Implications of structural and thermodynamic studies of HLA-B27 subtypes exhibiting differential association with ankylosing spondylitis.

Structural and thermodynamic properties of HLA-B27 molecules provide the basis for their function within the immune system and are probably also central for the understanding of the pathology of HLA-B27-associated diseases such as ankolysing spondylitis (AS). Several HLA-B27 alleles are AS-associated, whereas some are not, although the protein encoded by the former may differ in only a single amino acid exchange from those specified by the latter. This indicates that subtype-specific polymorphic residues play a key role in determining whether an HLA-B27 subtype is AS-associated or not and open the possibility to correlate structural, thermodynamic and functional characteristics ofa given subtype with the disease association. Our studies involved X-ray crystallography and various other biophysical techniques to examine how several different peptides are accommodated within the binding groove of the molecules. The HLA-B*2705 and HLA-B*2709 subtypes, whose products differ in only a single amino acid residue of their heavy chains from each other, were primarily chosen for these analyses, but our studies have recently also been extended to the closely related subtypes HLA-B*2703, HLA-B*2704 and HLA-B*2706. The analyses reveal that structural and thermodynamic differences between HLA-B27 complexes may exist, depending on the peptide that is displayed. Furthermore, aviralpeptide and two self-peptides were found that exhibit HLA-B27 subtype-dependent molecular mimicry, thereby providing a molecular basis to account for the subtype-dependent presence of autoreactive T-cells. Although these results do not exclude other theories for the pathogenesis of AS, they support the arthritogenic peptide hypothesis which envisages molecular mimicry between HLA-B27-presented foreign and self-peptides to explain the cross-reactivity of autoreactive T-cells that are found in HLA-B*2705-positive individuals, in particular when they suffer from AS.

Early stages of misfolding and association of beta2-microglobulin: insights from infrared spectroscopy and dynamic light scattering.

Conformational changes associated with the assembly of recombinant beta 2-microglobulin in vitro under acidic conditions were investigated using infrared spectroscopy and static and dynamic light scattering. In parallel, the morphology of the different aggregated species obtained under defined conditions was characterized by electron microscopy. The initial salt-induced aggregate form of beta 2-microglobulin, composed of small oligomers (dimers to tetramers), revealed the presence of beta-strands organized in an intramolecular-like fashion. Further particle growth was accompanied by the formation of intermolecular beta-sheet structure and led to short curved forms. An increase in temperature by only 25 degrees C was able to disaggregate these assemblies, followed by the formation of longer filamentous structures. In contrast, a rise in temperature up to 100 degrees C was associated with a reorganization of the short curved forms at the level of secondary structure and the state of assembly, leading to a species with a characteristic infrared spectrum different from those of all the other aggregates observed before, suggesting a unique overall structure. The infrared spectral features of this species were nearly identical to those of beta 2-microglobulin assemblies formed at low ionic strength with agitation, indicating the presence of fibrils, which was confirmed by electron microscopy. The observed spectroscopic changes suggest that the heat-triggered conversion of the short curved assemblies into fibrils involves a reorganization of the beta-strands from an antiparallel arrangement to a parallel arrangement, with the latter being characteristic of amyloid fibrils of beta 2-microglobulin.

Molecular determinants of major histocompatibility complex class I complex stability: shaping antigenic features through short and long range electrostatic interactions.

