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Biomedical subjects

R T Clubb

Publications and source records attributed to R T Clubb.

At least 19 recordsLinked to original sources
PubMed2001

Structure of sortase, the transpeptidase that anchors proteins to the cell wall of Staphylococcus aureus.

Surface proteins of Gram-positive bacteria play important roles during the pathogenesis of human infections and require sortase for anchoring to the cell-wall envelope. Sortase cleaves surface proteins at the LPXTG motif and catalyzes the formation of an amide bond between the carboxyl group of threonine (T) and the amino group of cell-wall crossbridges. The NMR structure of sortase reveals a unique beta-barrel structure, in which the active-site sulfhydryl of cysteine-184 is poised for ionization by histidine-120, presumably enabling the resultant thiolate to attack the LPXTG peptide. Calcium binding near the active site stimulates catalysis, possibly by altering the conformation of a surface loop that recognizes newly translocated polypeptides. The structure suggests a mechanistic relationship to the papain/cathepsin proteases and should facilitate the design of new antiinfective agents.

Aminoacyltransferases↗
PubMed2001

An efficient NMR experiment for analyzing sugar-puckering in unlabeled DNA: application to the 26-kDa dead ringer-DNA complex.

We present a new NMR experiment for estimating the type and degree of sugar-puckering in high-molecular-weight unlabeled DNA molecules. The experiment consists of a NOESY sequence preceded by a constant-time scalar coupling period. Two subexperiments are compared, each differing in the amount of time the (3)J(H3'H2') and (3)J(H3'H2") couplings are active on the H3' magnetization. The resultant data are easy to analyze, since a comparison of the signal intensities of any resolved NOE cross peak originating from H3' atoms of the duplex can be used to estimate the sum of the (3)J(H3'H2') and (3)J(H3'H2") couplings and thus the puckering type of the deoxyribose ring. Isotope filters to eliminate signals of the (13)C-labeled component in the F1-dimension are implemented, facilitating analyses of high-molecular-weight protein-DNA complexes containing (13)C-labeled protein and unlabeled DNA. The utility of the experiment is demonstrated on the 26-kDa Dead Ringer protein-DNA complex and reveals that the DNA uniformly adopts the S-type configuration when bound to protein.

Carbon Isotopes↗
PubMed2001

Improved NMR spectra of a protein-DNA complex through rational mutagenesis and the application of a sensitivity optimized isotope-filtered NOESY experiment.

The NMR spectra of the complex between the DNA-binding domain of the Dead ringer protein (DRI-DBD, Gly262-Gly398) and its DNA binding site (DRI-DBD:DNA, 26 kDa) have been optimized by biochemical and spectroscopic means. First, we demonstrate the utility of a modified 2D [F1,F2] 13C-filtered NOESY experiment that employs a 1J(HC) versus chemical shift optimized adiabatic 13C inversion pulse [Zwahlen, C. et al. (1997) J. Am. Chem. Soc., 119, 6711-6721]. The new sequence is shown to be more sensitive than previously published pulse schemes (up to 40% in favorable cases) and its utility is demonstrated using two protein-DNA complexes. Second, we demonstrate that the targeted replacement of an interfacial aromatic residue in the DRI-DBD:DNA complex substantially reduces line broadening within its NMR spectra. The spectral changes are dramatic, salvaging a protein-DNA complex that was originally ill suited for structural analysis by NMR. This biochemical approach is not a general method, but may prove useful in the spectral optimization of other protein complexes that suffer from interfacial line broadening caused by dynamic changes in proximal aromatic rings.

Binding Sites↗
PubMed2001

The Mu repressor-DNA complex contains an immobilized 'wing' within the minor groove.

We have determined the solution structure of the complex between the 'winged-helix' enhancer binding domain of the Mu repressor protein and its cognate DNA site. The structure reveals an unusual use for the 'wing' which becomes immobilized upon DNA binding where it makes intermolecular hydrogen bond contacts deep within the minor groove. Although the wing is mobile in the absence of DNA, it partially negates the large entropic penalty associated with its burial by maintaining a small degree of structural order in the DNA-free state. Extensive contacts are also formed between the recognition helix and the DNA, which reads the major groove of a highly conserved region of the binding site through a single base-specific hydrogen bond and van der Waals contacts.

Amino Acid Sequence↗
PubMed2000

Major groove recognition by three-stranded beta-sheets: affinity determinants and conserved structural features.

