Biomedical subjects

I D Campbell

Publications and source records attributed to I D Campbell.

At least 19 recordsLinked to original sources

PubMed2003

Studies of protein-ligand interactions by NMR.

Solution-state NMR has become an accepted method for studying the structure of small proteins in solution. This has resulted in over 3000 NMR-based co-ordinate sets being deposited in the Protein Databank. It is becoming increasingly apparent, however, that NMR is also a very powerful tool for accessing interactions between macromolecules and various ligands. These interactions can be assessed at a wide variety of levels, e.g. qualitative screening of libraries of pharmaceuticals and 'chemical shift mapping'. Dissociation constants can sometimes be obtained in such cases. Another example would be the complete three-dimensional structure determination of a protein-ligand complex. Here we briefly describe a few of the principles involved and illustrate the method with recent examples.

Bacterial Proteins↗

PubMed2003

Modular proteins at the cell surface.

Many proteins, including cell-surface receptors, extracellular matrix (ECM) proteins and intracellular signalling systems, are constructed from a relatively small number of domains or modules. Modularity provides biological systems with a convenient way of presenting binding sites on a stable protein scaffold, in the correct position for function; it also allows regulation by module rearrangement. Knowledge about modular proteins is increasing rapidly because of good databases and more systematic approaches to protein expression and structure determination. There have been a number of important recent structures of modular proteins at the cell surface, including the low-density-lipoprotein receptor, epidermal growth factor receptor and an integrin. These and other studies show how the main task of structural biology has moved from determination of module structures to the task of assessing how modular proteins are regulated and how they bind their various ligands. These aspects are illustrated here by recent studies of modular proteins carried out in our laboratory and elsewhere. Examples will include studies of ECM proteins, such as fibronectin, and proteins associated with focal adhesion complexes that involve fibronectin, integrins and various intracellular modular proteins, such as focal adhesion kinase, Src family kinases, talin and paxillin.

Cytoplasm↗

PubMed2001

NMR studies of the interactions of SpoIIAA with its partner proteins that regulate sporulation in Bacillus subtilis.

SpoIIAA is a key component in the network of interactions that regulate the first sporulation-specific transcription factor, sigma(F), in Bacillus subtilis. In its unphosphorylated form SpoIIAA is either phosphorylated by or forms a non-covalent complex with SpoIIAB, whereas in its phosphorylated form it is dephosphorylated by SpoIIE. In this work we present NMR studies of the SpoIIAA(2).SpoIIAB complex and of mutant proteins that are deficient in their ability to interact with SpoIIAB or SpoIIE. The NMR studies of the SpoIIAA(2).SpoIIAB complex allowed us to define a contiguous patch that is perturbed upon complex formation. By examining the chemical shift perturbations in the mutant proteins we have identified more specific areas that contain residues critical for the SpoIIAB and SpoIIE interactions.

Adenylyl Imidodiphosphate↗

PubMed2001

The eighth FIII domain of human fibronectin promotes integrin alpha5beta1 binding via stabilization of the ninth FIII domain.

Binding of the extracellular matrix molecule fibronectin to the integrin receptor alpha(5)beta(1) elicits downstream signaling pathways that modulate cell function. Fibronectin-alpha(5)beta(1) interaction occurs via the conserved RGD sequence in the tenth FIII (FIII10) domain of fibronectin. A synergistic site containing the sequence PHSRN in the adjacent FIII9 domain has also been identified. Here we investigate the function of the eighth FIII domain in integrin-mediated cell adhesion using a wide range of methods, including biochemical, biological, and biophysical assays of integrin binding, cell adhesion, and protein denaturation. Mutation of the FIII9 synergistic site (PHSRN to PHAAA) in FIII9-10 reduced the binding activity for integrin alpha(5)beta(1) to levels observed for FIII10 alone, but the corresponding mutant in FIII8-9-10 showed no loss of binding activity. Cell adhesion assays also demonstrated enhanced functional activity of constructs containing FIII8. Equilibrium chemical denaturation studies indicated that FIII8 confers conformational stability upon FIII9, but only if the exposed loops, PHSRN and VKNEED on FIII9 and FIII8, respectively, are intact. These results demonstrate that the loss of integrin binding activity, observed upon alteration of the PHSRN synergistic site of FIII9-10, results partly from a loss of conformational stability of FIII9. Our data suggest a mechanism for integrin alpha(5)beta(1)-fibronectin interaction, which in addition to the primary RGD binding event, involves a conformation-sensitive scanning by the integrin for accessible sites on the ligand, whereupon full activation of downstream signaling occurs.

