Trigger Conformational Changes Research Articles

Modulation of HIF-2 Function with Small Molecules: Did Nature Beat Us to It?

Abstract SCI-39Hypoxia inducible factors (HIFs) govern a mammalian cellular hypoxic response, regulating the transcription of hundreds of genes in response to low oxygen levels. Though this pathway performs critical functions under physiological conditions, increased levels of HIFs are also highly correlated with several disease states. As such, the development of artificial compounds that directly and selectively regulate HIF function is of interest. We have used a combination of biophysical, biochemical, and chemical approaches to study the structure and function of the Per-ARNT-Sim (PAS) domains of human HIFs, which serve as critical protein/protein interaction modules in assembling the functional HIF heterodimer. These studies have revealed the presence of a putative ligand-binding site within one of the HIF-2α PAS domains. A series of biased and unbiased approaches were undertaken to identify compounds that bind at this site and trigger conformational changes within the domain, leading to dissociation of the HIF-2 heterodimer in vitro and in living cells. Consequently, the expression of many hypoxia-inducible genes can be coordinately downregulated by a single agent. Despite its high amino acid identity, HIF-1α lacks a comparable ligand-binding site, indicating that these agents will be useful in deciphering differences in the biological roles of the HIF-1 and HIF-2 isoforms. The presence of a preformed cavity within HIF-2α is particularly as exciting as it suggests the potential for direct physiological regulation of HIF-2 by endogenous metabolite(s). Together, these studies offer further insight into HIF function and regulation while providing chemical tools that advance therapeutic applications. Disclosures:No relevant conflicts of interest to declare.

Mechanism of Diphtheria Toxin Catalytic Domain Delivery to the Eukaryotic Cell Cytosol and the Cellular Factors that Directly Participate in the Process

Research on diphtheria and anthrax toxins over the past three decades has culminated in a detailed understanding of their structure function relationships (e.g., catalytic (C), transmembrane (T), and receptor binding (R) domains), as well as the identification of their eukaryotic cell surface receptor, an understanding of the molecular events leading to the receptor-mediated internalization of the toxin into an endosomal compartment, and the pH triggered conformational changes required for pore formation in the vesicle membrane. Recently, a major research effort has been focused on the development of a detailed understanding of the molecular interactions between each of these toxins and eukaryotic cell factors that play an essential role in the efficient translocation of their respective catalytic domains through the trans-endosomal vesicle membrane pore and delivery into the cell cytosol. In this review, I shall focus on recent findings that have led to a more detailed understanding of the mechanism by which the diphtheria toxin catalytic domain is delivered to the eukaryotic cell cytosol. While much work remains, it is becoming increasingly clear that the entry process is facilitated by specific interactions with a number of cellular factors in an ordered sequential fashion. In addition, since diphtheria, anthrax lethal factor and anthrax edema factor all carry multiple coatomer I complex binding motifs and COPI complex has been shown to play an essential role in entry process, it is likely that the initial steps in catalytic domain entry of these divergent toxins follow a common mechanism.

The Unique Transmembrane Hairpin of Flavivirus Fusion Protein E Is Essential for Membrane Fusion

The fusion of enveloped viruses with cellular membranes is mediated by proteins that are anchored in the lipid bilayer of the virus and capable of triggered conformational changes necessary for driving fusion. The flavivirus envelope protein E is the only known viral fusion protein with a double membrane anchor, consisting of two antiparallel transmembrane helices (TM1 and TM2). TM1 functions as a stop-transfer sequence and TM2 as an internal signal sequence for the first nonstructural protein during polyprotein processing. The possible role of this peculiar C-terminal helical hairpin in membrane fusion has not been investigated so far. We addressed this question by studying TM mutants of tick-borne encephalitis virus (TBEV) recombinant subviral particles (RSPs), an established model system for flavivirus membrane fusion. The engineered mutations included the deletion of TM2, the replacement of both TM domains (TMDs) by those of the related Japanese encephalitis virus (JEV), and the use of chimeric TBEV-JEV membrane anchors. Using these mutant RSPs, we provide evidence that TM2 is not just a remnant of polyprotein processing but, together with TM1, plays an active role in fusion. None of the TM mutations, including the deletion of TM2, affected early steps of the fusion process, but TM interactions apparently contribute to the stability of the postfusion E trimer and the completion of the merger of the membranes. Our data provide evidence for both intratrimer and intertrimer interactions mediated by the TMDs of E and thus extend the existing models of flavivirus membrane fusion.

