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L P McIntosh

Publications and source records attributed to L P McIntosh.

At least 37 records · Page 2Linked to original sources
PubMed1998

Assigning the NMR spectra of aromatic amino acids in proteins: analysis of two Ets pointed domains.

The measurement of interproton nuclear Overhauser enhancements (NOEs) and dihedral angle restraints of aromatic amino acids is a critical step towards determining the structure of a protein. The complete assignment of the resonances from aromatic rings and the subsequent resolution and identification of their associated NOEs, however, can be a difficult task. Shown here is a strategy for assigning the 1H, 13C, and 15N signals from the aromatic side chains of histidine, tryptophan, tyrosine, and phenylalanine using a suite of homo- and hetero-nuclear scalar and NOE correlation experiments, as well as selective deuterium isotope labelling. In addition, a comparison of NOE information obtained from homonuclear NOE spectroscopy (NOESY) and 13C-edited NOESY-heteronuclear single quantum correlation experiments indicates that high-resolution homonuclear two-dimensional NOESY spectra of selectively deuterated proteins are invaluable for obtaining distance restraints to the aromatic residues.

Amino Acids, Cyclic↗
PubMed1997

High-resolution NMR structure and backbone dynamics of the Bacillus subtilis response regulator, Spo0F: implications for phosphorylation and molecular recognition.

NMR has been employed for structural and dynamic studies of the bacterial response regulator, Spo0F. This 124-residue protein is an essential component of the sporulation phosphorelay signal transduction pathway in Bacillus subtilis. Three-dimensional 1H, 15N, and 13C experiments have been used to obtain full side chain assignments and the 1511 distance, 121 dihedral angle, and 80 hydrogen bonding restraints required for generating a family of structures (14 restraints per residue). The structures give a well-defined (alpha/beta)5 fold for residues 4-120 with average rms deviations of 0.59 A for backbone heavy atoms and 1.02 A for all heavy atoms. Analyses of backbone 15N relaxation measurements demonstrate relative rigidity in most regions of regular secondary structure with a generalized order parameter (S2) of 0.9 +/- 0.05 and a rotational correlation time (taum) of 7.0 +/- 0.5 ns. Loop regions near the site of phosphorylation have higher than average rms deviation values and T1/T2 ratios suggesting significant internal motion or chemical exchange at these sites. Additionally, multiple conformers are observed for the beta4-alpha4 loop and beta-strand 5 region. These conformers may be related to structural changes associated with phosphorylation and also indicative of the propensity this recognition surface has for differential protein interactions. Comparison of Spo0F structural features to those of other response regulators reveals subtle differences in the orientations of secondary structure in the putative recognition surfaces and the relative charge distribution of residues surrounding the site of phosphorylation. These may be important in providing specificity for protein-protein interactions and for determining the lifetimes of the phosphorylated state.

Bacillus subtilis↗
PubMed1997

Complete measurement of the pKa values of the carboxyl and imidazole groups in Bacillus circulans xylanase.

Electrostatic interactions in proteins can be dissected experimentally by determining the pKa values of their constituent ionizable amino acids. To complement previous studies of the glutamic acid and histidine residues in Bacillus circulans xylanase (BCX), we have used NMR methods to measure the pKa s of the seven aspartic acids and the C-terminus of this protein. The pKa s of these carboxyls are all less than the corresponding values observed with random coil polypeptides, indicating that their ionization contributes favorably to the stability of the folded enzyme. In general, the aspartic acids with the most reduced pKa s are those with limited exposure to the solvent and a high degree of conservation among homologous xylanases. Most dramatically, Asp 83 and Asp 101 have pKa s < 2 and thus remain deprotonated in native BCX under all conditions examined. Asp 83 is completely buried, forming a strong salt bridge with Arg 136. In contrast, Asp 101 is located on the surface of the protein, stabilized in the deprotonated form by an extensive network of hydrogen bonds involving an internal water molecule and the neutral side-chain and main-chain atoms of Ser 100 and Thr 145. These data provide a complete experimental database for theoretical studies of the ionization behavior of BCX under acidic conditions.