We present the results of a rational mutagenesis and binding-affinity study of the three-stranded beta-sheet-DNA interface in the complex formed by the amino-terminal DNA-binding domain of the Tn916 integrase protein and its cognate binding site. The relative importance of interfacial contacts present in its NMR-derived solution structure have been tested through mutagenesis, fluorescence anisotropy, and intrinsic quenching DNA-binding assays. We find that seven protein-DNA hydrogen bonds (two base-specific and five to phosphate groups) significantly contribute to the level of affinity. These interactions span the entire DNA-binding surface on the protein, but primarily originate from residues in only two strands of the sheet and loop L2. Interestingly, we show that highly populated, precisely defined intermolecular hydrogen bonds in the ensemble of conformers are invariably important for DNA-binding, implying that NMR-derived solution structures provide direct insight into the energetics of recognition. Unusual three-stranded beta-sheet-DNA interfaces have recently been discovered in three unrelated protein-DNA complexes. A comparative analysis of these structures reveals similar sheet positioning, the presence of two invariant interfacial contacts to the phosphodiester backbone, and two semi-conserved base-specific hydrogen bonds. Two of these conserved contacts significantly contribute to the affinity of the integrase-DNA complex, suggesting that the three-stranded beta-sheet DNA-binding motif exhibits conserved principles of recognition.

Amino Acid Motifs↗
PubMed1999

Solution structure of the DNA binding domain from Dead ringer, a sequence-specific AT-rich interaction domain (ARID).

The Dead ringer protein from Drosophila melanogaster is a transcriptional regulatory protein required for early embryonic development. It is the founding member of a large family of DNA binding proteins that interact with DNA through a highly conserved domain called the AT-rich interaction domain (ARID). The solution structure of the Dead ringer ARID (residues Gly262-Gly398) was determined using NMR spectroscopy. The ARID forms a unique globular structure consisting of eight alpha-helices and a short two-stranded anti-parallel beta-sheet. Amino acid sequence homology indicates that ARID DNA binding proteins are partitioned into three structural classes: (i) minimal ARID proteins that consist of a core domain formed by six alpha-helices; (ii) ARID proteins that supplement the core domain with an N-terminal alpha-helix; and (iii) extended-ARID proteins, which contain the core domain and additional alpha-helices at their N- and C-termini. Studies of the Dead ringer-DNA complex suggest that the major groove of DNA is recognized by a helix-turn-helix (HTH) motif and the adjacent minor grooves are contacted by a beta-hairpin and C-terminal alpha-helix. Primary homology suggests that all ARID-containing proteins contact DNA through the HTH and hairpin structures, but only extended-ARID proteins supplement this binding surface with a terminal helix.

Amino Acid Sequence↗
PubMed1999

NMR structure and functional studies of the Mu repressor DNA-binding domain.

The repressor protein of bacteriophage Mu establishes and maintains lysogeny by shutting down transposition functions needed for phage DNA replication. It interacts with several repeated DNA sequences within the early operator, preventing transcription from two divergent promoters. It also directly represses transposition by competing with the MuA transposase for an internal activation sequence (IAS) that is coincident with the operator and required for efficient transposition. The transposase and repressor proteins compete for the operator/IAS region using homologous DNA-binding domains located at their amino termini. Here we present the solution structure of the amino-terminal DNA-binding domain from the repressor protein determined by heteronuclear multidimensional nuclear magnetic resonance spectroscopy. The structure of the repressor DNA-binding domain provides insights into the molecular basis of several temperature sensitive mutations and, in combination with complementary experiments using flourescence anisotropy, surface plasmon resonance, and circular dichroism, defines the structural and biochemical differences between the transposase and repressor DNA-binding modules. We find that the repressor and enhancer domains possess similar three-dimensional structures, thermostabilities, and intrinsic affinities for DNA. This latter result suggests that the higher affinity of the full-length repressor relative to that of the MuA transposase protein originates from cooperative interactions between repressor protomers and not from intrinsic differences in their DNA-binding domains. In addition, we present the results of nucleotide and amino acid mutagenesis which delimits the minimal repressor DNA-binding module and coarsely defines the nucleotide dependence of repressor binding.

Amino Acid Sequence↗
PubMed1999

NMR structure of the Tn916 integrase-DNA complex.