Binding Sites↗

PubMed2001

NMR analysis of structure and dynamics of the cytosolic tails of integrin alpha IIb beta 3 in aqueous solution.

The structural and dynamic properties of the cytosolic tails of the adhesion receptor integrin alphaIIbbeta3, fused to a coiled-coil construct via (Gly)(3) linkers, were studied in aqueous solution by nuclear magnetic resonance (NMR) spectroscopy. Both tails were largely flexible and unstructured, although, in the beta3 tail, residues Arg(724)-Ala(735) have a propensity to form a helical structure and residues Asn(744)-Tyr(747) (NPLY) have a propensity to adopt reverse-turn conformations. The mutation beta3(Y747A) disrupted this reverse-turn tendency and markedly reduced the affinity of the head domain of the cytoskeletal protein, talin for the beta3 tail. Omission of the (Gly)(3) linker connecting the coiled-coiled helices and the integrin tails lead to helix propagation into the beta3 tail extending up to eight residues. A variety of different tail constructs were made and studied to reveal tail-tail interactions, but surprisingly no significant interactions between both tails could be detected within the context of our constructs. These results provide structural insight into a highly conserved beta tail motif (NPXY/F) required for integrin signaling and highlight a second transiently structured region (residues Arg(724)-Ala(735)), which might also be of functional significance.

Amino Acid Sequence↗

PubMed2001

Binding of a peptide from a Streptococcus dysgalactiae MSCRAMM to the N-terminal F1 module pair of human fibronectin involves both modules.

Host invasion by a number of pathogenic bacteria such as staphylococci and streptococci involves binding to fibronectin, a ubiquitous extracellular matrix protein. On the bacterial side, host extracellular matrix adherence is mediated by MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) which, in some cases, have been identified to be important virulence factors. In this study we used nuclear magnetic resonance spectroscopy to characterize the interaction of B3, a synthetic peptide derived from an adhesin of Streptococcus dysgalactiae, with the N-terminal module pair 1F12F1 of human fibronectin. 1F12F1 chemical shift changes occurring on formation of the 1F12F1/B3 complex indicate that both modules bind to the peptide and that a similar region of each module is involved. A similar surface of the 4F15F1 module pair had previously been identified as the binding site for a fibronectin-binding peptide from Staphylococcus aureus.

Adhesins, Bacterial↗

PubMed2001

A membrane-distal segment of the integrin alpha IIb cytoplasmic domain regulates integrin activation.

Previous evidence suggests that interactions between integrin cytoplasmic domains regulate integrin activation. We have constructed and validated recombinant structural mimics of the heterodimeric alpha(IIb)beta(3) cytoplasmic domain. The mimics elicited polyclonal antibodies that recognize a combinatorial epitope(s) formed in mixtures of the alpha(IIb) and beta(3) cytoplasmic domains but not present in either isolated tail. This epitope(s) is present within intact alpha(IIb)beta(3), indicating that interaction between the tails can occur in the native integrin. Furthermore, the combinatorial epitope(s) is also formed by introducing the activation-blocking beta(3)(Y747A) mutation into the beta(3) tail. A membrane-distal heptapeptide sequence in the alpha(IIb) tail ((997)RPPLEED) is responsible for this effect on beta(3). Membrane-permeant palmitoylated peptides, containing this alpha(IIb) sequence, specifically blocked alpha(IIb)beta(3) activation in platelets. Thus, this region of the alpha(IIb) tail causes the beta(3) tail to resemble that of beta(3)(Y747A) and suppresses activation of the integrin.

Amino Acid Sequence↗

PubMed2001

The hairpin structure of the (6)F1(1)F2(2)F2 fragment from human fibronectin enhances gelatin binding.

The solution structure of the (6)F1(1)F2(2)F2 fragment from the gelatin-binding region of fibronectin has been determined (Protein Data Bank entry codes 1e88 and 1e8b). The structure reveals an extensive hydrophobic interface between the non-contiguous (6)F1 and (2)F2 modules. The buried surface area between (6)F1 and (2)F2 ( approximately 870 A(2)) is the largest intermodule interface seen in fibronectin to date. The dissection of (6)F1(1)F2(2)F2 into the (6)F1(1)F2 pair and (2)F2 results in near-complete loss of gelatin-binding activity. The hairpin topology of (6)F1(1)F2(2)F2 may facilitate intramolecular contact between the matrix assembly regions flanking the gelatin-binding domain. This is the first high-resolution study to reveal a compact, globular arrangement of modules in fibronectin. This arrangement is not consistent with the view that fibronectin is simply a linear 'string of beads'.

