Thermodynamics of DNA binding and distortion by the hyperthermophile chromatin protein Sac7d

Protein and DNA Conformational Changes Contribute to Specificity of Cre Recombinase

Cre, a conservative site-specific tyrosine recombinase, is a powerful gene editing tool in the laboratory. Expanded applications in human health are hindered by a lack of understanding of the mechanism by which Cre selectively binds and recombines its cognate loxP sequences. This knowledge is essential for retargeting the enzyme to new sites and for mitigating the effects of off-target recombination. Prior studies have suggested that in addition to a few base-specific contacts to cognate loxP DNA, the enzyme's specificity is enhanced by (1) autoinhibition involving a conformational change in the protein's C-terminal helix and (2) indirect readout via binding-coupled conformational changes in the target DNA. We used isothermal titration calorimetry (ITC), circular dichroism (CD), and heteronuclear NMR spectroscopy to investigate DNA site recognition by wild-type Cre and a deletion mutant lacking the C-terminal helix. ITC of Cre and a C-terminal deletion variant against cognate and noncognate DNA recombinase binding elements (RBEs) reveal that the C-terminus reduces DNA binding affinity by 6-fold toward cognate DNA. Additionally, ITC revealed highly unfavorable binding enthalpy, which, when combined with evidence from CD and NMR of structural differences between cognate and noncognate complexes, supports a model in which binding-coupled DNA bending provides a unique structure-thermodynamic signature of cognate complexes. Together, these findings advance our understanding of site recognition by Cre recombinase.

DNA Mimic Foldamer Recognition of a Chromosomal Protein

Helical aromatic oligoamide foldamers bearing anionic side chains that mimic the overall shape and charge surface distribution of DNA were synthesized. Their interactions with chromosomal protein Sac7d, a non-sequence-selective DNA-binder that kinks DNA, were investigated by Surface Plasmon Resonance (SPR), Isothermal Titration Calorimetry (ITC), Circular Dichroism spectroscopy (CD), melting curve analysis, Atomic Force Microscopy (AFM), and Nuclear Magnetic Resonance (NMR), as well as by single crystal X-ray crystallography. The foldamers were shown to bind to Sac7d better than a DNA duplex of comparable length. The interaction is diastereoselective and takes place at the DNA binding site. Crystallography revealed that the DNA mimic foldamers have a binding mode of their own and that they can bind to Sac7d without being kinked.

Asymmetric dimerization in a transcription factor superfamily is promoted by allosteric interactions with DNA

Transcription factors, such as nuclear receptors achieve precise transcriptional regulation by means of a tight and reciprocal communication with DNA, where cooperativity gained by receptor dimerization is added to binding site sequence specificity to expand the range of DNA target gene sequences. To unravel the evolutionary steps in the emergence of DNA selection by steroid receptors (SRs) from monomeric to dimeric palindromic binding sites, we carried out crystallographic, biophysical and phylogenetic studies, focusing on the estrogen-related receptors (ERRs, NR3B) that represent closest relatives of SRs. Our results, showing the structure of the ERR DNA-binding domain bound to a palindromic response element (RE), unveil the molecular mechanisms of ERR dimerization which are imprinted in the protein itself with DNA acting as an allosteric driver by allowing the formation of a novel extended asymmetric dimerization region (KR-box). Phylogenetic analyses suggest that this dimerization asymmetry is an ancestral feature necessary for establishing a strong overall dimerization interface, which was progressively modified in other SRs in the course of evolution.

Beyond Antibodies as Binding Partners: The Role of Antibody Mimetics in Bioanalysis

The emergence of novel binding proteins or antibody mimetics capable of binding to ligand analytes in a manner analogous to that of the antigen-antibody interaction has spurred increased interest in the biotechnology and bioanalytical communities. The goal is to produce antibody mimetics designed to outperform antibodies with regard to binding affinities, cellular and tumor penetration, large-scale production, and temperature and pH stability. The generation of antibody mimetics with tailored characteristics involves the identification of a naturally occurring protein scaffold as a template that binds to a desired ligand. This scaffold is then engineered to create a superior binder by first creating a library that is then subjected to a series of selection steps. Antibody mimetics have been successfully used in the development of binding assays for the detection of analytes in biological samples, as well as in separation methods, cancer therapy, targeted drug delivery, and in vivo imaging. This review describes recent advances in the field of antibody mimetics and their applications in bioanalytical chemistry, specifically in diagnostics and other analytical methods.

The archaeal "7 kDa DNA-binding" proteins: extended characterization of an old gifted family

The “7 kDa DNA-binding” family, also known as the Sul7d family, is composed of chromatin proteins from the Sulfolobales archaeal order. Among them, Sac7d and Sso7d have been the focus of several studies with some characterization of their properties. Here, we studied eleven other proteins alongside Sac7d and Sso7d under the same conditions. The dissociation constants of the purified proteins for binding to double-stranded DNA (dsDNA) were determined in phosphate-buffered saline at 25 °C and were in the range from 11 μM to 22 μM with a preference for G/C rich sequences. In accordance with the extremophilic origin of their hosts, the proteins were found highly stable from pH 0 to pH 12 and at temperatures from 85.5 °C to 100 °C. Thus, these results validate eight putative “7 kDa DNA-binding” family proteins and show that they behave similarly regarding both their function and their stability among various genera and species. As Sac7d and Sso7d have found numerous uses as molecular biology reagents and artificial affinity proteins, this study also sheds light on even more attractive proteins that will facilitate engineering of novel highly robust reagents.