Aspartic Acid↗
PubMed1996

Structure of the N-terminal cellulose-binding domain of Cellulomonas fimi CenC determined by nuclear magnetic resonance spectroscopy.

Multidimensional heteronuclear nuclear magnetic resonance (NMR) spectroscopy was used to determine the tertiary structure of the 152 amino acid N-terminal cellulose-binding domain from Cellulomonas fimi 1,4-beta-glucanase CenC (CBDN1). CBDN1 was studied in the presence of saturating concentrations of cellotetraose, but due to spectral overlap, the oligosaccharide was not included in the structure calculations. A total of 1705 interproton nuclear Overhauser effect (NOE), 56 phi, 88 psi, 42 chi 1, 9 chi 2 dihedral angle, and 88 hydrogen-bond restraints were used to calculate 25 final structures. These structures have a rmsd from the average of 0.79 +/- 0.11 A for all backbone atoms excluding disordered termini and 0.44 +/- 0.05 A for residues with regular secondary structures. CBDN1 is composed of 10 beta-strands, folded into two antiparallel beta-sheets with the topology of a jelly-roll beta-sandwich. The strands forming the face of the protein previously determined by chemical shift perturbations to be responsible for cellooligosaccharide binding [Johnson, P. E., Tomme, P., Joshi, M. D., & McIntosh, L. P. (1996) Biochemistry 35, 13895-13906] are shorter than those forming the opposite side of the protein. This results in a 5-stranded binding cleft, containing a central strip of hydrophobic residues that is flanked on both sides by polar hydrogen-bonding groups. The presence of this cleft provides a structural explanation for the unique selectivity of CBDN1 for amorphous cellulose and other soluble oligosaccharides and the lack of binding to crystalline cellulose. The tertiary structure of CBDN1 is strikingly similar to that of the bacterial 1,3-1,4-beta-glucanases, as well as other sugar-binding proteins with jelly-roll folds.

Actinomycetales↗
PubMed1996

Interaction of soluble cellooligosaccharides with the N-terminal cellulose-binding domain of Cellulomonas fimi CenC 2. NMR and ultraviolet absorption spectroscopy.

The N-terminal cellulose-binding domain (CBDN1) from Cellulomonas fimi beta-1,4-glucanase CenC binds amorphous but not crystalline cellulose. To investigate the structural and thermodynamic bases of cellulose binding, NMR and difference ultraviolet absorbance spectroscopy were used in parallel with calorimetry (Tomme, P., Creagh, A. L., Kilburn, D. G., & Haynes, C. A., (1996) Biochemistry 35, 13885-13894) to characterize the interaction of soluble cellooligosaccharides with CBDN1. Association constants, determined from the dependence of the amide 1H and 15N chemical shifts of CBDN1 upon added sugar, increase from 180 +/- 60 M-1 for cellotriose to 4,200 +/- 720 M-1 for cellotetraose, 34,000 +/- 7,600 M-1 for cellopentaose, and an estimate of 50,000 M-1 for cellohexaose. This implies that the CBDN1 cellulose-binding site spans approximately five glucosyl units. On the basis of the observed patterns of amide chemical shift changes, the cellooligosaccharides bind along a five-stranded beta-sheet that forms a concave face of the jelly-roll beta-sandwich structure of CBDN1. This beta-sheet contains a strip of hydrophobic side chains flanked on both sides by polar residues. NMR and difference ultraviolet absorbance measurements also demonstrate that tyrosine, but not tryptophan, side chains may be involved in oligosaccharide binding. These results lead to a model in which CBDN1 interacts with soluble cellooligosaccharides and, by inference, with single polysaccharide chains in regions of amorphous cellulose, primarily through hydrogen bonding to the equatorial hydroxyl groups of the pyranose rings. Van der Waals stacking of the sugar rings against the apolar side chains may augment binding. CBDN1 stands in marked contrast to previously characterized CBDs that absorb to crystalline cellulose via a flat binding surface dominated by exposed aromatic rings.