Structural basis for the coupling between activation and inactivation gates in K+ channels

The coupled interplay between activation and inactivation gating is a functional hallmark of K(+) channels. This coupling has been experimentally demonstrated through ion interaction effects and cysteine accessibility, and is associated with a well defined boundary of energetically coupled residues. The structure of the K(+) channel KcsA in its fully open conformation, in addition to four other partial channel openings, richly illustrates the structural basis of activation-inactivation gating. Here, we identify the mechanistic principles by which movements on the inner bundle gate trigger conformational changes at the selectivity filter, leading to the non-conductive C-type inactivated state. Analysis of a series of KcsA open structures suggests that, as a consequence of the hinge-bending and rotation of the TM2 helix, the aromatic ring of Phe 103 tilts towards residues Thr 74 and Thr 75 in the pore-helix and towards Ile 100 in the neighbouring subunit. This allows the network of hydrogen bonds among residues Trp 67, Glu 71 and Asp 80 to destabilize the selectivity filter, allowing entry to its non-conductive conformation. Mutations at position 103 have a size-dependent effect on gating kinetics: small side-chain substitutions F103A and F103C severely impair inactivation kinetics, whereas larger side chains such as F103W have more subtle effects. This suggests that the allosteric coupling between the inner helical bundle and the selectivity filter might rely on straightforward mechanical deformation propagated through a network of steric contacts. Average interactions calculated from molecular dynamics simulations show favourable open-state interaction-energies between Phe 103 and the surrounding residues. We probed similar interactions in the Shaker K(+) channel where inactivation was impaired in the mutant I470A. We propose that side-chain rearrangements at position 103 mechanically couple activation and inactivation in KcsA and a variety of other K(+) channels.

Topological Layers in the HIV-1 gp120 Inner Domain Regulate gp41 Interaction and CD4-Triggered Conformational Transitions

The entry of human immunodeficiency virus (HIV-1) into cells is initiated by binding of the gp120 exterior envelope glycoprotein to the receptor, CD4. How does CD4 binding trigger conformational changes in gp120 that allow the gp41 transmembrane envelope glycoprotein to mediate viral-cell membrane fusion? The transition from the unliganded to the CD4-bound state is regulated by two potentially flexible topological layers (layers 1 and 2) in the gp120 inner domain. Both layers apparently contribute to the noncovalent association of unliganded gp120 with gp41. After CD4 makes initial contact with the gp120 outer domain, layer 1-layer 2 interactions strengthen gp120-CD4 binding by reducing the off rate. Layer 1-layer 2 interactions also destabilize the activated state induced on HIV-1 by treatment with soluble CD4. Thus, despite lack of contact with CD4, the gp120 inner-domain layers govern CD4 triggering by participating in conformational transitions within gp120 and regulating the interaction with gp41.

Determining the catalytic role of remote substrate binding interactions in ketosteroid isomerase

A fundamental difference between enzymes and small chemical catalysts is the ability of enzymes to use binding interactions with nonreactive portions of substrates to accelerate chemical reactions. Remote binding interactions can localize substrates to the active site, position substrates relative to enzymatic functional groups and other substrates, trigger conformational changes, induce local destabilization, and modulate an active site environment by solvent exclusion. We investigated the role of remote substrate binding interactions in the reaction catalyzed by the enzyme ketosteroid isomerase (KSI), which catalyzes a double bond migration of steroid substrates through a dienolate intermediate that is stabilized in an oxyanion hole. Comparison of a single-ring and multiple-ring substrate allowed the catalytic contribution of binding interactions with the distal substrate rings to be determined. The value of k(cat)/K(M) for a single-ring substrate is reduced 27,000-fold relative to a multiple-ring steroid substrate, suggesting that remote binding interactions with the steroid substrate contribute substantially to the KSI reaction. Nevertheless, the reaction rates for KSI-bound single- and multiple-ring substrates (k(cat)) are within 2-fold. Further, oxyanion hole mutations have the same effect on reactions of the single- and multiple-ring substrates. These results suggest that remote binding interactions contribute >5 kcal/mol to catalysis by KSI but that local rather than remote interactions dictate the catalytic contributions from KSI's general base and oxyanion hole.