Designer Histone Complexes: Controlling Protein-DNA Interactions with Protein Charge as an "All-or-None" Digital Switch

An artificial histone is synthesized that functions as a DNA-protein digital switch, where DNA binding is all or none, controlled by a sharp threshold of protein charge. A non-DNA-binding protein, glucose oxidase (GOx), was chemically modified by attaching an increasing number of triethylenetetramine (TETA) side chains to its glutamate/aspartate groups to obtain a small library of covalently modified GOx(n) derivatives. The parameter n denotes the net charge on the protein at pH 7, which was increased from -62 (pristine GOx) to +75 by attaching an increasing number of TETA residues to the protein. All GOx(n) derivatives retained their secondary structure to a good extent, as monitored by UV circular dichroism (CD) spectroscopy, and they also retained oxidase activities to a significant extent. The interaction of the GOx(n) with calf thymus DNA was examined by isothermal titration calorimetry (ITC). Pristine GOx of -62 charge at pH 7 in 10 mM Tris-HCl and 50 mM NaCl buffer had no affinity for the negatively charged DNA helix, and GOx(n) with n < +30 had no affinity for DNA either. However, binding has been turned on abruptly when n ≥ +30 with binding constants (K_b) ranging from (1.5 ± 0.7) × 10⁷ to (7.3 ± 2.8) × 10⁷ M^-1 for n values of +30 and +75, respectively, and this type of "all-or-none" binding based on protein charge is intriguing. Furthermore, thermodynamic analysis of the titration data revealed that binding is entirely entropy-driven with ΔS ranging from 0.09 ± 0.007 to 0.19 ± 0.008 kcal/mol K with enthalpic penalties of 17.0 ± 2.3 and 46.1 ± 2.1 kcal/mol, respectively. The binding had intrinsic propensities (ΔG) ranging from -9.8 ± 0.14 to -10.7 ± 0.25 kcal/mol, independent of n. DNA binding distorted protein-DNA secondary structure, as evidenced by CD spectroscopy, but oxidase activity of GOx(n)/DNA complexes has been unaffected. This is the very first example of an artificial histone (GOx(n)) where the protein charge functioned as a DNA-binding switch; protein charge is in turn under complete chemical control while preserving the biological activity of the protein. The new insight gained here could be useful in the design of novel "on-off" protein switches.

Thermodynamic characterization of the interaction between the human Y-box binding protein YB-1 and nucleic acids

The calorimetric analyses demonstrate the RNA- and DNA-binding manner of YB-1 and their specific binding and the assisted affinity enhancement.

A unified framework based on the binding polynomial for characterizing biological systems by isothermal titration calorimetry

Isothermal titration calorimetry (ITC) has become the gold-standard technique for studying binding processes due to its high precision and sensitivity, as well as its capability for the simultaneous determination of the association equilibrium constant, the binding enthalpy and the binding stoichiometry. The current widespread use of ITC for biological systems has been facilitated by technical advances and the availability of commercial calorimeters. However, the complexity of data analysis for non-standard models is one of the most significant drawbacks in ITC. Many models for studying macromolecular interactions can be found in the literature, but it looks like each biological system requires specific modeling and data analysis approaches. The aim of this article is to solve this lack of unity and provide a unified methodological framework for studying binding interactions by ITC that can be applied to any experimental system. The apparent complexity of this methodology, based on the binding polynomial, is overcome by its easy generalization to complex systems.

Effect of temperature on the intrinsic flexibility of DNA and its interaction with architectural proteins

The helical structure of double-stranded DNA is destabilized by increasing temperature. Above a critical temperature (the melting temperature), the two strands in duplex DNA become fully separated. Below this temperature, the structural effects are localized. Using tethered particle motion in a temperature-controlled sample chamber, we systematically investigated the effect of increasing temperature on DNA structure and the interplay between this effect and protein binding. Our measurements revealed that (1) increasing temperature enhances DNA flexibility, effectively leading to more compact folding of the double-stranded DNA chain, and (2) temperature differentially affects different types of DNA-bending chromatin proteins from mesophilic and thermophilic organisms. Thus, our findings aid in understanding genome organization in organisms thriving at moderate as well as extreme temperatures. Moreover, our results underscore the importance of carefully controlling and measuring temperature in single-molecule DNA (micromanipulation) experiments.