Actinomycetales↗
PubMed1996

The pKa of the general acid/base carboxyl group of a glycosidase cycles during catalysis: a 13C-NMR study of bacillus circulans xylanase.

The 20 kDa xylanase from Bacillus circulans carries out hydrolysis of xylan via a two-step mechanism involving a covalent glycosyl-enzyme intermediate. In this double-displacement reaction, Glu78 functions as a nucleophile to form the intermediate, while Glu172 acts as a general acid catalyst during glycosylation, protonating the departing aglycone, and then as a general base during deglycosylation, deprotonating the attacking water. The dual role of Glu172 places specific demands upon its ionization states and hence pKa values. 13C-NMR titrations of xylanase, labeled with [delta-13C]glutamic acid, have revealed pKa values of 4.6 and 6.7 for Glu78 and Glu172, respectively. These agree well with the apparent pKa values obtained from a study of the pH dependence of kcat/Km and demonstrate that, at the enzyme's pH optimum of 5.7, the nucleophile Glu78 is deprotonated and the general acid Glu172 initially protonated. Remarkably, the pKa for Glu172 drops to 4.2 in a trapped covalent glycosyl-enzyme intermediate, formed by reaction with 2', 4'-dinitrophenyl 2-deoxy-2-fluoro-beta-xylobioside [Miao et al. (1994) Biochemistry 33, 7027-7032]. A similar pKa is measured for Glu172 when a glutamine is present at position 78. This large decrease in pKa of approximately 2.5 units is consistent with the role of Glu172 as a general base catalyst in the deglycosylation step and appears to be a consequence of both reduced electrostatic repulsion due to neutralization of Glu78 and a conformational change in the protein. Such "pKa cycling" during catalysis is likely to be a common phenomenon in glycosidases.

Amino Acid Sequence↗
PubMed1996

Solution structure of the ETS domain from murine Ets-1: a winged helix-turn-helix DNA binding motif.

Ets-1 is the prototypic member of the ets family of transcription factors. This family is characterized by the conserved ETS domain that mediates specific DNA binding. Using NMR methods, we have determined the structure of a fragment of murine Ets-1 composed of the 85 residue ETS domain and a 25 amino acid extension that ends at its native C-terminus. The ETS domain folds into a helix-turn-helix motif on a four-stranded anti-parallel beta-sheet scaffold. This structure places Ets-1 in the winged helix-turn-helix (wHTH) family of DNA binding proteins and provides a model for interpreting the sequence conservation of the ETS domain and the specific interaction of Ets-1 with DNA. The C-terminal sequence of Ets-1, which is mutated in the v-Ets oncoprotein, forms an alpha-helix that packs anti-parallel to the N-terminal helix of the ETS domain. In this position, the C-terminal helix is poised to interact directly with an N-terminal inhibitory region in Ets-1 as well as the wHTH motif. This explains structurally the concerted role of residues flanking the ETS domain in the intramolecular inhibition of Ets-1 DNA binding.

Amino Acid Sequence↗
PubMed1996

Structural coupling of the inhibitory regions flanking the ETS domain of murine Ets-1.

Several members of the ets gene family of transcription factors show negative regulation of DNA binding by intramolecular interactions. A structural mechanism for this auto-inhibition is investigated using a 161-residue N-terminal deletion mutant of murine Ets-1, Ets-1 delta N280. This protein shows a similar reduced affinity for DNA as native Ets-1 because it contains the ETS domain in context of the flanking amino- and carboxy-terminal regions that together mediate repression of DNA binding. The secondary structure of Ets-1 delta N280 was determined using NMR chemical shift, NOE, J coupling, and amide hydrogen exchange information. In addition to the winged helix-turn-helix ETS domain, Ets-1 delta N280 contains two alpha-helices in the amino-terminal inhibitory region and one alpha-helix in the carboxy-terminal inhibitory region. Chemical shift comparisons were made between this protein and an activated form of Ets-1 lacking the amino-terminal inhibitory region. The spectral differences demonstrate that the amino- and carboxy-terminal inhibitory sequences are structurally coupled to one another, thus explaining the observation that both regions are required for the repression of DNA binding. Furthermore, these data show that the inhibitory sequences also interact directly with the first helix of the intervening ETS domain, thereby providing a pathway for the repression of DNA binding. These results lead to a model of an inhibitory module in Ets-1 composed of both the amino- and carboxy-terminal regions interfaced with the ETS domain. This establishes the structural framework for understanding the intramolecular inhibition of Ets-1 DNA binding.