Mechanism of action of N-phenyl- N′-(2-chloroethyl)ureas in the colchicine-binding site at the interface between α- and β-tubulin

Computational tools such as CoMSIA and CoMFA models reported in a recent study revealed the structure–activity relationships ruling the interactions occurring between hydrophobic N-phenyl- N′-(2-chloroethyl)ureas (CEU) and the colchicine-binding site (C-BS) on β ΙΙ-tubulin. Here, we describe the mechanisms involved in the covalent binding of three subsets of CEU derivatives to the C-BS. The FlexiDock experiments confirmed that the interaction of non-covalent portions of the CEU auxophore moiety of CEU is involved in the binding of the drug to the C-BS facilitate the nucleophilic attack of Glu-β198 rather than Cys-β239. In addition, these studies suggest that Cys-β239 together with Asn-α99, Ser-α176, Thr-α177, Leu-β246, Asn-β247, Ala-β248, Lys-β252 and Asn-β256 are implicated in the stabilization of a C-BS–CEU complex prior to the acylation of Glu-β198 by CEU. Our molecular models propose the formation of a stabilized C-BS–CEU complex before the completion of the Glu-β198 acylation; acylation triggering conformational changes of β-tubulin, microtubule depolymerization and anoikis. The computational models presented here might be useful to the design of selective and more potent C-BS inhibitors. Of interest, in vivo acylation of acidic amino acid residues by xenobiotics is an unusual reaction and may open new approaches for the design of irreversible protein inhibitors such as tubulin.

Strong carotenoid-to-peptide interaction immediately after triplet excitation triggering conformational changes in photo-reaction center-bound 15-cis-spheroidene as revealed by submicrosecond time-resolved Raman spectroscopy

Intermolecular interactions and structural changes of 15-cis-spheroidene in the binding pocket of reaction center from Rhodobacter sphaeroides 2.4.1 have been studied by submicrosecond time-resolved Raman spectroscopy at physiological temperature. Immediately after triplet excitation, strong enhancement of the 13-methyl asymmetric deformation Raman line of the carotenoid (Car) at 1458cm−1 has been observed. The intensity enhancement can be explained in terms of the severe steric interaction between the Car 13-methyl group and the peptide CO oxygen of Gly M178 caused by the isomerization of the bound 15-cis-spheroidene toward all-trans. Subsequent conformational changes of the Car have also been observed.

Characterization of a Structural Intermediate of Flavivirus Membrane Fusion

Viral membrane fusion proceeds through a sequence of steps that are driven by triggered conformational changes of viral envelope glycoproteins, so-called fusion proteins. Although high-resolution structural snapshots of viral fusion proteins in their prefusion and postfusion conformations are available, it has been difficult to define intermediate structures of the fusion pathway because of their transient nature. Flaviviruses possess a class II viral fusion protein (E) mediating fusion at acidic pH that is converted from a dimer to a trimer with a hairpin-like structure during the fusion process. Here we show for tick-borne encephalitis virus that exposure of virions to alkaline instead of acidic pH traps the particles in an intermediate conformation in which the E dimers dissociate and interact with target membranes via the fusion peptide without proceeding to the merger of the membranes. Further treatment to low pH, however, leads to fusion, suggesting that these monomers correspond to an as-yet-elusive intermediate required to convert the prefusion dimer into the postfusion trimer. Thus, the use of nonphysiological conditions allows a dissection of the flavivirus fusion process and the identification of two separate steps, in which membrane insertion of multiple copies of E monomers precedes the formation of hairpin-like trimers. This sequence of events provides important new insights for understanding the dynamic process of viral membrane fusion.

Concordance of Residual Dipolar Couplings, Backbone Order Parameters and Crystallographic B-factors for a Small α/β Protein: A Unified Picture of High Probability, Fast Atomic Motions in Proteins

Using ensemble refinement of the third immunoglobulin binding domain (GB3) of streptococcal protein G (a small alpha/beta protein of 56 residues), we demonstrate that backbone (N-H, N-C', Calpha-Halpha, Calpha-C') residual dipolar coupling data in five independent alignment media, generalized order parameters from 15N relaxation data, and B-factors from a high-resolution (1.1A), room temperature crystal structure are entirely consistent with one another within experimental error. The optimal ensemble size representation is between four and eight, as assessed by complete cross-validation of the residual dipolar couplings. Thus, in the case of GB3, all three observables reflect the same low-amplitude anisotropic motions arising from fluctuations in backbone phi/psi torsion angles in the picosecond to nanosecond regime in both solution and crystalline environments, yielding a unified picture of fast, high-probability atomic motions in proteins. An understanding of these motions is crucial for understanding the impact of protein dynamics on protein function, since they provide part of the driving force for triggered conformational changes that occur, for example, upon ligand binding, signal transduction and enzyme catalysis.

