Solution structure and backbone dynamics of the cysteine 103 to serine mutant of the N-terminal domain of DsbD from Neisseria meningitidis

The DsbD protein is essential for electron transfer from the cytoplasm to the periplasm of Gram-negative bacteria. Its N-terminal domain dispatches electrons coming from cytoplasmic thioredoxin (Trx), via its central transmembrane and C-terminal domains, to its periplasmic partners: DsbC, DsbE/CcmG, and DsbG. Previous structural studies described the latter proteins as Trx-like folds possessing a characteristic C-X-X-C motif able to generate a disulfide bond upon oxidation. The Escherichia coli nDsbD displays an immunoglobulin-like fold in which two cysteine residues (Cys103 and Cys109) allow a disulfide bond exchange with its biological partners.We have determined the structure in solution and the backbone dynamics of the C103S mutant of the N-terminal domain of DsbD from Neisseria meningitidis. Our results highlight significant structural changes concerning the beta-sheets and the local topology of the active site compared with the oxidized form of the E. coli nDsbD. The structure reveals a "cap loop" covering the active site, similar to the oxidized E. coli nDsbD X-ray structure. However, regions featuring enhanced mobility were observed both near to and distant from the active site, revealing a capacity of structural adjustments in the active site and in putative interaction areas with nDsbD biological partners. Results are discussed in terms of functional consequences.     

The X-ray structure of the N-terminal domain of PILB from Neisseria meningitidis reveals a thioredoxin-fold

The secreted form of the PilB protein was recently shown to be bound to the outer membrane of Neisseria gonorrhoeae and proposed to be involved in survival of the pathogen to the host's oxidative burst. PilB is composed of three domains. The central and the C-terminal domains display methionine sulfoxide reductase (Msr) A and B activities respectively, i.e. the ability to reduce specifically the S and the R enantiomers of the sulfoxide function of the methionine sulfoxides, which are easily formed upon oxidation of methionine residues. The N-terminal domain of PilB (Dom1(PILB)) of N.meningitidis, which possesses a CXXC motif, was recently shown to recycle the oxidized forms of the PilB Msr domains in vitro, as the Escherichia coli thioredoxin (Trx) 1 does. The X-ray structure of Dom1(PILB) of N.meningitidis determined here shows a Trx-fold, in agreement with the biochemical properties of Dom1(PILB). However, substantial structural differences with E.coli Trx1 exist. Dom1(PILB) displays more structural homologies with the periplasmic disulfide oxidoreductases involved in cytochrome maturation pathways in bacteria. The active site of the reduced form of Dom1(PILB) reveals a high level of stabilization of the N-terminal catalytic cysteine residue and a hydrophobic environment of the C-terminal recycling cysteine in the CXXC motif, consistent with the pK(app) values measured for Cys67 (<6) and Cys70 (9.3), respectively. Compared to cytochrome maturation disulfide oxidoreductases and to Trx1, one edge of the active site is covered by four additional residues (99)FLHE(102). The putative role of the resulting protuberance is discussed in relation to the disulfide reductase properties of Dom1(PILB).     

Solution structure of the immunodominant domain of protective antigen GNA1870 of Neisseria meningitidis

GNA1870, a 28-kDa surface-exposed lipoprotein of Neisseria meningitidis recently discovered by reverse vaccinology, is one of the most potent antigens of Meningococcus and a promising candidate for a universal vaccine against a devastating disease. Previous studies of epitope mapping and genetic characterization identified residues critical for bactericidal response within the C-terminal domain of the molecule. To elucidate the conformation of protective epitopes, we used NMR spectroscopy to obtain the solution structure of the immunodominant 18-kDa C-terminal portion of GNA1870. The structure consists of an eight-stranded antiparallel beta-barrel overlaid by a short alpha-helix with an unstructured N-terminal end. Residues previously shown to be important for antibody recognition were mapped on loops facing the same ridge of the molecule. The sequence similarity of GNA1870 with members of the bacterial transferrin receptor family allows one to predict the folding of this class of well known bacterial antigens, providing the basis for the rational engineering of high affinity B cell epitopes.     