Binding Sites↗

PubMed2001

The role of the Src homology 3-Src homology 2 interface in the regulation of Src kinases.

The regulatory fragment of Src kinases, comprising Src homology (SH) 3 and SH2 domains, is responsible for controlled repression of kinase activity. We have used a multidisciplinary approach involving crystallography, NMR, and isothermal titration calorimetry to study the regulatory fragment of Fyn (FynSH32) and its interaction with a physiological activator: a fragment of focal adhesion kinase that contains both phosphotyrosine and polyproline motifs. Although flexible, the preferred disposition of SH3 and SH2 domains in FynSH32 resembles the inactive forms of Hck and Src, differing significantly from LckSH32. This difference, which results from variation in the SH3-SH2 linker sequences, will affect the potential of the regulatory fragments to repress kinase activity. This surprising result implies that the mechanism of repression of Src family members may vary, explaining functional distinctions between Fyn and Lck. The interaction between FynSH32 and focal adhesion kinase is restricted to the canonical SH3 and SH2 binding sites and does not affect the dynamic independence of the two domains. Consequently, the interaction shows no enhancement by an avidity effect. Such an interaction may have evolved to gain specificity through an extended recognition site while maintaining rapid dissociation after signaling.

Amino Acid Sequence↗

PubMed2001

Solution structure of the LDL receptor EGF-AB pair: a paradigm for the assembly of tandem calcium binding EGF domains.

BACKGROUND: From the observed structure and sequence of a pair of calcium binding (cb) epidermal growth factor-like (EGF) domains from human fibrillin-1, we proposed that many tandem cbEGF domains adopt a conserved relative conformation. The low-density lipoprotein receptor (LDLR), which is functionally unrelated to fibrillin-1, contains a single pair of EGF domains that was chosen for study in the validation of this hypothesis. The LDLR is the protein that is defective in familial hypercholesterolaemia, a common genetic disorder that predisposes individuals to cardiovascular complications and premature death. RESULTS: Here, we present the solution structure of the first two EGF domains from the LDL receptor, determined using conventional NMR restraints and residual dipolar couplings. The cbEGF domains have an elongated, rod-like arrangement, as predicted. The new structure allows a detailed assessment of the consequences of mutations associated with familial hypercholesterolaemia to be made. CONCLUSIONS: The validation of the conserved arrangement of EGF domains in functionally distinct proteins has important implications for structural genomics, since multiple tandem cbEGF pairs have been identified in many essential proteins that are implicated in human disease. Our results provide the means to use homology modeling to probe structure-function relationships in this diverse family of proteins and may hold the potential for the design of novel diagnostics and therapies in the future.

Amino Acid Sequence↗

PubMed2001

NMR studies of the sporulation protein SpoIIAA: implications for the regulation of the transcription factor sigmaF in Bacillus subtilis.

SpoIIAA participates in a four-component mechanism for phosphorylation-dependent transcription control at the outset of sporulation. We report the refinement of the solution structure of SpoIIAA by using the automated iterative NOE assignment method ARIA. To complement the structural data, the protein dynamics were determined by measuring the T1, T2 and NOE of the backbone 15N-nuclei. The refined structure permits a discussion of the structural features that are important for the function of SpoIIAA in the regulation of the sporulation sigma factor sigmaF, and for homologous regulatory pathways present in B. subtilis and in other bacilli.

Bacillus subtilis↗

PubMed2001

Bacillus subtilis mutations that alter the pathway of phosphorylation of the anti-anti-sigmaF factor SpoIIAA lead to a Spo- phenotype.

Sigma-F, the first sporulation-specific transcription factor of Bacillus subtilis, is regulated by an anti-sigma factor SpoIIAB, which can also act as a protein kinase that phosphorylates the anti-anti-sigma factor SpoIIAA. The time course of phosphorylation reaction is biphasic, a fact that has been interpreted in terms of a mechanism for sequestering SpoIIAB away from sigmaF and thus allowing activation of sigmaF when needed. Site-directed mutagenesis of SpoIIAA has allowed us to isolate two mutants that cannot activate sigmaF and which are therefore Spo-. The two mutant SpoIIAA proteins, SpoIIAAL61A and SpoIIAAL90A, are phosphorylated with linear kinetics; in addition they are less able to form the stable non-covalent complex that wild-type SpoIIAA makes with SpoIIAB in the presence of ADP. The phosphorylated form of SpoIIAAL90A was hydrolysed by the specific phosphatase SpoIIE at the same rate as wild-type SpoIIAA-P, but the rate of hydrolysis of SpoIIAAL61A-P was much slower. The secondary structure and the global fold of the mutant proteins were unchanged from the wild type. The results are interpreted in terms of a model for the wild type in which SpoIIAB, after phosphorylating SpoIIAA, is released in a form that is tightly bound to ADP and which then makes a ternary complex with an unreacted SpoIIAA. We propose that it is the inability to make this ternary complex that deprives the mutant cells of a means of keeping SpoIIAB from inhibiting sigmaF.