The High Resolution Crystal Structure of a Native Thermostable Serpin Reveals the Complex Mechanism Underpinning the Stressed to Relaxed Transition

Serpins fold into a native metastable state and utilize a complex conformational change to inhibit target proteases. An undesirable result of this conformational flexibility is that most inhibitory serpins are heat sensitive, forming inactive polymers at elevated temperatures. However, the prokaryote serpin, thermopin, from Thermobifida fusca is able to function in a heated environment. We have determined the 1.8 A x-ray crystal structure of thermopin in the native, inhibitory conformation. A structural comparison with the previously determined 1.5 A structure of cleaved thermopin provides detailed insight into the complex mechanism of conformational change in serpins. Flexibility in the shutter region and electrostatic interactions at the top of the A beta-sheet (the breach) involving the C-terminal tail, a unique structural feature of thermopin, are postulated to be important for controlling inhibitory activity and triggering conformational change, respectively, in the native state. Here we have discussed the structural basis of how this serpin reconciles the thermodynamic instability necessary for function with the stability required to withstand elevated temperatures.

Promoter opening by sigma(54) and sigma(70) RNA polymerases: sigma factor-directed alterations in the mechanism and tightness of control.

Transcription control at the melting step is not yet understood. Here, band shift, cross-linking, and transcription experiments on diverse DNA probes were used with two bacterial RNA polymerase holoenzymes that differ in how they regulate melting. Data indicated that both sigma(54) and sigma(70) holoenzymes assume a default closed form that cannot establish single-strand binding. Upon activation the enzymes are converted to an open form that can bind simultaneously to the upstream fork junction and to the melted transcription start site. The key difference is that sigma(54) imposes tighter regulation by creating a complex molecular switch at -12/-11; the current data show that this switch can be thrown by activator. In this case an ATP-bound enhancer protein causes sigma(54) to alter its cross-linking pattern near -11 and also causes a reorganization of holoenzyme: DNA interactions, detected by electrophoretic mobility-shift assay. At a temperature-dependent sigma(70) promoter, elevated temperature alone can assist in triggering conformational changes that enhance the engagement of single-strand DNA. Thus, the two sigma factors modify the same intrinsic opening pathway to create quite different mechanisms of transcriptional regulation.

Intra- vs intersubunit communication in the homodimeric restriction enzyme EcoRV: Thr 37 and Lys 38 involved in indirect readout are only important for the catalytic activity of their own subunit.

EcoRV is a dimer of two identical subunits which together form one binding site for the double-stranded DNA substrate. Concerted cleavage of both strands of the duplex requires intersubunit communication to synchronize the two catalytic centers of EcoRV. Here we address the question of how contacts to the DNA backbone trigger conformational changes which lead to the activation of both catalytic centers. The structure of the specific EcoRV-DNA complex shows that a region including amino acids Thr 37 and Lys 38 is involved in interactions with the DNA backbone and is a candidate for intersubunit communication. Homodimeric EcoRV T37A and K38A variants have a 1000-fold reduced catalytic activity. To examine whether Thr 37 and Lys 38 of one subunit affect the catalytic center in the same subunit and/or in the other subunit, we have produced heterodimeric variants containing a Thr 37 --> Ala or Lys 38 --> Ala substitution in one subunit combined with a wild type (wt) subunit (wt/T37A and wt/K38A) or with a subunit which contains an amino acid substitution (Asp 90 --> Ala) in the active site (D90A/T37A and D90A/K38A). Cleavage experiments with supercoiled pAT153 show that wt/T37A and wt/K38A preferentially nick the DNA. A steady-state kinetic analysis of the cleavage of an oligodeoxynucleotide substrate shows that the activity of wt/T37A and wt/K38A is half of that of wild type EcoRV, whereas D90A/T37A and D90A/K38A are almost inactive. These results demonstrate that Thr 37 and Lys 38 affect primarily the catalytic center in their own subunit and that both subunits of EcoRV can be activated independently of each other. We suggest that Thr 37 and Lys 38 control the catalytic activity of the active site in their own subunit by positioning alpha-helix B.

A role for the acidic trimer repeat region of transcription factor sigma 54 in setting the rate and temperature dependence of promoter melting in vivo.

sigma 54 is an atypical sigma factor involved in enhancer-dependent transcription in Escherichia coli. In vivo assays were developed for following the kinetics and thermodynamics of sigma 54-dependent melting of the glnAP2 promoter start site. These assays were applied to a series of three sigma 54 polymerases containing zero, one, and two copies of the highly acidic trimer repeat region. The results showed that at least one copy of this acid region is required for a complete melting transition at physiological temperature, but is not required for a low temperature melting transition. The rate of melting the promoter in vivo increased with number of copies of this region. Taken together with other observations, the experiments point to a role for the acid region in triggering conformational changes that lead to specific promoter melting. The acid region is required for these changes to occur fully at physiological temperature and influences the rate at which they occur.