Sulfolobus chromatin proteins modulate strand displacement by DNA polymerase B1

Strand displacement by a DNA polymerase serves a key role in Okazaki fragment maturation, which involves displacement of the RNA primer of the preexisting Okazaki fragment into a flap structure, and subsequent flap removal and fragment ligation. We investigated the role of Sulfolobus chromatin proteins Sso7d and Cren7 in strand displacement by DNA polymerase B1 (PolB1) from the hyperthermophilic archaeon Sulfolobus solfataricus. PolB1 showed a robust strand displacement activity and was capable of synthesizing thousands of nucleotides on a DNA-primed 72-nt single-stranded circular DNA template. This activity was inhibited by both Sso7d and Cren7, which limited the flap length to 3–4 nt at saturating concentrations. However, neither protein inhibited RNA displacement on an RNA-primed single-stranded DNA minicircle by PolB1. Strand displacement remained sensitive to modulation by the chromatin proteins when PolB1 was in association with proliferating cell nuclear antigen. Inhibition of DNA instead of RNA strand displacement by the chromatin proteins is consistent with the finding that double-stranded DNA was more efficiently bound and stabilized than an RNA:DNA duplex by these proteins. Our results suggest that Sulfolobus chromatin proteins modulate strand displacement by PolB1, permitting efficient removal of the RNA primer while inhibiting excessive displacement of the newly synthesized DNA strand during Okazaki fragment maturation.

Theoretical modeling of masking DNA application in aptamer-facilitated biomarker discovery

In aptamer-facilitated biomarker discovery (AptaBiD), aptamers are selected from a library of random DNA (or RNA) sequences for their ability to specifically bind cell-surface biomarkers. The library is incubated with intact cells, and cell-bound DNA molecules are separated from those unbound and amplified by the polymerase chain reaction (PCR). The partitioning/amplification cycle is repeated multiple times while alternating target cells and control cells. Efficient aptamer selection in AptaBiD relies on the inclusion of masking DNA within the cell and library mixture. Masking DNA lacks primer regions for PCR amplification and is typically taken in excess to the library. The role of masking DNA within the selection mixture is to outcompete any nonspecific binding sequences within the initial library, thus allowing specific DNA sequences (i.e., aptamers) to be selected more efficiently. Efficient AptaBiD requires an optimum ratio of masking DNA to library DNA, at which aptamers still bind specific binding sites but nonaptamers within the library do not bind nonspecific binding sites. Here, we have developed a mathematical model that describes the binding processes taking place within the equilibrium mixture of masking DNA, library DNA, and target cells. An obtained mathematical solution allows one to estimate the concentration of masking DNA that is required to outcompete the library DNA at a desirable ratio of bound masking DNA to bound library DNA. The required concentration depends on concentrations of the library and cells as well as on unknown cell characteristics. These characteristics include the concentration of total binding sites on the cell surface, N, and equilibrium dissociation constants, K(nsL) and K(nsM), for nonspecific binding of the library DNA and masking DNA, respectively. We developed a theory that allows the determination of N, K(nsL), and K(nsM) based on measurements of EC50 values for cells mixed separately with the library and masking DNA (EC50 is the concentration of fluorescently labeled DNA at which half of the maximum fluorescence signal from DNA-bound cells is reached). We also obtained expressions for signals from bound DNA (measured by flow cytometry) in terms of N, K(nsL), and K(nsM). These expressions can be used for the verification of N, K(nsL), and K(nsM) values found from EC50 measurements. The developed procedure was applied to MCF-7 breast cancer cells, and corresponding values of N, K(nsL), and K(nsM) were established for the first time. The concentration of masking DNA required for AptaBiD with MCF-7 breast cancer cells was also estimated.

Influence of hyperthermophilic protein Cren7 on the stability and conformation of DNA: insights from molecular dynamics simulation and free energy analysis

Cren7, a novel chromatin protein highly conserved among crenarchaea, plays an important role in genome packaging and gene regulation. However, the detail dynamical structural characteristic of the Cren7-DNA complex and the detail study of the DNA in the complex have not been done. Focused on two specific Cren7-DNA complexes (PDB codes 3LWH and 3LWI ), we applied molecular dynamics (MD) simulations at four different temperatures (300, 350, 400, and 450 K) and the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) free energy calculation at 300 and 350 K to examine the role of Cren7 protein in enhancing the stability of DNA duplexes via protein-DNA interactions, and to study the structural transition in DNA. The simulation results indicate that Cren7 stabilizes DNA duplex in a certain temperature range in the binary complex compared with the unbound DNA molecules. At the same time, DNA molecules were found to undergo B-like to A-like form transitions with increased temperature. The results of statistical analyses of the H-bond and hydrophobic contacts show that some residues have significant influence on the structure of DNA molecules. Our work can give important information to understand the interactions of proteins with nucleic acids and other ligands.

Insights into the thermal stabilization and conformational transitions of DNA by hyperthermophile protein Sso7d: molecular dynamics simulations and MM-PBSA analysis

In the assembly of DNA-protein complex, the DNA kinking plays an important role in nucleoprotein structures and gene regulation. Molecular dynamics (MD) simulations were performed on specific protein-DNA complexes in this study to investigate the stability and structural transitions of DNA depending on temperature. Furthermore, we introduced the molecular mechanics/Poisson-Boltzmann surface area (MM-PBSA) approach to analyze the interactions between DNA and protein in hyperthermophile. Focused on two specific Sso7d-DNA complexes (PDB codes: 1BNZ and 1BF4), we performed MD simulations at four temperatures (300, 360, 420, and 480 K) and MM-PBSA at 300 and 360 K to illustrate detailed information on the changes of DNA. Our results show that Sso7d stabilizes DNA duplex over a certain temperature range and DNA molecules undergo B-like to A-like form transitions in the binary complex with the temperature increasing, which are consistent with the experimental data. Our work will contribute to a better understanding of protein-DNA interaction.