Solution structure of the lipoyl domain of the chimeric dihydrolipoyl dehydrogenase P64K from Neisseria meningitidis.

The antigenic P64K protein from the pathogenic bacterium Neisseria meningitidis is found in the outer membrane of the cell, and consists of two parts: an 81-residue N-terminal region and a 482-residue C-terminal region. The amino-acid sequence of the N-terminal region is homologous with the lipoyl domains of the dihydrolipoyl acyltransferase (E2) components, and that of the C-terminal region with the dihydrolipoyl dehydrogenase (E3) components, of 2-oxo acid dehydrogenase multienzyme complexes. The two parts are separated by a long linker region, similar to the linker regions in the E2 chains of 2-oxo acid dehydrogenase complexes, and it is likely this region is conformationally flexible. A subgene encoding the P64K lipoyl domain was created and over-expressed in Escherichia coli. The product was capable of post-translational modification by the lipoate protein ligase but not aberrant modification by the biotin protein ligase of E. coli. The solution structure of the apo-domain was determined by means of heteronuclear NMR spectroscopy and found to be a flattened beta barrel composed of two four-stranded antiparallel beta sheets. The lysine residue that becomes lipoylated is in an exposed beta turn that, from a [1H]-15N heteronuclear Overhauser effect experiment, appears to enjoy substantial local motion. This structure of a lipoyl domain derived from a dihydrolipoyl dehydrogenase resembles that of lipoyl domains normally found as part of the dihydrolipoyl acyltransferase component of 2-oxo acid dehydrogenase complexes and will assist in furthering the understanding of its function in a multienzyme complex and in the membrane-bound P64K protein itself.     

Solution structure of the COMMD1 N-terminal domain

COMMD1 is the prototype of a new protein family that plays a role in several important cellular processes, including NF-kappaB signaling, sodium transport, and copper metabolism. The COMMD proteins interact with one another via a conserved C-terminal domain, whereas distinct functions are predicted to result from a variable N-terminal domain. The COMMD proteins have not been characterized biochemically or structurally. Here, we present the solution structure of the N-terminal domain of COMMD1 (N-COMMD1, residues 1-108). This domain adopts an alpha-helical structure that bears little resemblance to any other helical protein. The compact nature of N-COMMD1 suggests that full-length COMMD proteins are modular, consistent with specific functional properties for each domain. Interactions between N-COMMD1 and partner proteins may occur via complementary electrostatic surfaces. These data provide a new foundation for biochemical characterization of COMMD proteins and for probing COMMD1 protein-protein interactions at the molecular level.     

Structure of the OmpA-like domain of RmpM from Neisseria meningitidis

RmpM is a putative peptidoglycan binding protein from Neisseria meningitidis that has been shown to interact with integral outer membrane proteins such as porins and TonB-dependent transporters. Here we report the 1.9 A crystal structure of the C-terminal domain of RmpM. The 150-residue domain adopts a betaalphabetaalphabetabeta fold, as first identified in Bacillus subtilis chorismate mutase. The C-terminal RmpM domain is homologous to the periplasmic, C-terminal domain of Escherichia coli OmpA; these domains are thought to be responsible for non-covalent interactions with peptidoglycan. From the structure of the OmpA-like domain of RmpM, we suggest a putative peptidoglycan binding site and identify residues that may be essential for binding. Both the crystal structure and solution experiments indicate that RmpM may exist as a dimer. This would promote more efficient peptidoglycan binding, by allowing RmpM to interact simultaneously with two glycan chains through its C-terminal, OmpA-like binding domain, while its (structurally uncharacterized) N-terminal domain could stabilize oligomers of porins and TonB-dependent transporters in the outer membrane.     