Amino Acid Sequence↗
PubMed1996

Secondary structure and NMR assignments of Bacillus circulans xylanase.

Bacillus circulans xylanase (BCX) is a member of the family of low molecular weight endo-beta-(1,4)-xylanases. The main-chain 1H, 13C, and 15N resonances of this 20.4-kDa enzyme were assigned using heteronuclear NMR experiments recorded on a combination of selectively and uniformly labeled protein samples. Using chemical shift, NOE, J coupling, and amide hydrogen exchange information, 14 beta-strands, arranged in a network of three beta-sheets, and a single alpha-helix were identified in BCX. The NMR-derived secondary structure and beta-sheet topology agree closely with that observed in the crystal structure of this protein. The HN of Ile 118 has a strongly upfield-shifted resonance at 4.03 ppm, indicative of a potential amide-aromatic hydrogen bond to the indole ring of Trp 71. This interaction, which is conserved in all low molecular weight xylanases of known structure, may play an important role in establishing the active site conformation of these enzymes. Following hen egg white and bacteriophage T4 lysozymes, B. circulans xylanase represents the third family of beta-glycanases for which extensive NMR assignments have been reported. These assignments provide the background for detailed studies of the mechanism of carbohydrate recognition and hydrolysis by this bacterial xylanase.

Amino Acid Sequence↗
PubMed1996

Probing the role of tryptophan residues in a cellulose-binding domain by chemical modification.

The cellulose-binding domain (CBDCex) of the mixed function glucanase-xylanase Cex from Cellulomonas fimi contains five tryptophans, two of which are located within the beta-barrel structure and three exposed on the surface (Xu GY et al., 1995, Biochemistry 34:6993-7009). Although all five tryptophans can be oxidized by N-bromosuccinimide (NBS), stopped-flow measurements show that three tryptophans react faster than the other two. NMR analysis during the titration of CBDCex with NBS shows that the tryptophans on the surface of the protein are fully oxidized before there is significant reaction with the two buried tryptophans. Additionally, modification of the exposed tryptophans does not affect the conformation of the backbone of CBDCex, whereas complete oxidation of all five tryptophans denatures the polypeptide. The modification of the equivalent of one and two tryptophans by NBS reduces binding of CBDCex to cellulose by 70% and 90%, respectively. This confirms the direct role of the exposed aromatic residues in the binding of CBDCex to cellulose. Although adsorption to cellulose does afford some protection against NBS, as evidenced by the increased quantity of NBS required to oxidize all of the tryptophan residues, the polypeptide can still be oxidized completely when adsorbed. This suggests that, whereas the binding appears to be irreversible overall [Ong E et al., 1989, Bio/Technology 7:604-607], each of the exposed tryptophans interacts reversibly with cellulose.

Binding Sites↗
PubMed1996

Characterization of a buried neutral histidine residue in Bacillus circulans xylanase: NMR assignments, pH titration, and hydrogen exchange.