A single amino acid exchange between the major histocompatibility complex molecules HLA-B(*)2705 and HLA-B(*)2709 (Asp-116/His) is responsible for the emergence of distinct HLA-B27-restricted T cell repertoires in individuals harboring either of these two subtypes and could correlate with their differential association with the autoimmune disease ankylosing spondylitis. By using fluorescence depolarization and pK(a) calculations, we investigated to what extent electrostatic interactions contribute to shape antigenic differences between these HLA molecules complexed with viral, self, and non-natural peptide ligands. In addition to the established main anchor of peptides binding to HLA-B27, arginine at position 2 (pArg-2), and the secondary anchors at the peptide termini, at least two further determinants contribute to stable peptide accommodation. 1) The interaction of Asp-116 with arginine at peptide position 5, as found in pLMP2 (RRRWRRLTV; viral) and pVIPR (RRKWRRWHL; self), and with lysine in pOmega, as found in gag (KRWIILGLNK; viral), additionally stabilizes the B(*)2705 complexes by approximately 5 and approximately 27 kJ/mol, respectively, in comparison with B(*)2709. 2) The protonation state of the key residues Glu-45 and Glu-63 in the B-pocket, which accommodates pArg-2, affects peptide binding strength in a peptide- and subtype-dependent manner. In B(*)2705/pLMP2, protonation of Glu-45/Glu-63 reduces the interaction energy of pArg-2 by approximately 24 kJ/mol as compared with B(*)2705/pVIPR. B(*)2705/pVIPR is stabilized by a deprotonated Glu-45/Glu-63 pair, evoked by allosteric interactions with pHis-8. The mutual electrostatic interactions of peptide and HLA molecule, including peptide- and subtype-dependent protonation of key residues, modulate complex stability and antigenic features of the respective HLA-B27 subtype.

HLA-B27 subtypes differentially associated with disease exhibit conformational differences in solution.

Human leukocyte antigen (HLA) class I molecules consist of a heavy chain, beta(2)-microglobulin, and a peptide that are noncovalently bound. Certain HLA-B27 subtypes are associated with ankylosing spondylitis (such as HLA-B*2705), whereas others (such as HLA-B*2709) are not. Both differ in only one residue (Asp116 and His116, respectively) in the F pocket that accommodates the peptide C-terminus. An isotope-edited IR spectroscopy study of these HLA-B27 subtypes complexed with the self-peptide RRKWRRWHL was carried out, revealing that the heavy chain is more flexible in the HLA-B*2705 than in the HLA-B*2709 subtype. In agreement with these experimental data, molecular dynamics simulations showed an increased flexibility of the HLA-B*2705 binding groove in comparison with that of the HLA-B*2709 subtype. This difference correlates with an opening of the HLA-B*2705 binding groove, accompanied by a partial detachment of the C-terminal peptide anchor. These combined results demonstrate how the deeply embedded polymorphic heavy-chain residue 116 influences the flexibility of the peptide binding groove in a subtype-dependent manner, a feature that could also influence the recognition of the HLA-B27 complexes by effector cells.

Helicobacter exploits integrin for type IV secretion and kinase activation.

Integrins are important mammalian receptors involved in normal cellular functions as well as pathogenesis of chronic inflammation and cancer. We propose that integrins are exploited by the gastric pathogen and type-1 carcinogen Helicobacter pylori for injection of the bacterial oncoprotein cytotoxin-associated gene A (CagA) into gastric epithelial cells. Virulent H. pylori express a type-IV secretion pilus that injects CagA into the host cell; CagA then becomes tyrosine-phosphorylated by Src family kinases. However, the identity of the host cell receptor involved in this process has remained unknown. Here we show that the H. pylori CagL protein is a specialized adhesin that is targeted to the pilus surface, where it binds to and activates integrin alpha5beta1 receptor on gastric epithelial cells through an arginine-glycine-aspartate motif. This interaction triggers CagA delivery into target cells as well as activation of focal adhesion kinase and Src. Our findings provide insights into the role of integrins in H.-pylori-induced pathogenesis. CagL may be exploited as a new molecular tool for our further understanding of integrin signalling.

Conformation and stability of the Streptococcus pyogenes pSM19035-encoded site-specific beta recombinase, and identification of a folding intermediate.

Solution properties of beta recombinase were studied by circular dichroism and fluorescence spectroscopy, size exclusion chromatography, analytical ultracentrifugation, denaturant-induced unfolding and thermal unfolding experiments. In high ionic strength buffer (1 M NaCl) beta recombinase forms mainly dimers, and strongly tends to aggregate at ionic strength lower than 0.3 M NaCl. Urea and guanidinium chloride denaturants unfold beta recombinase in a two-step process. The unfolding curves have bends at approximately 5 M and 2.2 M in urea and guanidinium chloride-containing buffers. Assuming a three-state unfolding model (N2-->2I-->2U), the total free energy change from 1 mol of native dimers to 2 mol of unfolded monomers amounts to deltaG(tot) = 17.9 kcal/mol, with deltaG(N2-->2I) = 4.2 kcal/mol for the first transition and deltaG(I-->U) = 6.9 kcal/mol for the second transition. Using sedimentation-equilibrium analytical ultracentrifugation, the presence of beta recombinase monomers was indicated at 5 M urea, and the urea dependence of the circular dichroism at 222 nm strongly suggests that folded monomers represent the unfolding intermediate.