Two-dimensional structure of the Opc invasin from Neisseria meningitidis

A two-dimensional structural model was devised for the Opc outer membrane protein invasin which contains 10 transmembrane strands and five surface-exposed loops. One continuous epitope recognized by three monoclonal antibodies was localized to the tip of loop 2 by synthetic peptides and site-directed mutagenesis while a second, discontinuous epitope recognized by a fourth antibody was localized to loops 4 and 5 by insertion mutagenesis. These monoclonal antibodies are bactericidal and inhibit adhesion and invasion. Most of the T-cell epitopes defined by Wiertz et al. (1996) were localized to the transmembrane strands. Oligonucleotides encoding a foreign epitope (∇) from Semliki Forest virus were inserted into Bgl II restriction sites created by site-directed mutagenesis. The ∇ epitopes inserted in all five predicted loops were recognized on the cell surface of live Escherichia coli bacteria by a monoclonal antibody and are exposed while ∇ epitopes in the N-terminus or three predicted turns were not. The results thus confirm important predictions of the model and define five permissive sites within surface-exposed loops which can be used to insert foreign epitopes.     

The N-terminal domain of PILB from Neisseria meningitidis is a disulfide reductase that can recycle methionine sulfoxide reductases

The PilB protein of the Neisseria genus comprises three domains. Two forms have been recently reported to be produced in vivo. One form, containing the three domains, is secreted from the bacterial cytoplasm to the outer membrane, whereas the second form, which is cytoplasmic, only contains the central and the C-terminal domains. The secreted form was shown to be involved in survival under oxidative conditions. Although previous studies indicated that the central and the C-terminal domains display methionine sulfoxide reductase A and B activities, respectively, no function was described so far for the N-terminal domain. In the present study, the N-terminal domain of the PilB of Neisseria meningitidis was produced as a folded entity, and its biochemical and enzymatic properties have been determined. The data show that the N-terminal domain possesses a disulfide redox-active site with a redox potential in the range of that of thioredoxin. Moreover, the N-terminal domain, either as an isolated form or included in PilB, recycles the oxidized forms of the methionine sulfoxide reductases like thioredoxin. These results, which show that the N-terminal domain exhibits a disulfide reductase activity and probably has a thioredoxin-fold, are discussed in relation to its possible functional role in Neisseria.     

Heteronuclear NMR assignments and secondary structure of the coiled coil trimerization domain from cartilage matrix protein in oxidized and reduced forms.

The C-terminal oligomerization domain of chicken cartilage matrix protein is a trimeric coiled coil comprised of three identical 43-residue chains. NMR spectra of the protein show equivalent magnetic environments for each monomer, indicating a parallel coiled coil structure with complete threefold symmetry. Sequence-specific assignments for 1H-, 15N-, and 13C-NMR resonances have been obtained from 2D 1H NOESY and TOCSY spectra, and from 3D HNCA, 15N NOESY-HSQC, and HCCH-TOCSY spectra. A stretch of alpha-helix encompassing five heptad repeats (35 residues) has been identified from intra-chain HN-HN and HN-H alpha NOE connectivities. 3JHNH alpha coupling constants, and chemical shift indices. The alpha-helix begins immediately downstream of inter-chain disulfide bonds between residues Cys 5 and Cys 7, and extends to near the C-terminus of the molecule. The threefold symmetry of the molecule is maintained when the inter-chain disulfide bonds that flank the N-terminus of the coiled coil are reduced. Residues Ile 21 through Glu 36 show conserved chemical shifts and NOE connectivities, as well as strong protection from solvent exchange in the oxidized and reduced forms of the protein. By contrast, residues Ile 10 through Val 17 show pronounced chemical shift differences between the oxidized and reduced protein. Strong chemical exchange NOEs between HN resonances and water indicate solvent exchange on time scales faster than 10 s, and suggests a dynamic fraying of the N-terminus of the coiled coil upon reduction of the disulfide bonds. Possible roles for the disulfide crosslinks of the oligomerization domain in the function of cartilage matrix protein are proposed.     