Archaeal chromatin proteins

Archaea, along with Bacteria and Eukarya, are the three domains of life. In all living cells, chromatin proteins serve a crucial role in maintaining the integrity of the structure and function of the genome. An array of small, abundant and basic DNA-binding proteins, considered candidates for chromatin proteins, has been isolated from the Euryarchaeota and the Crenarchaeota, the two major phyla in Archaea. While most euryarchaea encode proteins resembling eukaryotic histones, crenarchaea appear to synthesize a number of unique DNA-binding proteins likely involved in chromosomal organization. Several of these proteins (e.g., archaeal histones, Sac10b homologs, Sul7d, Cren7, CC1, etc.) have been extensively studied. However, whether they are chromatin proteins and how they function in vivo remain to be fully understood. Future investigation of archaeal chromatin proteins will lead to a better understanding of chromosomal organization and gene expression in Archaea and provide valuable information on the evolution of DNA packaging in cellular life.

Thermodynamics of nucleic acid "shape readout" by an aminosugar

Recognition of nucleic acids is important for our understanding of nucleic acid structure as well as for our understanding of nucleic acid-protein interactions. In addition to the direct readout mechanisms of nucleic acids such as H-bonding, shape recognition of nucleic acids is being increasingly recognized as playing an equally important role in DNA recognition. Competition dialysis, UV, flourescent intercalator displacement (FID), computational docking, and calorimetry studies were conducted to study the interaction of neomycin with a variety of nucleic acid conformations (shapes). At pH 5.5, the results suggest the following. (1) Neomycin binds three RNA structures [16S A site rRNA, poly(rA)·poly(rA), and poly(rA)·poly(rU)] with high affinities (K(a) ~ 10(7) M(-1)). (2) The binding of neomycin to A-form GC-rich oligomer d(A(2)G(15)C(15)T(2))(2) has an affinity comparable to those of RNA structures. (3) The binding of neomycin to DNA·RNA hybrids shows a 3-fold variance that can be attributed to their structural differences [for poly(dA)·poly(rU), K(a) = 9.4 × 10(6) M(-1), and for poly(rA)·poly(dT), K(a) = 3.1 × 10(6) M(-1)]. (4) The interaction of neomycin with DNA triplex poly(dA)·2poly(dT) yields a binding affinity (K(a)) of 2.4 × 10(5) M(-1). (5) Poly(dA-dT)(2) shows the lowest association constant for all nucleic acids studied (K(a) < 10(5)). (6) Neomycin binds to G-quadruplexes with K(a) values of ~10(4)-10(5) M(-1). (7) Computational studies show that the decrease in major groove width in the B to A transition correlates with increasing neomycin affinity. Neomycin's affinity for various nucleic acid structures can be ranked as follows: RNAs and GC-rich d(A(2)G(15)C(15)T(2))(2) structures > poly(dA)·poly(rU) > poly(rA)·poly(dT) > T·A-T triplex, G-quadruplex, B-form AT-rich, or GC-rich DNA sequences. The results illustrate the first example of a small molecule-based "shape readout" of different nucleic acid conformations.

E. coli SSB tetramer binds the first and second molecules of (dT)(35) with heat capacities of opposite sign

We have previously shown that formation of a 1:1 fully wrapped complex of Escherichia coli SSB tetramer with (dT)(70) displays a temperature-dependent sign reversal of the binding heat capacity (ΔC(P)). Here we examine SSB binding to shorter oligodeoxynucleotides ((dX)(35)) to probe whether this effect requires binding of one or two (dX)(35) molecules per SSB tetramer. We find that the ΔC(P) for the first molecule of (dX)(35) is always negative. However, a sign reversal of ΔC(P) from negative to positive occurs with increasing temperature for binding of the second (dX)(35). This striking behavior of ΔC(P) for the second (dX)(35) appears linked to conformational changes within the ssDNA-SSB complex that are required to form a fully wrapped (SSB)(65) binding mode. These results also underscore that binding heat capacities of macromolecular interactions have multiple origins that cannot be understood simply on the basis of examining static structures.

Molecular simulations on the thermal stabilization of DNA by hyperthermophilic chromatin protein Sac7d, and associated conformational transitions

Sac7d belongs to a family of chromosomal proteins, which are crucial for thermal stabilization of DNA at higher growth temperatures. It is capable of binding DNA nonspecifically, and is responsible for the increase in the melting temperature of DNA in the bound form up to 85 °C. Molecular dynamics (MD) simulations were performed at different temperatures on two protein-DNA complexes of Sac7d. Various structural and energetic parameters were calculated to examine the DNA stability and to investigate the conformational changes in DNA and the protein-DNA interactions. Room temperature simulations indicated very good agreement with the experimental structures. The protein structure is nearly unchanged at both 300 and 360 K, and only up to five base pairs of the DNA are stabilized by Sac7d at 360 K. However, the MD simulations on DNA alone systems show that they lose their helical structures at 360 K further supporting the role of Sac7d in stabilizing the oligomers. At higher temperatures (420 and 480 K), DNA undergoes denaturation in the presence and the absence of the protein. The DNA molecules were found to undergo B- to A-form transitions consistent with experimental studies, and the extent of these transitions are examined in detail. The extent of sampling B- and A-form regions was found to show temperature and sequence dependence. Multiple MD simulations yielded similar results validating the proposed model. Interaction energy calculations corresponding to protein-DNA binding indicates major contribution due to DNA backbone, explaining the nonspecific interactions of Sac7d.