Role of the N-terminal region and of beta-sheet residue Thr29 on the activity of the omega2 global regulator from the broad-host range Streptococcus pyogenes plasmid pSM19035.

The dimeric regulatory protein wild-type omega (wt omega2) binds to arrays of 7-bp sequences (heptads) present in the operator DNA region of copy control and partition functions of plasmid pSM19035. Each omega2 protein probably binds with an antiparallel beta-sheet structure in the major groove of the 7-bp subsite of the operator DNA. Exchange of threonine at position 29 to alanine (T29A) drastically affects the activity of variant protein omega2T29A both in vivo and in vitro, and reduces the thermodynamic stability deltaG(o)u, but does not change the conformation. Likewise, the binding affinity to DNA is reduced and the association of the two monomeric subunits of the omega2T29A dimer is weakened, as manifested by an increase in the dissociation constant from 3.2 microM for wt omega2 to 6.3 microM for omega2T29A. Denatured dimers are formed upon thermal unfolding of wt omega2 and omega2T29A at ca. 45 microM (D(n)<-->D(u)). Removal of 8 (omega2deltaN8), or even 18 (omega2deltaN18) N-terminal amino acids has no obvious effect either on the core structure or on the activity in comparison to wt omega2. The stability of variants omega2deltaN8 and omega2deltaN18 is similar to that of wt omega2, and their binding to operator DNA is not impaired.

The structure of the TRAPP subunit TPC6 suggests a model for a TRAPP subcomplex.

The TRAPP (transport protein particle) complexes are tethering complexes that have an important role at the different steps of vesicle transport. Recently, the crystal structures of the TRAPP subunits SEDL and BET3 have been determined, and we present here the 1.7 Angstroms crystal structure of human TPC6, a third TRAPP subunit. The protein adopts an alpha/beta-plait topology and forms a dimer. In spite of low sequence similarity, the structure of TPC6 strikingly resembles that of BET3. The similarity is especially prominent at the dimerization interfaces of the proteins. This suggests heterodimerization of TPC6 and BET3, which is shown by in vitro and in vivo association studies. Together with TPC5, another TRAPP subunit, TPC6 and BET3 are supposed to constitute a family of paralogous proteins with closely similar three-dimensional structures but little sequence similarity among its members.

Arc repressor-operator DNA interactions and contribution of Phe10 to binding specificity.

Solution properties of Arc repressors (wild-type and F10H variant) from Salmonella bacteriophage P22 and their complexes with operator DNA (Arc-wt-DNA and Arc-F10H-DNA) were characterized by circular dichroism, fluorescence, and Raman difference spectroscopy and compared with the crystal structures of free and DNA-bound Arc repressors (wild-type and F10V variant). From the crystal structure of Arc-wt-operator DNA complex, it is known that amino acids Phe10/10' flip out of the hydrophobic protein core, and in the Arc-F10V-DNA complex, the methyl groups of Val10/10' rotate toward the DNA. Arc-wt and Arc-F10H significantly perturb the Raman signatures of the operator DNA upon complex formation. The two proteins induce similar changes in the DNA spectra. Raman markers in the difference spectra (spectrum of the complex minus spectra of DNA and Arc) indicate binding of Arc in the major groove, several direct contacts, e.g., hydrogen bonds of protein residues with bases, and slight perturbations of the deoxyribose ring systems that are consistent with bending of the operator DNA. Trp14, the only one tryptophan of Arc repressor monomers, serves as a very sensitive tool for changes of the hydrophobic core of the protein. The Raman spectra identify in the free Arc-F10H variant a largely different chi(2,1) rotation angle of Trp14 compared to that in wild-type Arc. In the Arc-wt-DNA and Arc-F10H-DNA complexes, however, the Trp14 chi(2,1) rotation angles are similar in both proteins. Furthermore, in both complexes, a strengthening of the van der Waals interactions of the aromatic ring of Trp14 is indicated compared to these interactions in the free proteins. According to the fluorescence and Raman data, His10 is buried in the hydrophobic core of free Arc-F10H, resembling the "core" conformation of Phe10 in Arc-wt, but His10 is looped out in the complex with DNA resembling the "bound" conformation of Phe10 in the Arc-wt-operator DNA complex.

