cell death signals originating at other sites.^{12, 13, 14, 15} On the other hand, the inhibition of CII may also directly initiate cell death, as suggested by our previous results with vitamin E (VE) analogs such as the mitochondrially targeted VE succinate (MitoVES). This compound inhibits CII activity leading to ROS generation and cell death induction in cancer cells, as evidenced by the suppression of tumor growth in experimental animal models.^{16, 17, 18, 19, 20} The efficacy of MitoVES is greatly reduced in the absence of functional CII, and computer modeling along with other corroborative evidence suggests that MitoVES binds to the Q_p site of CII.¹⁶ However, this is only circumstantial evidence with respect to cell death induction, as cells lacking electron flux within CII due to a structural defect should not be able to produce CII-derived ROS. Accordingly, not only the direct cell death initiation upon CII inhibition will be compromised in this situation, but also the indirect signal amplification mentioned above will be affected.In the present study, we combined site-directed mutagenesis of Q_p site amino-acid residues with the use of Q_p site inhibitors MitoVES, thenoyltrifluoroacetone (TTFA) and Atpenin A5 to assess the link between Q_p site inhibition and cell death initiation. We show that for MitoVES and TTFA, the potency of Q_p site inhibition correlates with the extent of ROS production and cell death induction in respiration-competent CII variants, and that the induced cell death is dependent on CII-derived ROS.Atpenin, however, did not induce cell death, possibly due to the rapid accumulation of succinate in intact cells, incompatible with ROS generation from CII. These results provide evidence for the role of CII in cell death initiation and establish the Q_p site as a target for cell death induction.     

Structure and thermodynamic characterization of the EphB4/Ephrin-B2 antagonist peptide complex reveals the determinants for receptor specificity

The Eph receptor tyrosine kinases and their ligands, the ephrins, regulate numerous biological processes in developing and adult tissues and have been implicated in cancer progression and in pathological forms of angiogenesis. We report the crystal structure of the EphB4 receptor in complex with a highly specific antagonistic peptide at a resolution of 1.65 angstroms. The peptide is situated in a hydrophobic cleft of EphB4 corresponding to the cleft in EphB2 occupied by the ephrin-B2 G-H loop, consistent with its antagonistic properties. Structural analysis identifies several residues within the EphB4 binding cleft that likely determine the ligand specificity of this receptor, while isothermal titration calorimetry experiments with truncated forms of the peptide define the amino acid residues of the peptide that are critical for receptor binding. These studies reveal structural features that will aid drug discovery initiatives to develop EphB4 antagonists for therapeutic applications.     

Molecular mechanisms of apoptosis. Structure of cytochrome c-cardiolipin complex

One of the functions of cytochrome c in living cells is the initiation of apoptosis by catalyzing lipid peroxidation in the inner mitochondrial membrane, which involves cytochrome c bound with acidic lipids, especially cardiolipin. In this paper the results of studies of cytochrome c-cardiolipin complex structure carried out by different authors mainly on unilamellar cardiolipin-containing phospholipid liposomes are critically analyzed. The principal conclusion from the published papers is that cytochrome c-cardiolipin complex is formed by attachment of a cytochrome c molecule to the membrane surface via electrostatic interactions and the subsequent penetration of one of the fatty-acid cardiolipin chains into the protein globule, this being associated with hydrophobic interactions that break the >Fe…S(Met80) coordinate bond and giving rise to appearance of cytochrome c peroxidase activity. Nevertheless, according to data obtained in our laboratory, cytochrome c and cardiolipin form spherical nanoparticles in which protein is surrounded by a monolayer of cardiolipin molecules. Under the action of cooperative forces, the protein in the globule expands greatly in volume, its conformation is modified, and the protein becomes a peroxidase. In extended membranes, such as giant monolayer liposomes, and very likely in biological membranes, the formation of nanospheres of cytochrome c-cardiolipin complex causes fusion of membrane sections and dramatic chaotization of the whole membrane structure. The subsequent disintegration of the outer mitochondrial membrane is accompanied by cytochrome c release from the mitochondria and triggering of a cascade of programmed cell death reactions.     

Involvement of phage phi29 DNA polymerase and terminal protein subdomains in conferring specificity during initiation of protein-primed DNA replication

To initiate ϕ29 DNA replication, the DNA polymerase has to form a complex with the homologous primer terminal protein (TP) that further recognizes the replication origins of the homologous TP-DNA placed at both ends of the linear genome. By means of chimerical proteins, constructed by swapping the priming domain of the related ϕ29 and GA-1 TPs, we show that DNA polymerase can form catalytically active heterodimers exclusively with that chimerical TP containing the N-terminal part of the homologous TP, suggesting that the interaction between the polymerase TPR-1 subdomain and the TP N-terminal part is the one mainly responsible for the specificity between both proteins. We also show that the TP N-terminal part assists the proper binding of the priming domain at the polymerase active site. Additionally, a chimerical ϕ29 DNA polymerase containing the GA-1 TPR-1 subdomain could use GA-1 TP, but only in the presence of ϕ29 TP-DNA as template, indicating that parental TP recognition is mainly accomplished by the DNA polymerase. The sequential events occurring during initiation of bacteriophage protein-primed DNA replication are proposed.     

Functional identification of nucleotides conferring substrate specificity to retroviral integrase reactions.