Bacillus circulans xylanase contains two histidines, one of which (His 156) is solvent exposed, whereas the other (His 149) is buried within its hydrophobic core. His 149 is involved in a network of hydrogen bonds with an internal water and Ser 130, as well as a potential weak aromatic-aromatic interaction with Tyr 105. These three residues, and their network of interactions with the bound water, are conserved in four homologous xylanases. To probe the structural role played by His 149, NMR spectroscopy was used to characterize the histidines in BCX. Complete assignments of the 1H, 13C, and 15N resonances and tautomeric forms of the imidazole rings were obtained from two-dimensional heteronuclear correlation experiments. An unusual spectroscopic feature of BCX is a peak near 12 ppm arising from the nitrogen bonded 1H epsilon 2 of His 149. Due to its solvent inaccessibility and hydrogen bonding to an internal water molecule, the exchange rate of this proton (4.0 x 10(-5) s-1 at pH*7.04 and 30 degrees C) is retarded by > 10(6)-fold relative to an exposed histidine. The pKa of His 156 is unperturbed at approximately 6.5, as measured from the pH dependence of the 15N- and 1H-NMR spectra of BCX. In contrast, His 149 has a pKa < 2.3, existing in the neutral N epsilon 2H tautomeric state under all conditions examined. BCX unfolds at low pH and 30 degrees C, and thus His 149 is never protonated significantly in the context of the native enzyme. The structural importance of this buried histidine is confirmed by the destablizing effect of substituting a phenylalanine or glutamine at position 149 in BCX.

Bacillus↗
PubMed1995

Modulation of transcription factor Ets-1 DNA binding: DNA-induced unfolding of an alpha helix.

Conformational changes, including local protein folding, play important roles in protein-DNA interactions. Here, studies of the transcription factor Ets-1 provided evidence that local protein unfolding also can accompany DNA binding. Circular dichroism and partial proteolysis showed that the secondary structure of the Ets-1 DNA-binding domain is unchanged in the presence of DNA. In contrast, DNA allosterically induced the unfolding of an alpha helix that lies within a flanking region involved in the negative regulation of DNA binding. These findings suggest a structural basis for the intramolecular inhibition of DNA binding and a mechanism for the cooperative partnerships that are common features of many eukaryotic transcription factors.

Base Sequence↗
PubMed1995

Structural features of the epsilon subunit of the Escherichia coli ATP synthase determined by NMR spectroscopy.

The tertiary fold of the epsilon subunit of the Escherichia coli F1F0 ATPsynthase (ECF1F0) has been determined by two- and three-dimensional heteronuclear (13C, 15N) NMR spectroscopy. The epsilon subunit exhibits a distinct two domain structure, with the N-terminal 84 residues of the protein forming a 10-stranded beta-structure, and with the C-terminal 48 amino acids arranged as two alpha-helices running antiparallel to one another (two helix hairpin). The beta-domain folds as a beta-sandwich with a hydrophobic interior between the two layers of the sandwich. The C-terminal two-helix hairpin folds back to the N-terminal domain and interacts with one side of the beta-domain. The arrangement of the epsilon subunit in the intact F1F0 ATP synthase involves interaction of the two helix hairpin with the F1 part, and binding of the open side of the beta-sandwich to the c subunits of the membrane-embedded F0 part.

Amino Acid Sequence↗
PubMed1994

Secondary structure of the ETS domain places murine Ets-1 in the superfamily of winged helix-turn-helix DNA-binding proteins.

The members of the ets gene family of transcription factors are characterized by a conserved 85-residue DNA-binding region, termed the ETS domain, that lacks sequence homology to structurally characterized DNA-binding motifs. The secondary structure of the ETS domain of murine Ets-1 was determined on the basis of NMR chemical shifts, NOE and J-coupling constraints, amide hydrogen exchange, circular dichroism, and FT-IR spectroscopy. The ETS domain is composed of three alpha-helices (H) and four beta-strands (S) arranged in the order H1-S1-S2-H2-H3-S3-S4. The four-stranded antiparallel beta-sheet is the scaffold for a putative helix-turn-helix DNA recognition motif formed by helices 2 and 3. The 25 residues extending beyond the ETS domain to the native C-terminus of the truncated Ets-1 also contain a helical segment. On the basis of the similarity of this topology with that of catabolite activator protein (CAP), heat shock factor (HSF), and hepatocyte nuclear factor (HNF-3 gamma), we propose that ets proteins are members of the superfamily of winged helix-turn-helix DNA-binding proteins.