Thermodynamic and structural equivalence of two HLA-B27 subtypes complexed with a self-peptide.

The F pocket of major histocompatibility complex (in humans HLA) class I molecules accommodates the C terminus of the bound peptide. Residues forming this pocket exhibit considerable polymorphism, and a single difference (Asp116 in HLA-B*2705 and His116 in HLA-B*2709 heavy chains) confers differential association of these two HLA-B27 subtypes to the autoimmune disease ankylosing spondylitis. As peptide presentation by HLA molecules is of central importance for immune responses, we performed thermodynamic (circular dichroism, differential scanning calorimetry, fluorescence polarization) and X-ray crystallographic analyses of both HLA-B27 subtypes complexed with the epidermal growth factor response factor 1-derived self-peptide TIS (RRLPIFSRL) to understand the impact of the Asp116His exchange on peptide display. This peptide is known to be presented in vivo by both subtypes, and as expected for a self-peptide, TIS-reactive cytotoxic T lymphocytes are absent in the respective individuals. The thermodynamic analyses reveal that both HLA-B27:TIS complexes exhibit comparable, relatively high thermostability (Tm approximately 60 degrees C) and undergo multi-step unfolding reactions, with dissociation of the peptide in the first step. As shown by X-ray crystallography, only subtle structural differences between the subtypes were observed regarding the architecture of their F pockets, including the presence of distinct networks of water molecules. However, no consistent structural differences were found between the peptide presentation modes. In contrast to other peptides displayed by the two HLA-subtypes which show either structural or dynamical differences in their peptide presentation modes, the TIS-complexed HLA-B*2705 and HLA-B*2709 subtypes are an example for thermodynamic and structural equivalence, in agreement with functional data.

The microglia-activating potential of thrombin: the protease is not involved in the induction of proinflammatory cytokines and chemokines.

The serine protease thrombin is known as a blood coagulation factor. Through limited cleavage of proteinase-activated receptors it can also control growth and functions in various cell types, including neurons, astrocytes, and microglia (brain macrophages). A number of previous studies indicated that thrombin induces the release of proinflammatory cytokines and chemokines from microglial cells, suggesting another important role for the protease beyond hemostasis. In the present report, we provide evidence that this effect is not mediated by any proteolytic or non-proteolytic mechanism involving thrombin proper. Inhibition of the enzymatic thrombin activity did not affect the microglial release response. Instead the cyto-/chemokine-inducing activity solely resided in a high molecular weight protein fraction that could be isolated in trace amounts even from apparently homogenous alpha- and gamma-thrombin preparations. High molecular weight material contained thrombin-derived peptides as revealed by mass spectrometry but was devoid of thrombin-like enzymatic activity. Separated from the high molecular weight fraction by fast protein liquid chromatography, enzymatically intact alpha- and gamma-thrombin failed to trigger any release. Our findings may force a revision of the notion that thrombin itself is a direct proinflammatory release signal for microglia. In addition, they could be relevant for the study of other cellular activities and their assignment to this protease.

Recognition of DNA by omega protein from the broad-host range Streptococcus pyogenes plasmid pSM19035: analysis of binding to operator DNA with one to four heptad repeats.