The long terminal repeats (LTRs) that flank the retroviral DNA genome play a distinct role in the integration process by acting as specific substrates for the integrase (IN). The role of LTR sequences in providing substrate recognition and specificity to integration reactions was investigated for INs from human immunodeficiency virus type 1 (HIV-1), Moloney murine leukemia virus (M-MuLV), human T-cell leukemia virus type 1 (HTLV-1), and human T-cell leukemia virus type 2 (HTLV-2). Overall, these INs required specific LTR sequences for optimal catalysis of 3'-processing reactions, as opposed to strand transfer and disintegration reactions. It is of particular note that in strand transfer reactions the sites of integration were similar among the four INs. In the 3'-processing reaction, sequence specificity for each IN was traced to the three nucleotides proximal to the conserved CA. Reactions catalyzed by M-MuLV IN were additionally influenced by upstream regions. The nucleotide requirements for optimal catalysis differed for each IN. HIV-1 IN showed a broad range of substrate specificities, while HTLV-1 IN and HTLV-2 IN had more defined sequence requirements. M-MuLV IN exhibited greater activity with the heterologous LTR substrates than with its own wild-type substrate. This finding was further substantiated by the high levels of activity catalyzed by the IN on modified M-MuLV LTRs. This work suggests that unlike the other INs examined, M-MuLV IN has evolved with an IN-LTR interaction that is suboptimal.     

Structure of the antithrombin-thrombin-heparin ternary complex reveals the antithrombotic mechanism of heparin

The maintenance of normal blood flow depends completely on the inhibition of thrombin by antithrombin, a member of the serpin family. Antithrombin circulates at a high concentration, but only becomes capable of efficient thrombin inhibition on interaction with heparin or related glycosaminoglycans. The anticoagulant properties of therapeutic heparin are mediated by its interaction with antithrombin, although the structural basis for this interaction is unclear. Here we present the crystal structure at a resolution of 2.5 A of the ternary complex between antithrombin, thrombin and a heparin mimetic (SR123781). The structure reveals a template mechanism with antithrombin and thrombin bound to the same heparin chain. A notably close contact interface, comprised of extensive active site and exosite interactions, explains, in molecular detail, the basis of the antithrombotic properties of therapeutic heparin.     

A proposed role for IF-3 and EF-T in maintaining the specificity of prokaryotic initiation complex formation

Initiation factor-free 30S subunits of E. coli ribosomes bind aminoacyl-tRNAs more efficiently than fMet-tRNA _inff ^supMet . Elongator-tRNA binding was unaffected by IF-1 or IF-2 but was inhibited by IF-3. Their combination reduced this binding up to 40% and stimulated that of fMet-tRNA _inff ^supMet . Unexpectedly, EF-T also prevented elongator-tRNA binding by complexing both to the 30S and to the aminoacyl-tRNAs. Using AUGU₃ as mRNA, elongator-tRNAs competed with fMet-fRNA _inff ^supMet and with tRNA _inff ^supMet . fMet-tRNA _inff ^supMet reacted with puromycin after addition of 50S subunits suggesting that it occupied the P site. EF-T directed binding of phe-tRNA to the 30S.AUGU₃ complex at the A site only if fMet-tRNA _inff ^supMet or tRNA _inff ^supMet filled the P/E site. We propose that one function of EF-T may be to prevent the entry of aminoacyl-tRNAs into the 30S particle during initiation. The possibility that a special site for fMet-tRNA resides on 16S rRNA is also discussed.     

The mechanisms and kinetics of initiation of blood coagulation by the extrinsic tenase complex

Biophysics - The system of hemostasis includes coagulation of blood plasma and formation of platelet aggregate. Plasma clotting is a cascade of proteolytic reactions, triggered by the contact of...     

Structure and reaction mechanisms of multifunctional mitochondrial cytochrome bc1 complex.

The cytochrome bc1 complex from bovine heart mitochondria is a multi-functional enzyme complex. In addition to electron and proton transfer activity, the complex also processes an activatable peptidase activity and a superoxide generating activity. The crystal structure of the complex exists as a closely interacting functional dimer. There are 13 transmembrane helices in each monomer, eight of which belong to cytochrome b, and five of which belong to cytochrome c1, Rieske iron-sulfur protein (ISP), subunits 7, 10 and 11, one each. The distances of 21 A between bL heme and bH heme and of 27 A between bL heme and the iron-sulfur cluster (FeS), accommodate well the observed fast electron transfers between the involved redox centers. However, the distance of 31 A between heme c1 and FeS, makes it difficult to explain the high electron transfer rate between them. 3D structural analyses of the bc1 complexes co-crystallized with the Qu site inhibitors suggest that the extramembrane domain of the ISP may undergo substantial movement during the catalytic cycle of the complex. This suggestion is further supported by the decreased in the cytochrome bc1 complex activity and the increased in activation energy for mutants with increased rigidity in the neck region of ISP.     

Structure analysis reveals the flexibility of the ADAMTS-5 active site

A ((1S,2R)-2-hydroxy-2,3-dihydro-1H-inden-1-yl) succinamide derivative (here referred to as Compound 12) shows significant activity toward many matrix metalloproteinases (MMPs), including MMP-2, MMP-8, MMP-9, and MMP-13. Modeling studies had predicted that this compound would not bind to ADAMTS-5 (a disintegrin and metalloproteinase with thrombospondin motifs-5) due to its shallow S1' pocket. However, inhibition analysis revealed it to be a nanomolar inhibitor of both ADAMTS-4 and -5. The observed inconsistency was explained by analysis of crystallographic structures, which showed that Compound 12 in complex with the catalytic domain of ADAMTS-5 (cataTS5) exhibits an unusual conformation in the S1' pocket of the protein. This first demonstration that cataTS5 can undergo an induced conformational change in its active site pocket by a molecule like Compound 12 should enable the design of new aggrecanase inhibitors with better potency and selectivity profiles.