Calorimetric and spectroscopic studies of aminoglycoside binding to AT-rich DNA triple helices

Calorimetric and fluorescence techniques were used to characterize the binding of aminoglycosides-neomycin, paromomycin, and ribostamycin, with 5'-dA(12)-x-dT(12)-x-dT(12)-3' intramolecular DNA triplex (x = hexaethylene glycol) and poly(dA).2poly(dT) triplex. Our results demonstrate the following features: (1) UV thermal analysis reveals that the T(m) for triplex decreases with increasing pH value in the presence of neomycin, while the T(m) for the duplex remains unchanged. (2) The binding affinity of neomycin decreases with increased pH, although there is an increase in observed binding enthalpy. (3) ITC studies conducted in two buffers (sodium cacodylate and MOPS) yield the number of protonated drug amino groups (Deltan) as 0.29 and 0.40 for neomycin and paromomycin interaction with 5'-dA(12)-x-dT(12)-x-dT(12)-3', respectively. (4) The specific heat capacity change (DeltaC(p)) determined by ITC studies is negative, with more negative values at lower salt concentrations. From 100 mM to 250 mM KCl, the DeltaC(p) ranges from -402 to -60 cal/(mol K) for neomycin. At pH 5.5, a more positive DeltaC(p) is observed, with a value of -98 cal/(mol K) at 100 mM KCl. DeltaC(p) is not significantly affected by ionic strength. (5) Salt dependence studies reveal that there are at least three amino groups of neomycin participating in the electrostatic interactions with the triplex. (6) FID studies using thiazole orange were used to derive the AC(50) (aminoglycoside concentration needed to displace 50% of the dye from the triplex) values. Neomycin shows a seven fold higher affinity than paromomycin and eleven fold higher affinity than ribostamycin at pH 6.8. (7) Modeling studies, consistent with UV and ITC results, show the importance of an additional positive charge in triplex recognition by neomycin. The modeling and thermodynamic studies indicate that neomycin binding to the DNA triplex depends upon significant contributions from charge as well as shape complementarity of the drug to the DNA triplex Watson-Hoogsteen groove.

Characterization of the ligand and DNA binding properties of a putative archaeal regulator ST1710

While a rich collection of bacterium-like regulating proteins has been identified in the archaeal genome, few of them have been studied at the molecular level. In this study, we characterized the ligand and DNA binding properties of a putative regulator ST1710 from the archaeon Sulfolobus tokodaii. ST1710 is homologous to the multiple-antibiotic resistance repressor (MarR) family bacterial regulators. The protein consists of a ligand binding site, partially overlapping with a winged helix-turn-helix DNA binding site. We characterized the interactions between ST1710 and three ligands, salicylate, carbonyl cyanide m-chlorophenylhydrazone (CCCP), and ethidium, which bind to bacterial MarRs. The binding affinities of the ligands for ST1710 were comparable to their affinities for the bacterial MarRs. The ligand binding was temperature sensitive and caused conformational changes in ST1710. To investigate the effect of ligand binding on the interaction between ST1710 and DNA, we fluorescently labeled a 47mer dsDNA (ST1) containing a putative ST1710 recognition site and determined the dissociation constant between ST1 and ST1710 using the fluorescence polarization method. The binding affinity almost doubled from 10 degrees C (Kd = 618 +/- 34 nM) to 30 degreesC (Kd = 334 +/- 15 nM), and again from 30 to 50 degrees C (Kd = 189 +/- 9 nM). This result suggests that under the natural living condition (80 degrees C) of S. tokodaii, the binding affinity might increase even further. The presence of CCCP and salicylate suppressed ST1710-ST1 interaction, indicating that ST1710 functioned as a repressor.

Analysis of cooperativity by isothermal titration calorimetry

Cooperative binding pervades Nature. This review discusses the use of isothermal titration calorimetry (ITC) in the identification and characterisation of cooperativity in biological interactions. ITC has broad scope in the analysis of cooperativity as it determines binding stiochiometries, affinities and thermodynamic parameters, including enthalpy and entropy in a single experiment. Examples from the literature are used to demonstrate the applicability of ITC in the characterisation of cooperative systems.

Artificial binding proteins (Affitins) as probes for conformational changes in secretin PulD

The DNA-binding protein Sac7d was previously modified to bind with high affinity to the N domain of the outer membrane secretin PulD from the bacterium Klebsiella oxytoca. Here, we show that binding of the Sac7d derivatives (affitins) to PulD is sensitive to conformational changes caused by denaturant and by the zwitterionic detergent Zwittergent 3-14 routinely used to extract secretins from outer membranes. This sensitivity to the conformational state of PulD allowed us to use the affitins as probes for the native structure of PulD and to devise protocols for examining in vitro synthesized protein in nonionic detergent and for the affinity purification of native PulD using affitins as ligands. When fused to periplasmic PhoA, three affitins inhibited PulD multimerization in vivo and caused loss of function. In two cases, this was likely to be due to dimerization of the affitin by the bound PhoA, as the effect was absent when the affitins were fused to monomeric MalE. In the third case, the MalE and PhoA moieties probably interfered sterically with PulD protomer interactions and, thereby, inhibited multimerization. None of the affitins tested interacted with PulD at sites of protomer interaction or blocked the secretin channel through which exoproteins cross the outer membrane in the Type II secretion pathway of which PulD is a key component.