pSM19035-encoded omega protein forms a dimer (omega2) that binds to a set of 7-bp repeats with sequence 5'-NATCACN-3'. Upon binding to its cognate sites, omega2 regulates transcription of genes required for copy number control and stable inheritance of plasmids, and promotes accurate plasmid segregation. Protein omega2 binds poorly to one heptad but the affinity to DNA increases with two and more unspaced heptads in direct or inverted orientation. DNA titration of increasing numbers of heptads with omega2, monitored by circular dichroism measurements, indicates the binding of one omega2 to one heptad (omega2:heptad stoichiometry of 1:1). Spacing of two directly or inversely oriented heptads by 1 to 7 bp reduces the affinity of the protein for its cognate target site. The binding affinity of omega2 for two directly repeated heptads was severely reduced if one of the base pairs of the core 5'-ATCAC-3' sequence of one of the heptads was individually substituted by any other base pair. Hydroxyl radical footprinting shows a protection pattern at the 5'-ATCAC-3' core. These data suggest that each heptad defines an operator half-site and that tight binding of the symmetric omega2 to the central 5'-TCA-3' core of symmetric or asymmetric targets (differently oriented heptads) is probably achieved by structural changes of DNA and/or protein or both.

Thermodynamic and structural analysis of peptide- and allele-dependent properties of two HLA-B27 subtypes exhibiting differential disease association.

Selected HLA-B27 subtypes are associated with spondyloarthropathies, but the underlying mechanism is not understood. To explain this association in molecular terms, a comparison of peptide-dependent dynamic and structural properties of the differentially disease-associated subtypes HLA-B*2705 and HLA-B*2709 was carried out. These molecules differ only by a single amino acid at the floor of the peptide binding groove. The thermostabilities of a series of HLA-B27 molecules complexed with nonameric and decameric peptides were determined and revealed substantial differences depending on the subtype as well as the residues at the termini of the peptides. In addition we present the crystal structure of the B*2709 subtype complexed with a decameric peptide. This structure provides an explanation for the preference of HLA-B27 for a peptide with an N-terminal arginine as secondary anchor and the lack of preference for tyrosine as peptide C terminus in B*2709. The data show that differences in thermodynamic properties between peptide-complexed HLA-B27 subtypes are correlated with a variety of structural properties.

Design and characterization of a hybrid miniprotein that specifically inhibits porcine pancreatic elastase.

Studying protease/peptide inhibitor interactions is a useful tool for understanding molecular recognition in general and is particularly relevant for the rational design of inhibitors with therapeutic potential. An inhibitory peptide (PMTLEYR) derived from the third domain of turkey ovomucoid inhibitor and optimized for specific porcine pancreatic elastase inhibition was introduced into an inhibitor scaffold to increase the proteolytic stability of the peptide. The trypsin-specific squash inhibitor EETI II from Ecballium elaterium was chosen as the scaffold. The resulting hybrid inhibitor HEI-TOE I (hybrid inhibitor from E. elaterium and the optimized binding loop of the third domain of turkey ovomucoid inhibitor) shows a specificity and affinity to porcine pancreatic elastase similar to the free inhibitory peptide but with significantly higher proteolytic stability. Isothermal titration calorimetry revealed that elastase binding of HEI-TOE I occurs with a small unfavorable positive enthalpy contribution, a large favorable positive entropy change, and a large negative heat capacity change. In addition, the inhibitory peptide and the hybrid inhibitor HEI-TOE I protected endothelial cells against degradation following treatment with porcine pancreatic elastase.

Interaction of epitope-related and -unrelated peptides with anti-p24 (HIV-1) monoclonal antibody CB4-1 and its Fab fragment.

The binding of four epitope-related peptides and three library-derived, epitope-unrelated peptides of different lengths (10-14 amino acids) and sequence by anti-p24 (HIV-1) monoclonal antibody CB4-1 and its Fab fragment was studied by isothermal titration calorimetry. The binding constants K(A) at 25 degrees C vary between 5.1 x 10(7) M (-1) for the strongest and 1.4 x 10(5) M (-1) for the weakest binder. For each of the peptides complex formation is enthalpically driven and connected with unfavorable entropic contributions; however, the ratio of enthalpy and entropy contributions to deltaG(0) differs markedly for the individual peptides. A plot of -deltaH(0) vs -TdeltaS(0) shows a linear correlation of the data for a wide variety of experimental conditions as expected for a process with deltaC(p) much larger than deltaS(0). The dissimilarity of deltaC(p) and deltaS(0) also explains why deltaH(0) and TdeltaS(0) show similar temperature dependences resulting in relatively small changes of deltaG(0) with temperature. The heat capacity changes deltaC(p) upon antibody-peptide complex formation determined for three selected peptides vary only in a small range, indicating basic thermodynamic similarity despite different key residues interacting in the complexes. Furthermore, the comparison of van't Hoff and calorimetric enthalpies point to a non-two-state binding mechanism. Protonation effects were excluded by measurements in buffers of different ionization enthalpies. Differences in the solution conformation of the peptides as demonstrated by circular dichroic measurements do not explain different binding affinities of the peptides; specifically a high helix content in solution is not essential for high binding affinity despite the helical epitope conformation in the crystal structure of p24.