Amino Acid Sequence↗
PubMed1993

Perturbation of Trp 138 in T4 lysozyme by mutations at Gln 105 used to correlate changes in structure, stability, solvation, and spectroscopic properties.

In order to correlate between spectroscopic and structural changes in a protein, the environment of Trp 135 in T4 lysozyme was deliberately perturbed by the replacement of Gln 105 with alanine (Q105A), glycine (Q105G), and glutamic acid (Q105E). In wild-type lysozyme, Trp 135 is buried, but the indole nitrogen is hydrogen-bonded to the side-chain of Gln 105. In the Q105G and Q105A mutant structures, the indole nitrogen becomes accessible to solvent. Crystallographic analysis shows that the structures of all of the mutants are similar to wild-type. There are, however, distinct rearrangements of the local solvent structure in response to the new side-chains. There are also small but significant changes in the relative orientations of the two domains of the protein that appear to result from a series of small, concerted movements of side-chains adjacent to residue 105. Evaluation of the fluorescence and phosphorescence of the mutant proteins in terms of their observed three-dimensional structures shows that large spectral changes do not necessarily imply large changes in structure or in static solvent accessibility. Increases in polar relaxation about the excited state of tryptophan may be the result of only small increases in local dynamics or solvent exposure. 1H-NMR was also used to monitor the effects of the substitutions on Trp 138. In Q105E, but not in Q105G, Q105A and WT, the H epsilon chemical shift of Trp 138 is very pH-dependent, apparently reflecting the titration of Glu 105 which has a spectroscopically determined pKa of 6.0. The elevation of the pKa of Glu 105 in Q105E is also reflected in the pH dependence of the stability of this mutant.

Bacteriophage T4↗
PubMed1990

Assignment of the backbone 1H and 15N NMR resonances of bacteriophage T4 lysozyme.

The proton and nitrogen (15NH-H alpha-H beta) resonances of bacteriophage T4 lysozyme were assigned by 15N-aided 1H NMR. The assignments were directed from the backbone amide 1H-15N nuclei, with the heteronuclear single-multiple-quantum coherence (HSMQC) spectrum of uniformly 15N enriched protein serving as the master template for this work. The main-chain amide 1H-15N resonances and H alpha resonances were resolved and classified into 18 amino acid types by using HMQC and 15N-edited COSY measurements, respectively, of T4 lysozymes selectively enriched with one or more of alpha-15N-labeled Ala, Arg, Asn, Asp, Gly, Gln, Glu, Ile, Leu, Lys, Met, Phe, Ser, Thr, Trp, Tyr, or Val. The heteronuclear spectra were complemented by proton DQF-COSY and TOCSY spectra of unlabeled protein in H2O and D2O buffers, from which the H beta resonances of many residues were identified. The NOE cross peaks to almost every amide proton were resolved in 15N-edited NOESY spectra of the selectively 15N enriched protein samples. Residue specific assignments were determined by using NOE connectivities between protons in the 15NH-H alpha-H beta spin systems of known amino acid type. Additional assignments of the aromatic proton resonances were obtained from 1H NMR spectra of unlabeled and selectively deuterated protein samples. The secondary structure of T4 lysozyme indicated from a qualitative analysis of the NOESY data is consistent with the crystallographic model of the protein.

Amino Acid Sequence↗
PubMed1990

Secondary structure of a leucine zipper determined by nuclear magnetic resonance spectroscopy.

Previous work has shown that a synthetic peptide corresponding to the leucine zipper region of the yeast transcriptional activator GCN4 forms a stable dimer of alpha-helices and that the helices are oriented in a parallel manner. Two-dimensional nuclear magnetic resonance spectroscopy (NMR) is used here to demonstrate that the helix is continuous for at least 32 of the 33 residues in the peptide. The results also indicate that the dimer is symmetric. It is therefore unlikely that the interdigitation model for the structure of leucine zippers is correct, since interdigitation of leucine residues in a parallel dimer would lead to an asymmetric structure. The data are consistent with a coiled-coil structure.

Amino Acid Sequence↗