Structure of the Staphylococcus aureus AgrA LytTR domain bound to DNA reveals a beta fold with an unusual mode of binding

The LytTR domain is a DNA-binding motif found within the AlgR/AgrA/LytR family of transcription factors that regulate virulence factor and toxin gene expression in pathogenic bacteria. This previously uncharacterized domain lacks sequence similarity with proteins of known structure. The crystal structure of the DNA-binding domain of Staphylococcus aureus AgrA complexed with a DNA pentadecamer duplex has been determined at 1.6 A resolution. The structure establishes a 10-stranded beta fold for the LytTR domain and reveals its mode of interaction with DNA. Residues within loop regions of AgrA contact two successive major grooves and the intervening minor groove on one face of the oligonucleotide duplex, inducing a substantial bend in the DNA. Loss of DNA binding upon substitution of key interacting residues in AgrA supports the observed binding mode. This mode of protein-DNA interaction provides a potential target for future antimicrobial drug design.

DNA binding mode transitions of Escherichia coli HU(alphabeta): evidence for formation of a bent DNA--protein complex on intact, linear duplex DNA

Escherichia coli HU(alphabeta), a major nucleoid-associated protein, organizes chromosomal DNA and facilitates numerous DNA transactions. Using isothermal titration calorimetry, fluorescence resonance energy transfer and a series of DNA lengths (8 bp, 15 bp, 34 bp, 38 bp and 160 bp) we established that HU(alphabeta) interacts with duplex DNA using three different nonspecific binding modes. Both the HU to DNA molar ratio ([HU]/[DNA]) and DNA length dictate the dominant HU binding mode. On sufficiently long DNA (> or =34 bp), at low [HU]/[DNA], HU populates a noncooperative 34 bp binding mode with a binding constant of 2.1+/-0.4x10(6) M(-1), and a binding enthalpy of +7.7+/-0.6 kcal/mol at 15 degrees C and 0.15 M Na(+). With increasing [HU]/[DNA], HU bound in the noncooperative 34 bp mode progressively converts to two cooperative (omega approximately 20) modes with site sizes of 10 bp and 6 bp. These latter modes exhibit smaller binding constants (1.1+/-0.2x10(5) M(-1) for the 10 bp mode, 3.5+/-1.4x10(4) M(-1) for the 6 bp mode) and binding enthalpies (4.2+/-0.3 kcal/mol for the 10 bp mode, -1.6+/-0.3 kcal/mol for the 6 bp mode). As DNA length increases to 34 bp or more at low [HU]/[DNA], the small modes are replaced by the 34 bp binding mode. Fluorescence resonance energy transfer data demonstrate that the 34 bp mode bends DNA by 143+/-6 degrees whereas the 6 bp and 10 bp modes do not. The model proposed in this study provides a novel quantitative and comprehensive framework for reconciling previous structural and solution studies of HU, including single molecule (force extension measurement), fluorescence, and electrophoretic gel mobility-shift assays. In particular, it explains how HU condenses or extends DNA depending on the relative concentrations of HU and DNA.

Prevalence of temperature-dependent heat capacity changes in protein-DNA interactions

A large, negative DeltaCp of DNA binding is a thermodynamic property of the majority of sequence-specific DNA-protein interactions, and a common, but not universal property of non-sequence-specific DNA binding. In a recent study of the binding of Taq polymerase to DNA, we showed that both the full-length polymerase and its "Klentaq" large fragment bind to primed-template DNA with significant negative heat capacities. Herein, we have extended this analysis by analyzing this data for temperature-variable heat capacity effects (DeltaDeltaCp), and have similarly analyzed an additional 47 protein-DNA binding pairs from the scientific literature. Over half of the systems examined can be easily fit to a function that includes a DeltaDeltaCp parameter. Of these, 90% display negative DeltaDeltaCp values, with the result that the DeltaCp of DNA binding will become more negative with rising temperature. The results of this collective analysis have potentially significant consequences for current quantitative theories relating DeltaCp values to changes in accessible surface area, which rely on the assumption of temperature invariance of the DeltaCp of binding. Solution structural data for Klentaq polymerase demonstrate that the observed heat capacity effects are not the result of a coupled folding event.

Microcalorimetry of biological macromolecules

The capabilities of contemporary differential scanning and isothermal titration microcalorimetry for studying the thermodynamics of protein unfolding/refolding and their association with partners, particularly target DNA duplexes, are considered. It is shown that the predenaturational changes of proteins must not be ignored in studying the thermodynamics of formation of their native structure and their complexes with partners, particularly their cognate DNA duplexes.

What drives proteins into the major or minor grooves of DNA?