The integrase protein catalyzes the excision and integration of the Tn916 conjugative transposon, a promiscuous genetic element that spreads antibiotic resistance in pathogenic bacteria. The solution structure of the N-terminal domain of the Tn916 integrase protein bound to its DNA-binding site within the transposon arm has been determined. The structure reveals an interesting mode of DNA recognition, in which the face of a three-stranded antiparallel beta-sheet is positioned within the major groove. A comparison to the structure of the homing endonuclease I-Ppol-DNA complex suggests that the three-stranded sheet may represent a new DNA-binding motif whose residue composition and position within the major groove are varied to alter specificity. The structure also provides insights into the mechanism of conjugative transposition. The DNA in the complex is bent approximately 35 degrees and may, together with potential interactions between bound integrase proteins at directly repeated sites, significantly bend the arms of the transposon.

Binding Sites↗
PubMed1998

Site-specific DNA binding using a variation of the double stranded RNA binding motif.

The integrase family of site-specific recombinases catalyze a diverse array of DNA rearrangements in archaebacteria, eubacteria and yeast. The solution structure of the DNA binding domain of the integrase protein from the conjugative transposon Tn916 has been determined using NMR spectroscopy. The structure provides the first insights into distal site DNA binding by a site-specific integrase and reveals that the N-terminal domain is structurally similar to the double stranded RNA binding domain (dsRBD). The results of chemical shift mapping experiments suggest that the integrase protein interacts with DNA using residues located on the face of its three stranded beta-sheet. This surface differs from the proposed RNA binding surface in dsRBDs, suggesting that different surfaces on the same protein fold can be used to bind DNA and RNA.

Binding Sites↗
PubMed1997

Solution structure of the Mu end DNA-binding ibeta subdomain of phage Mu transposase: modular DNA recognition by two tethered domains.

The phage Mu transposase (MuA) binds to the ends of the Mu genome during the assembly of higher order nucleoprotein complexes. We investigate the structure and function of the MuA end-binding domain (Ibetagamma). The three-dimensional solution structure of the Ibeta subdomain (residues 77-174) has been determined using multidimensional NMR spectroscopy. It comprises five alpha-helices, including a helix-turn-helix (HTH) DNA-binding motif formed by helices 3 and 4, and can be subdivided into two interacting structural elements. The structure has an elongated disc-like appearance from which protrudes the recognition helix of the HTH motif. The topology of helices 2-4 is very similar to that of helices 1-3 of the previously determined solution structure of the MuA Igamma subdomain and to that of the homeodomain family of HTH DNA-binding proteins. We show that each of the two subdomains binds to one half of the 22 bp recognition sequence, Ibeta to the more conserved Mu end distal half (beta subsite) and Igamma to the Mu end proximal half (gamma subsite) of the consensus Mu end-binding site. The complete Ibetagamma domain binds the recognition sequence with a 100- to 1000-fold higher affinity than the two subdomains independently, indicating a cooperative effect. Our results show that the Mu end DNA-binding domain of MuA has a modular organization, with each module acting on a specific part of the 22 bp binding site. Based on the present binding data and the structures of the Ibeta and Igamma subdomains, a model for the interaction of the complete Ibetagamma domain with DNA is proposed.

Amino Acid Sequence↗
PubMed1997

Solution structure of the I gamma subdomain of the Mu end DNA-binding domain of phage Mu transposase.

The MuA transposase of phase Mu is a large modular protein that plays a central role in transposition. We show that the Mu end DNA-binding domain, I beta gamma, which is responsible for binding the DNA attachment sites at each end of the Mu genome, comprises two subdomains, I beta and I gamma, that are structurally autonomous and do not interact with each other in the absence of DNA. The solution structure of the I gamma subdomain has been determined by multidimensional NMR spectroscopy. The structure of I gamma comprises a four helix bundle and, despite the absence of any significant sequence identity, the topology of the first three helices is very similar to that of the homeodomain family of helix-turn-helix DNA-binding proteins. The helix-turn-helix motif of I gamma, however, differs from that of the homeodomains in so far as the loop is longer and the second helix is shorter, reminiscent of that in the POU-specific domain.

Amino Acid Sequence↗
PubMed1996

The wing of the enhancer-binding domain of Mu phage transposase is flexible and is essential for efficient transposition.