Solution structure and backbone dynamics of the N-terminal region of the calcium regulatory domain from soybean calcium-dependent protein kinase alpha

Ca(2+)-dependent protein kinases (CDPKs) are vital Ca(2+)-signaling proteins in plants and protists which have both a kinase domain and a self-contained calcium regulatory calmodulin-like domain (CLD). Despite being very similar to CaM (>40% identity) and sharing the same fold, recent biochemical and structural evidence suggests that the behavior of CLD is distinct from its namesake, calmodulin. In this study, NMR spectroscopy is employed to examine the structure and backbone dynamics of a 168 amino acid Ca(2+)-saturated construct of the CLD (NtH-CLD) in which almost the entire C-terminal domain is exchange broadened and not visible in the NMR spectra. Structural characterization of the N-terminal domain indicates that the first Ca(2+)-binding loop is significantly more open than in a recently reported structure of the CLD complexed with a putative intramolecular binding region (JD) in the CDPK. Backbone dynamics suggest that parts of the third helix exhibit unusually high mobility, and significant exchange, consistent with previous findings that this helix interacts with the C-terminal domain. Dynamics data also show that the "tether" region, consisting of the first 11 amino acids of CLD, is highly mobile and these residues exhibit distinctive beta-type secondary structure, which may help to position the JD and CLD. Finally, the unusual global dynamic behavior of the protein is rationalized on the basis of possible interdomain rearrangements and the highly variable environments of the C- and N-terminal domains.     

A lipoamide dehydrogenase from Neisseria meningitidis has a lipoyl domain

A protein of molecular weight of 64 kDa (p64k) found in the outer membrane of Neisseria meningitidis shows a high degree of homology with both the lipoyl domain of the acetyltransferase and the entire sequence of the lipoamide dehydrogenase, the E2 and E3 components of the dehydrogenase multienzyme complexes, respectively. The alignment of the p64k with lipoyl domains and lipoamide dehydrogenases from different species is presented. The possible implications of this protein in binding protein-dependent transport are discussed. This is the first lipoamide dehydrogenase reported to have a lipoyl domain. © 1995 Wiley-Liss, Inc.     

The solution structure of a domain from the Neisseria meningitidis lipoprotein PilP reveals a new beta-sandwich fold

Type IV pili are long, thin fibres, which extend from the surface of the bacterial pathogen Neisseria meningitidis; they play a key role in adhesion and colonisation of host cells. PilP is a lipoprotein, suggested to be involved in the assembly and stabilization of an outer membrane protein, PilQ, which is required for pilus formation. Here we describe the expression of a recombinant fragment of PilP, spanning residues 20 to 181, and determination of the solution structure of a folded domain, spanning residues 85 to 163, by NMR. The N-terminal third of the protein, from residues 20 to 84, is apparently unfolded. Protease digestion yielded a 113 residue fragment that contained the folded domain. The domain adopts a simple beta-sandwich type fold, consisting of a three-stranded beta-sheet packed against a four-stranded beta-sheet. There is also a short segment of 3(10) helix at the N-terminal part of the folded domain. We were unable to identify any other proteins that are closely related in structure to the PilP domain, although the fold appears to be distantly related to the lipocalin family. Over 40 homologues of PilP have been identified in Gram-negative bacteria and the majority of conserved residues lie within the folded domain. The fourth beta-strand and adjacent loop regions contain a high proportion of conserved residues, including three glycine residues, which seem to play a role in linking the two beta-sheets. The two beta-sheets pack together to form a crevice, lined with conserved hydrophobic residues: we suggest that this feature could act as a binding site for a small ligand. The results show that PilP and its homologues have a conserved, folded domain at the C-terminal end of the protein that may be involved in mediating binding to hydrophobic ligands.     