The energetic profiles of a significant number of protein-DNA systems at 20 degrees C reveal that, despite comparable Gibbs free energies, association with the major groove is primarily an enthalpy-driven process, whereas binding to the minor groove is characterized by an unfavorable enthalpy that is compensated by favorable entropic contributions. These distinct energetic signatures for major versus minor groove binding are irrespective of the magnitude of DNA bending and/or the extent of binding-induced protein refolding. The primary determinants of their different energetic profiles appear to be the distinct hydration properties of the major and minor grooves; namely, that the water in the A+T-rich minor groove is in a highly ordered state and its removal results in a substantial positive contribution to the binding entropy. Since the entropic forces driving protein binding into the minor groove are a consequence of displacing water ordered by the regular arrangement of polar contacts, they cannot be regarded as hydrophobic.

Solution structure of the nonmethyl-CpG-binding CXXC domain of the leukaemia-associated MLL histone methyltransferase

Methylation of CpG dinucleotides is the major epigenetic modification of mammalian genomes, critical for regulating chromatin structure and gene activity. The mixed-lineage leukaemia (MLL) CXXC domain selectively binds nonmethyl-CpG DNA, and is required for transformation by MLL fusion proteins that commonly arise from recurrent chromosomal translocations in infant and secondary treatment-related acute leukaemias. To elucidate the molecular basis of nonmethyl-CpG DNA recognition, we determined the structure of the human MLL CXXC domain by multidimensional NMR spectroscopy. The CXXC domain has a novel fold in which two zinc ions are each coordinated tetrahedrally by four conserved cysteine ligands provided by two CGXCXXC motifs and two distal cysteine residues. We have identified the CXXC domain DNA binding interface by means of chemical shift perturbation analysis, cross-saturation transfer and site-directed mutagenesis. In particular, we have shown that residues in an extended surface loop are in close contact with the DNA. These data provide a template for the design of specifically targeted therapeutics for poor prognosis MLL-associated leukaemias.

Effects of monovalent anions on a temperature-dependent heat capacity change for Escherichia coli SSB tetramer binding to single-stranded DNA

We have previously shown that the linkage of temperature-dependent protonation and DNA base unstacking equilibria contribute significantly to both the negative enthalpy change (DeltaH(obs)) and the negative heat capacity change (DeltaC(p,obs)) for Escherichia coli SSB homotetramer binding to single-stranded (ss) DNA. Using isothermal titration calorimetry we have now examined DeltaH(obs) over a much wider temperature range (5-60 degrees C) and as a function of monovalent salt concentration and type for SSB binding to (dT)(70) under solution conditions that favor the fully wrapped (SSB)(65) complex (monovalent salt concentration >or=0.20 M). Over this wider temperature range we observe a strongly temperature-dependent DeltaC(p,obs). The DeltaH(obs) decreases as temperature increases from 5 to 35 degrees C (DeltaC(p,obs) <0) but then increases at higher temperatures up to 60 degrees C (DeltaC(p,obs) >0). Both salt concentration and anion type have large effects on DeltaH(obs) and DeltaC(p,obs). These observations can be explained by a model in which SSB protein can undergo a temperature- and salt-dependent conformational transition (below 35 degrees C), the midpoint of which shifts to higher temperature (above 35 degrees C) for SSB bound to ssDNA. Anions bind weakly to free SSB, with the preference Br(-) > Cl(-) > F(-), and these anions are then released upon binding ssDNA, affecting both DeltaH(obs) and DeltaC(p,obs). We conclude that the experimentally measured values of DeltaC(p,obs) for SSB binding to ssDNA cannot be explained solely on the basis of changes in accessible surface area (ASA) upon complex formation but rather result from a series of temperature-dependent equilibria (ion binding, protonation, and protein conformational changes) that are coupled to the SSB-ssDNA binding equilibrium. This is also likely true for many other protein-nucleic acid interactions.

Stability and flexibility in the structure of the hyperthermophile DNA-binding protein Sac7d

Sac7d is a chromatin protein from the hyperthermophile Sulfolobus acidocaldarius that severely kinks duplex DNA with negligible change in protein structure. In previous work, the overall stability of Sac7d has been well-characterized with a global analysis of the linkage of folding, protonation, and anion binding. We extend that work here with NMR measurements of global stability as well as the distribution of stability and flexibility in the solution structure. Native state amide hydrogen exchange has been used to identify the most-protected core amide protons which exchange through global unfolding. The pH and temperature dependence of stability defined by native state exchange is in excellent agreement with the free energy surface determined by a linkage analysis of the dependence of folding on pH, salt, and temperature. These results confirm that the deltaC(P) obtained from a Kirchhoff analysis of DSC data (i.e., deltaH vs Tm) is incorrect, and an accurate description of the protein stability curve for Sac7d requires a measure of the thermodynamic contributions of protonation and anion binding. Amide hydrogen exchange, along with generalized order parameters determined by 15N relaxation data, demonstrates considerable variation in stability throughout the structure with some of the least stable regions occurring at the N- and C-termini. The most stable and inflexible region of the backbone occurs primarily in the DNA-binding beta-sheet which is responsible for bending DNA.