A tetramer of the Mu transposase (MuA) pairs the recombination sites, cleaves the donor DNA, and joins these ends to a target DNA by strand transfer. Juxtaposition of the recombination sites is accomplished by the assembly of a stable synaptic complex of MuA protein and Mu DNA. This initial critical step is facilitated by the transient binding of the N-terminal domain of MuA to an enhancer DNA element within the Mu genome (called the internal activation sequence, IAS). Recently we solved the three-dimensional solution structure of the enhancer-binding domain of Mu phage transposase (residues 1-76, MuA76) and proposed a model for its interaction with the IAS element. Site-directed mutagenesis coupled with an in vitro transposition assay has been used to assess the validity of the model. We have identified five residues on the surface of MuA that are crucial for stable synaptic complex formation but dispensable for subsequent events in transposition. These mutations are located in the loop (wing) structure and recognition helix of the MuA76 domain of the transposase and do not seriously perturb the structure of the domain. Furthermore, in order to understand the dynamic behavior of the MuA76 domain prior to stable synaptic complex formation, we have measured heteronuclear 15N relaxation rates for the unbound MuA76 domain. In the DNA free state the backbone atoms of the helix-turn-helix motif are generally immobilized whereas the residues in the wing are highly flexible on the pico- to nanosecond time scale. Together these studies define the surface of MuA required for enhancement of transposition in vitro and suggest that a flexible loop in the MuA protein required for DNA recognition may become structurally ordered only upon DNA binding.

Bacteriophage mu↗
PubMed1995

Demonstration of positionally disordered water within a protein hydrophobic cavity by NMR.

The presence and location of water of hydration (that is, bound water) in the solution structure of human interleukin-1 beta (hIL-1 beta) was investigated with water-selective two-dimensional heteronuclear magnetic resonance spectroscopy. It is shown here that in addition to water at the surface of the protein and ordered internal water molecules involved in bridging hydrogen bonds, positionally disordered water is present within a large, naturally occurring hydrophobic cavity located at the center of the molecule. These water molecules of hydration have residency times in the range of 1 to 2 nanoseconds to 100 to 200 microseconds and can be readily detected by nuclear magnetic resonance (NMR). Thus, large hydrophobic cavities in proteins may not be truly empty, as analysis of crystal structures appears to show, but may contain mobile water molecules that are crystallographically invisible but detectable by NMR.

Electrochemistry↗
PubMed1995

Backbone dynamics of the oligomerization domain of p53 determined from 15N NMR relaxation measurements.

The backbone dynamics of the tetrameric p53 oligomerization domain (residues 319-360) have been investigated by two-dimensional inverse detected heteronuclear 1H-15N NMR spectroscopy at 500 and 600 MHz. 15N T1, T2, and heteronuclear NOEs were measured for 39 of 40 non-proline backbone NH vectors at both field strengths. The overall correlation time for the tetramer, calculated from the T1/T2 ratios, was found to be 14.8 ns at 35 degrees C. The correlation times and amplitudes of the internal motions were extracted from the relaxation data using the model-free formalism (Lipari G, Szabo A, 1982, J Am Chem Soc 104:4546-4559). The internal dynamics of the structural core of the p53 oligomerization domain are uniform and fairly rigid, with residues 327-354 exhibiting an average generalized order parameter (S2) of 0.88 +/- 0.08. The N- and C-termini exhibit substantial mobility and are unstructured in the solution structure of p53. Residues located at the N- and C-termini, in the beta-sheet, in the turn between the alpha-helix and beta-sheet, and at the C-terminal end of the alpha-helix display two distinct internal motions that are faster than the overall correlation time. Fast internal motions (< or = 20 ps) are within the extreme narrowing limit and are of uniform amplitude. The slower motions (0.6-2.2 ns) are outside the extreme narrowing limit and vary in amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer Graphics↗
PubMed1994

A novel class of winged helix-turn-helix protein: the DNA-binding domain of Mu transposase.

BACKGROUND: Mu transposase (MuA) is a multidomain protein encoded by the bacteriophage Mu genome. It is responsible for translocation of the Mu genome, which is the largest and most efficient transposon known. While the various domains of MuA have been delineated by means of biochemical methods, no data have been obtained to date relating to its tertiary structure. RESULTS: We have solved the three-dimensional solution structure of the DNA-binding domain (residues 1-76; MuA76) of MuA by multidimensional heteronuclear NMR spectroscopy. The structure consists of a three-membered alpha-helical bundle buttressed by a three-stranded antiparallel beta-sheet. Helices H1 and H2 and the seven-residue turn connecting them comprise a helix-turn-helix (HTH) motif. In addition, there is a long nine-residue flexible loop or wing connecting strands B2 and B3 of the sheet. NMR studies of MuA76 complexed with a consensus DNA site from the internal activation region of the Mu genome indicate that the wing and the second helix of the HTH motif are significantly perturbed upon DNA binding. CONCLUSIONS: While the general appearance of the DNA-binding domain of MuA76 is similar to that of other winged HTH proteins, the connectivity of the secondary structure elements is permuted. Hence, the fold of MuA76 represents a novel class of winged HTH DNA-binding domain.

Bacteriophage mu↗