The cystine knot promotes folding and not thermodynamic stability in vascular endothelial growth factor.

Cystine knots consist of three intertwined disulfide bridges and are considered major determinants of protein stability in proteins in which they occur. We questioned this function and observed that removal of individual disulfide bridges in human vascular endothelial growth factor (VEGF) does not reduce its thermodynamic stability but reduces its unexpected high thermal stability of 108 degrees C by up to 40 degrees C. In wild-type VEGF (deltaG(u,25)(0) = 5.1 kcal.mol(-1)), the knot is responsible for a large entropic stabilization of TdeltaS(u,25)(0) = -39.3 kcal mol(-1), which is compensated for by a deltaH(u,25)(0) of -34.2 kcal mol(-1). In the disulfide-deficient mutants, this entropic stabilization disappears, but instead of a decrease, we observe an increase in the thermodynamic stability by about 2 kcal.mol(-1). A detailed crystallographic analysis of the mutant structures suggests a role of the cystine knot motif in protein folding rather than in the stabilization of the folded state. When assuming that the sequential order of the disulfide bridge formation is conserved between VEGF and glycoprotein alpha-subunit, the crystal structure of the mutant C61A-C104A, which deviates by a root mean square deviation of more than 2.2 A from wild-type VEGF, identifies a true folding intermediate of VEGF.

NblA from Anabaena sp. PCC 7120 is a mostly alpha-helical protein undergoing reversible trimerization in solution.

The nblA family of genes encodes for small proteins necessary for the ordered degradation of phycobilisomes under certain stress conditions, a process known as chlorosis. Genes homologous to nblA seem to occur in all phycobilisome-containing organisms. However, to date, no molecular mechanism is known for the action of NblA, nor have the gene products been characterized to understand the physical properties of the molecule and thus help elucidate the mechanism on a structural basis. In this study we report on the first characterization of an NblA-homologous gene product. The chromosomal gene from the cyanobacterium Anabaena sp. PCC 7120 was cloned, heterologously expressed in Escherichia coli and purified to apparent homogeneity. This allowed the protein to be characterized by analytical ultracentrifugation and CD spectroscopy. These experiments show that the NblA protein has a mostly alpha-helical structure, undergoing an association reaction of folded monomers to form trimers in solution. No dimers are detectable.

In vitro and in vivo stability of the epsilon2zeta2 protein complex of the broad host-range Streptococcus pyogenes pSM19035 addiction system.

Streptococcus pyogenes pSM19035-encoded epsilon (10.7 kDa) and zeta (32.4 kDa) proteins are necessary to secure stable plasmid inheritance in bacteria, with zeta acting as toxin that kills plasmid-deprived cells and epsilon as an antitoxin that neutralises the activity of zeta. The epsilon and zeta proteins co-purify as a stable complex that, according to analytical ultracentrifugation and gel filtration, exists as epsilon2zeta2 heterotetramer in solution. Co-crystals of the epsilon2zeta2 complex contain epsilon and zeta in 1:1 molar ratio. Unfolding studies monitoring circular dichroic and fluorescence changes show that the zeta protein has a significantly lower thermodynamic stability than the epsilon protein both in free state and in the complex. Proteolytic studies indicate that zeta protein is more stable in the epsilon2zeta2 complex than in the free state. In vivo studies reveal a short half-life of the epsilon antitoxin (-18 min) and a long lifetime of the zeta toxin (>60 min). When transcription-translation of a plasmid containing the epsilon and zeta genes was inhibited, cell death was observed after a short lag phase that correlates with the disappearance of the epsilon protein from the background.