Optimizing Experimental Parameters in Isothermal Titration Calorimetry

In isothermal titration calorimetry, the statistical precisions with which the equilibrium constant (K) and reaction enthalpy (DeltaH degrees ) can be estimated from data for 1:1 binding depend on a number of quantities, key among them being the products c identical with K[M](0) and h identical with DeltaH degrees [M](0), the stoichiometry range (R(m)(), ratio of total titrant X to total titrate M after the last injection), and the number of injections of titrant. A study of the statistical errors as functions of these quantities leads to the following prescription for optimizing throughput and precision: (1) Make 10 injections of titrant. (2) Set the concentrations in accord with the empirical equation R(m)() = 6.4/c(0.2) + 13/c (but no smaller than 1.1). (3) Make the starting concentration [M](0) as large as possible within the large-signal limits of the instrumentation but limited to c < 10(3) for estimating K. With this procedure, both K and [M](0) are predicted to have relative standard errors <1% over large ranges of K. Systematic errors in the concentrations, [X](0) and [M](0), are fully compensated by the "site number" or stoichiometry parameter (n). On the other hand, altering and freezing any of the fit parameters leads to a deterioration of the fit quality and to predictable changes in the other parameters. Fit divergence at very small c is avoidable through a simple redefinition of the fit parameters; however, unless n can be fixed from other information, DeltaH degrees may be statistically ill-defined in this region.

Solution structure, stability, and flexibility of Sso10a: a hyperthermophile coiled-coil DNA-binding protein

Sso10a is one of a number of DNA-binding proteins from the hyperthermophile Sulfolobus solfataricus that has been associated with DNA packaging and chromatin regulation. Sequence analysis indicates that it is a member of a conserved group of archaeal transcription regulators (COG3432). We have determined the solution structure of Sso10a and show that it is a homodimer of winged-helix DNA-binding domains. The dimer interface consists of an extended antiparallel coiled coil, with the globular DNA-binding domains positioned at opposite ends of a solvent-exposed coiled-coil rod. NMR structure refinement of the elongated structure benefited not only from the inclusion of residual dipolar couplings from partially aligned samples but also the influence of anisotropic rotational diffusion on heteronuclear relaxation. An analysis of backbone mobility using (15)N relaxation rates indicated that the overall tertiary and quaternary structure is largely inflexible on the nanosecond to picosecond time scale. Amide hydrogen exchange data demonstrated that the most stable region of the protein extends from the core of the winged helices into the coiled coil. The positions of the globular heads relative to the coiled coil in solution deviate only slightly from that observed in a crystal structure. The most significant difference between the solution and crystal structures occurs in the putative DNA-binding helix-turn-helix (HTH) motif. This is the region of lowest stability in solution and a point of protein-protein contact in the crystal. Alternative conformations of the HTH motif may permit adjustment of the structure for optimal DNA binding.

Effect of Mutation of the Sac7d Intercalating Residues on the Temperature Dependence of DNA Distortion and Binding Thermodynamics†

Sac7d is a small chromatin protein from the hyperthermophile Sulfolobus acidocaldarius which kinks duplex DNA by approximately 66 degrees at a single base pair step with intercalation of V26 and M29 side chains. Site-directed mutagenesis coupled with calorimetric and spectroscopic data has been used to characterize the influence of the intercalating side chains on the structure and thermodynamics of the DNA complex from 5 to 85 degrees C. Two single-alanine substitutions (V26A and M29A) and five double-glycine, -alanine, -leucine, -phenylalanine, and -tryptophan substitutions of the surface residues have been created. NMR and fluorescence titrations indicated that the substitutions had little effect on the structure of the protein or DNA binding site size. Each of the mutant proteins demonstrated a temperature-dependent binding enthalpy which was correlated with a similar temperature dependence in the structure of the complex reflected by changes in fluorescence and circular dichroism. A positive heat capacity change (DeltaC(p)) for DNA binding was observed for only those mutants which also demonstrated a thermotropic structural transition in the complex, and the temperature range for the positive DeltaC(p) coincided with that observed for the structural transition. The thermodynamic data are interpreted using a model in which binding is linked to an endothermic distortion of the DNA in the complex. The results support the proposal that the unfavorable enthalpy of binding of Sac7d at 25 degrees C is due in part to the distortion of DNA.

Role of a Surface Tryptophan in Defining the Structure, Stability, and DNA Binding of the Hyperthermophile Protein Sac7d,

Sac7d is a small, chromatin protein from Sulfolobus acidocaldarius which induces a sharp kink in DNA with intercalation of valine and methionine side chains. The crystal structure of the protein-DNA complex indicates that a surface tryptophan (W24) plays a key role in DNA binding by hydrogen bonding to the DNA at the kink site. We show here that substitution of the solvent-exposed tryptophan with alanine (W24A) led to a significant loss in not only DNA binding affinity but also protein stability. The W24A substitution proved to be one of the most destabilizing surface substitutions in Sac7d. A global linkage analysis of the pH and salt dependence of stability indicated that the protein stability surface (DeltaG vs temperature, pH, and salt concentration) was lowered overall by 2 kcal/mol (from 0 to 100 degrees C, pH 0 to 7, and 0 to 0.3 M KCl). The lower free energy of unfolding could not be attributed to significant structural perturbations of surface electrostatic interactions. Residual dipolar coupling of partially aligned protein and the NMR solution structure of W24A confirmed that the surface substitution resulted in no significant change in structure. Stabilization of this hyperthermophile protein and its DNA complex by a surface cluster of hydrophobic residues involving W24 and the two intercalating side chains is discussed.