Solution structure and backbone dynamics of an N-terminal ubiquitin-like domain in the GLUT4-regulating protein, TUG

The GLUT4-regulating protein, TUG, functions to retain GLUT4-containing membrane vesicles intracellularly and, in response to insulin stimulation, releases these vesicles to the cellular exocytic machinery for translocation to the plasma membrane. As part of our on going effort to describe the molecular basis for TUG function, we have determined the tertiary structure and characterized the backbone dynamics for an N-terminal ubiquitin-like domain (TUG-UBL1) using NMR spectroscopy. A well-ordered conformation is observed for residues 10-83 of full-length TUG, and confirms a beta-grasp or ubiquitin-like topology. Although not required for in vitro association with GLUT4, the functional role of the TUG-UBL1 domain has not yet been described. We undertook a limited literature review of similar N-terminal UBL domains and noted that a majority participate in protein-protein interactions, generally functioning as adaptor modules to physically associate the over all activity of the protein with a specific cellular process, such as the ubiquitin-proteasome pathway. In consistent fashion, TUG-UBL1 is not expected to participate in a covalent protein modification reaction as it lacks the characteristic C-terminal diglycine ("GG") motif required for conjugation to an acceptor lysine, and also lacks the three most common acceptor lysine residues involved in polyubiquitination. Additionally, analysis of the TUG-UBL1 molecular surface reveals a lack of conservation of the "Ile-44 hydrophobic face" typically involved in ubiquitin recognition. Instead, we speculate on the possible significance of a concentrated area of negative electrostatic potential with increased backbone mobility, both of which are features suggestive of a potential protein-protein interaction site.     

Molecular dynamics study of Desulfovibrio africanus cytochrome c3 in oxidized and reduced forms

A 5-ns molecular dynamics study of a tetraheme cytochrome in fully oxidized and reduced forms was performed using the CHARMM molecular modeling program, with explicit water molecules, Langevin dynamics thermalization, Particle Mesh Ewald long-range electrostatics, and quantum mechanical determination of heme partial charges. The simulations used, as starting points, crystallographic structures of the oxidized and reduced forms of the acidic cytochrome c(3) from Desulfovibrio africanus obtained at pH 5.6. In this paper we also report structures for the two forms obtained at pH 8. In contrast to previous cytochrome c(3) dynamics simulations, our model is stable. The simulation structures agree reasonably well with the crystallographic ones, but our models show higher flexibility and the water molecules are more labile. We have compared in detail the differences between the simulated and experimental structures of the two redox states and observe that the hydration structure is highly dependent on the redox state. We have also analyzed the interaction energy terms between the hemes, the protein residues, and water. The direct electrostatic interaction between hemes is weak and nearly insensitive to the redox state, but the remaining terms are large and contribute in a complex way to the overall potential energy differences that we see between the redox states.     

Backbone dynamics of plastocyanin in both oxidation states. Solution structure of the reduced form and comparison with the oxidized state

A model-free analysis based on (15)N R(1), (15)N R(2), and (15)N-(1)H nuclear Overhauser effects was performed on reduced (diamagnetic) and oxidized (paramagnetic) forms of plastocyanin from Synechocystis sp. PCC6803. The protein backbone is rigid, displaying a small degree of mobility in the sub-nanosecond time scale. The loops surrounding the copper ion, involved in physiological electron transfer, feature a higher extent of flexibility in the longer time scale in both redox states, as measured from D(2)O exchange of amide protons and from NH-H(2)O saturation transfer experiments. In contrast to the situation for other electron transfer proteins, no significant difference in the dynamic properties is found between the two redox forms. A solution structure was also determined for the reduced plastocyanin and compared with the solution structure of the oxidized form in order to assess possible structural changes related to the copper ion redox state. Within the attained resolution, the structure of the reduced plastocyanin is indistinguishable from that of the oxidized form, even though small chemical shift differences are observed. The present characterization provides information on both the structural and dynamic behavior of blue copper proteins in solution that is useful to understand further the role(s) of protein dynamics in electron transfer processes.