Adenosine Diphosphate↗

PubMed2000

The conformation of calreticulin is influenced by the endoplasmic reticulum luminal environment.

In order to understand the dynamics of the endoplasmic reticulum (ER) luminal environment, we investigated the role of Ca(2+), Zn(2+), and ATP on conformational changes of calreticulin. Purified calreticulin was digested with trypsin in the presence or absence of Ca(2+), Zn(2+), and ATP. At low Ca(2+) concentration (<100 micrometer), calreticulin is rapidly and fully degraded by trypsin, indicating that under these conditions the protein is in a highly trypsin-susceptible conformation. Increasing Ca(2+) concentration up to 500 micrometer or 1 mm resulted in protection of the full-length calreticulin and in generation of the 27-kDa fragment highly resistant to trypsin digestion. The 27-kDa protease-resistant core of the protein represented the NH(2)-terminal half of calreticulin and was identified by its reactivity with specific antibodies and by NH(2)-terminal amino acid sequence analysis. Ca(2+)-dependent changes in calreticulin's sensitivity to proteolysis indicate that agonist-induced fluctuation in the free ER luminal Ca(2+) concentration may affect the protein conformation and function. Trypsin digestion of calreticulin in the presence of Zn(2+) resulted in the formation of a 17-kDa central protease-resistant core in the protein corresponding to the central region of the protein, indicating that under these conditions the N- and C-domains of the protein are in an extended conformation. Here we also show that calreticulin is an ATP-binding protein but that it does not contain detectable ATPase activity. Digestion of the protein with trypsin in the presence of Mg(2+)-ATP protects the full-length protein. These results indicate that calreticulin may undergo frequent, ion-induced conformation changes, which may affect its function and its ability to interact with other proteins in the lumen of the ER.

Adenosine Triphosphate↗

PubMed2000

Interface characterization of the type II module pair from fibronectin.

The lone (1)F2(2)F2 modular pair of fibronectin is found in the collagen-binding region. This exclusive localization suggests the (1)F2(2)F2 pair plays an important role in the recognition of collagen. However, no information is currently available about the interaction between the two F2 modules and, thus, the orientation of their putative collagen-binding sites with respect to one another. Comparison of a variety of high-resolution NMR parameters from the F2 modules in isolation and the (1)F2(2)F2 pair was used to establish the extent of interaction between the F2 modules in the pair. Chemical shifts of the F2 modules and the (1)F2(2)F2 pair indicate that the structures of the modules are preserved in the pair and that, with the exception of the covalent linkage, they do not interact. (15)N NMR relaxation data identify significant motion occurring in the linker region of the (1)F2(2)F2 pair, and analyses of the anisotropic diffusion properties of the (1)F2(2)F2 pair are consistent with the modules in the F2 pair tumbling independent of one another.

Binding Sites↗

PubMed2000

Localization and characterization of the hyaluronan-binding site on the link module from human TSG-6.

BACKGROUND: The interactions of hyaluronan (HA) with proteins are important in extracellular matrix integrity and leukocyte migration and are usually mediated by a domain termed a Link module. Although the tertiary structure of a Link module has been determined, the molecular basis of HA-protein interactions remains poorly understood. RESULTS: Isothermal titration calorimetry was used to characterize the interaction of the Link module from human TSG-6 (Link_TSG6) with HA oligosaccharides of defined length (HA(4)-HA(16)). All oligomers bound (except HA(4)) with K(d) values ranging from 0.2-0.5 microM at 25 degrees C. The reaction is exothermic with a favourable entropy and the thermodynamic profile is similar to those of other glycosaminoglycan-protein interactions. The HA(8) recognition site on Link_TSG6 was localized by comparing nuclear magnetic resonance (NMR) spectra from a 1:1 complex with free protein. Residues perturbed on HA binding include both amino acids that are likely to be directly involved in the interaction (i.e., Lys11, Tyr59, Asn67, Phe70, Lys72 and Tyr78) and those affected by a ligand-induced conformational change in the beta4/beta5 loop. The sidechain of Asn67 becomes more rigid in the complex suggesting that it is in close proximity to the binding site. CONCLUSIONS: In TSG-6 a single Link module is sufficient for a high-affinity interaction with HA. The HA-binding surface on Link_TSG6 is found in a similar position to that suggested previously for CD44, indicating that its location might be conserved across the Link module superfamily. Here we find no evidence for the involvement of linear sequence motifs in HA binding.