Binding and fluorescence studies of the relationship between neurophysin-peptide interaction and neurophysin self-association: an allosteric system exhibiting minimal cooperativity

The mechanism of peptide-enhanced neurophysin self-association was investigated to address questions raised by the crystal structure of a neurophysin-dipeptide complex. The dependence on protein concentration of the binding of a broad range of peptides to the principal hormone-binding site confirmed that occupancy of this site alone, and not a site that bridges the monomer-monomer interface, is the trigger for enhanced dimerization. For the binding of most peptides to the principal hormone-binding site on bovine neurophysin I, the affinity of each dimer site was at least 10 times that of monomer under the conditions used. No interactions between the two sites of the dimer were evident. Fluorescence polarization studies of pressure-induced dimer dissociation indicated that the volume change for this reaction was almost 4 times greater in the liganded than in the unliganded state, pointing to a significant alteration of the monomer-monomer interface upon peptide binding. Novel conformational changes in the vicinity of the single neurophysin tyrosine, Tyr-49, induced by pressures lower than required for subunit dissociation, were also observed. The bovine neurophysin I dimer therefore appears to represent an allosteric system in which there is thermodynamic and functional communication between each binding site and the monomer-monomer interface, but no communication across the interface to the binding site of the other subunit. A model for the peptide-enhanced dimerization is proposed in which intersubunit contacts between monomers reduce the large unfavorable free energy associated with binding-induced intrasubunit conformational change. Structural origins of the lack of communication across the interface are suggested on the basis of the low volume change associated with dimerization in the unliganded state and monomer-monomer contacts in the crystal structure. Potential roles for the peptide alpha-amino group and position 2 phenyl ring in triggering conformational change are discussed.

Coenzyme-induced conformational changes in glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus

The structure of apo-glyceraldehyde-3-phosphate dehydrogenase (GAPDHase) from Bacillus stearothermophilus has been refined using a restrained least-squares method. The final crystallographic R-factor is 0·177 for all 53,315 reflections between 7·0 and 2·5 Å. The resulting model has been analysed with respect to lattice interactions, molecular symmetry, temperature factors and solvent structure showing that, apart from local deviations due to intermolecular contact, the molecule exhibits a very high degree of local 222 symmetry. Analysis of differences between the structure of apo-GAPDHase and the previously refined holo-GAPDHase at 1·8 Å resolution reveals details of conformational change in the enzyme induced by cofactor binding. The change, which was previously described as a rigid-body rotation of the coenzyme-binding domain with respect to the catalytic domain, is of more complex nature and involves relative shifts of several structural elements in the coenzyme-binding domain and some small changes in the catalytic domain. A possible mechanism of this conformational change is proposed based on the comparison of the refined structures and model-building studies. According to this mechanism, the adenosine moiety of NAD can initially bind to the protein in the apo-enzyme conformation. Several attractive interactions resulting from the initial binding of the coenzyme trigger conformational changes in the molecule of GAPDHase that: (1) create the productive nicotinamide-moiety binding site; (2) improve enzyme-coenzyme interactions at the adenosine moiety; (3) modify the active site to optimize the positioning of catalytic residues and ion-binding sites. Implications of the proposed mechanism for existing experimental data on binding of NAD analogues to GAPDHase are discussed.

NAD-dependent hydrogenase from [formula omitted]: Does it have a regulatory centre?

Evidence is presented for the existence of a relatively high-potential regulatory centre in the NAD-dependent hydrogenase from the hydrogen oxidizing bacterium Alcaligenes eutrophus Z1 . Reduction of the hydrogenase to the redox potentials lower than −100 mV converts the enzyme into a catalytically active state that is remarcably stable to oxidants. Once activated, the enzyme does not loose its activity on intensive oxigenation for at least 3 hours. A novel hydrogenase ESR signal with a wide temperature optimum and a ∼−100 mV midpoint redox potential was detected. We suggest that the reduction of this redox centre trigger conformational changes in the inactive oxidized enzyme molecule, thus reorganizing the latter into the active one.

Cooperative action of the T4 tail fibers and baseplate in triggering conformational change and in determining host range

Thermolabile mutants of T4 altered in structural proteins of the baseplate (P5, P6, P7, P8, and P10) undergo normal conformational changes more easily than wild-type phage. Our results show that the tail fibers trigger the change in the baseplate that precipitates sheath contraction and that these mutants contract more easily than wild-type phage. The fact that some of the mutants have an extended host range suggests that they do not need as secure an attachment to the cell wall as wild-type phage for the cooperative action of the tail fibers to trigger the baseplate change.