Amino Acid Motifs↗

PubMed2000

Drosophila dumpy is a gigantic extracellular protein required to maintain tension at epidermal-cuticle attachment sites.

BACKGROUND: Growth and morphogenesis during development depend both on patterning genes, which assign positional information, and on genes that regulate mechanical forces. The dumpy gene of the fruit fly Drosophila melanogaster is an example of the latter class, with mutant phenotypes affecting size and shape of the limbs, thoracic cuticle, trachea and mouthparts. RESULTS: The genetically complex dumpy locus was found to span over 100 kb and encode a gigantic 2.5 MDa extracellular matrix protein. Dumpy represents an extreme form of modular protein evolution, containing 308 epidermal growth factor (EGF) modules, interspersed with a new module class, DPY, and terminating in a crosslinking zona pellucida domain and membrane anchor sequence. We determined the three-dimensional structure of the DPY module by nuclear magnetic resonance (NMR) spectroscopy and found that it forms a disulphide-stabilised beta sheet motif, capable of linking end-to-end with EGF modules to form a fibre. Consistent with its cuticle phenotypes, dumpy is expressed at several sites of cuticle-epidermal cell attachment, including the trachea and the muscle tendon cells, which mediate anchorage of the muscles to the cuticle. CONCLUSIONS: The dumpy gene encodes a gigantic extracellular molecule that we predict to be a membrane-anchored fibre of almost a micrometer in length. Insertion and crosslinking of this fibre within the cuticle may provide a strong anchor for the underlying tissue, allowing it to maintain mechanical tension at sites under stress. This would explain its contribution to tissue morphogenesis through its regulation of mechanical properties.

Amino Acid Sequence↗

PubMed2000

Identification of residues involved in the interaction of Staphylococcus aureus fibronectin-binding protein with the (4)F1(5)F1 module pair of human fibronectin using heteronuclear NMR spectroscopy.

Many pathogenic Gram-positive bacteria express cell surface proteins that bind to components of the extracellular matrix. This paper describes studies of the interaction between ligand binding repeats (D3 and D1-D4) of a fibronectin-binding protein from Staphylococcus aureus with a module pair ((4)F1(5)F1) from the N-terminal region of fibronectin. When D3 was added to isotope-labeled (4)F1(5)F1, (1)H, (15)N, and (13)C NMR chemical shift changes indicate that binding is primarily via residues in (4)F1, although a few residues in (5)F1 are also affected. Both hydrophobic and electrostatic interactions appear to be involved. The NMR data indicate that part of the D3 repeat converts from a disordered to a more ordered, extended conformation on binding to (4)F1(5)F1. In further NMR experiments, selective reduction of the intensity of D1-D4 resonances was observed on binding to (4)F1(5)F1, consistent with previous suggestions that in each of D1, D2, and D3 repeats, the main fibronectin binding site is in the C-terminal region of the repeat. In D1-D4, these regions also appear to go from a disordered to a more ordered conformation of fibronectin binding. Although the regions of the two proteins which interact had been previously identified, the findings presented here identify, for the first time, the specific residues in both proteins that are likely to be involved in the interaction.

Adhesins, Bacterial↗

PubMed2000

Backbone dynamics of a cbEGF domain pair in the presence of calcium.

Calcium binding (cb) epidermal growth factor-like (EGF) domains are found in a wide variety of extracellular proteins with diverse functions. In several proteins, including the fibrillins (1 and 2), the low-density lipoprotein receptor, the Notch receptor and related molecules, these domains are organised as multiple tandem repeats. The functional importance of calcium-binding by EGF domains has been underscored by the identification of missense mutations associated with defective calcium-binding, which have been linked to human diseases. Here, we present (15)N backbone relaxation data for a pair of cbEGF domains from fibrillin-1, the defective protein in the Marfan syndrome. The data were best fit using a symmetric top model, confirming the extended conformation of the cbEGF domain pair. Our data demonstrate that calcium plays a key role in stabilising the rigidity of the domain pair on the pico- to millisecond time-scale. Strikingly, the most dynamically stable region of the construct is centred about the domain interface. These results provide important insight into the properties of intact fibrillin-1, the consequences of Marfan syndrome causing mutations, and the ultrastructure of fibrillins and other extracellular matrix proteins.

Amino Acid Sequence↗