Stability of a receptor-binding active human immunodeficiency virus type 1 recombinant gp140 trimer conferred by intermonomer disulfide bonding of the V3 loop: differential effects of protein disulfide isomerase on CD4 and coreceptor binding.

Stable trimeric forms of human immunodeficiency virus recombinant gp140 (rgp140) are important templates for determining the structure of the glycoprotein to assist in our understanding of HIV infection and host immune response. Such information will aid the design of therapeutic drugs and vaccines. Here, we report the production of a highly stable and trimeric rgp140 derived from a HIV type 1 (HIV-1) subtype D isolate that may be suitable for structural studies. The rgp140 is functional in terms of binding to CD4 and three human monoclonal antibodies (17b, b12, and 2G12) that have broad neutralizing activities against a range of HIV-1 isolates from different subtypes. Treatment of rgp140 with protein disulfide isomerase (PDI) severely restricted 17b binding capabilities. The stable nature of the rgp140 was due to the lack of processing at the gp120/41 boundary and the presence of an intermonomer disulfide bond formed by the cysteines of the V3 loop. Further characterization showed the intermonomer disulfide bond to be a target for PDI processing. The relevance of these findings to the roles of the V3 domain and the timing of PDI action during the HIV infection process are discussed.

The crystal structure of the globular domain of sheep prion protein.

The prion protein PrP is a naturally occurring polypeptide that becomes transformed from a normal conformation to that of an aggregated form, characteristic of pathological states in fatal transmissible spongiform conditions such as Creutzfeld-Jacob Disease and Bovine Spongiform Encephalopathy. We report the crystal structure, at 2 A resolution, of residues 123-230 of the C-terminal globular domain of the ARQ allele of sheep prion protein (PrP). The asymmetric unit contains a single molecule whose secondary structure and overall organisation correspond to those structures of PrPs from various mammalian species determined by NMR. The globular domain shows a close association of helix-1, the C-terminal portion of helix-2 and the N-terminal portion of helix-3, bounded by the intramolecular disulphide bond, 179-214. The loop 164-177, between beta2 and helix-2 is relatively well structured compared to the human PrP NMR structure. Analysis of the sheep PrP structure identifies two possible loci for the initiation of beta-sheet mediated polymerisation. One of these comprises the beta-strand, residues 129-131 that forms an intra-molecular beta-sheet with residues 161-163. This strand is involved in lattice contacts about a crystal dyad to generate a four-stranded intermolecular beta-sheet between neighbouring molecules. The second locus involves the region 188-204, which modelling suggests is able to undergo a partial alpha-->beta switch within the monomer. These loci provide sites within the PrPc monomer that could readily give rise to early intermediate species on the pathway to the formation of aggregated PrPSc containing additional intermolecular beta-structure.

Structure of the human S100A12-copper complex: implications for host-parasite defence.

S100A12 is a member of the S100 family of EF-hand calcium-modulated proteins. Together with S100A8 and S100A9, it belongs to the calgranulin subfamily, i.e. it is mainly expressed in granulocytes, although there is an increasing body of evidence of expression in keratinocytes and psoriatic lesions. As well as being linked to inflammation, allergy and neuritogenesis, S100A12 is involved in host-parasite response, as are the other two calgranulins. Recent data suggest that the function of the S100-family proteins is modulated not only by calcium, but also by other metals such as zinc and copper. Previously, the structure of human S100A12 in low-calcium and high-calcium structural forms, crystallized in space groups R3 and P2(1), respectively, has been reported. Here, the structure of S100A12 in complex with copper (space group P2(1)2(1)2; unit-cell parameters a = 70.6, b = 119.0, c = 90.2 A) refined at 2.19 A resolution is reported. Comparison of anomalous difference electron-density maps calculated with data collected with radiation of wavelengths 1.37 and 1.65 A shows that each monomer binds a single copper ion. The copper binds at an equivalent site to that at which another S100 protein, S100A7, binds zinc. The results suggest that copper binding may be essential for the functional role of S100A12 and probably the other calgranulins in the early immune response.

Multiple structural states of S100A12: A key to its functional diversity.

S100A12 is a member of the S100 family of EF-hand calcium-binding proteins. Together with two other calgranulins, S100A8 and S100A9, it is mostly expressed in human granulocytes, although there is increasing evidence of expression in keratinocytes and psoriatic lesions. It is involved in host-parasite response, and linked to corneal autoimmune diseases connected with filarial parasite infestation. Interaction of S100A12 with a multiligand receptor for advanced glycation end products (RAGE) mediates inflammation. Human recombinant S100A12 was found to induce neuritogenesis of cultured hippocampal cells, similar to two other S100 proteins, S100B and S100A4. X-ray structure of S100A12 has been solved in two crystal forms: R3 and P2(1). In the R3 crystal form S100A12 is a dimer, and in the P2(1) crystal form the dimers are arranged as a hexamer. The hexameric form suggests its role in receptor oligomerisation. S100A12 binds copper at the predicted zinc/copper binding site, which is located close to the surface of the protein. We propose copper-mediated generation of reactive oxygen species by S100A12 as its function in host-parasite response.

The structure of substrate-free microbial ribonuclease binase and of its complexes with 3'GMP and sulfate ions.

The structures of Bacillus intermedius ribonuclease (binase), an extracellular 109-residue enzyme, and its complexes with 3'GMP and sulfate ions were solved at 1.65 and 2.0 A, respectively. The structures were refined using REFMAC. The crystal of free binase belongs to the space group C2, whereas the crystals of complexes belong to the space group P2(1)2(1)2(1). In both crystal lattices the asymmetric unit contains two molecules which form an identical dimer. The structure of the dimer is such that only one of its subunits can bind the nucleotide in the 3'GMP-binase complex, where the guanyl base is located in the recognition loop of the enzyme. In both binase complex structures the phosphate group of 3'GMP or one of the sulfate ions make an electrostatic interaction with the binase molecule at the catalytic site. A second phosphate-binding site was found in the structures of the complexes at the cleft formed by the loop 34-39, the main chain of Arg82 and the side chain of Trp34. Comparison of the complex and unliganded enzyme crystal structures shows that there are some small but distinct differences in the specificity loop (56-62) and in the loops 34-39 and 99-104 associated with the binding of the nucleotide and sulfate ions.

The structure of S100A12 in a hexameric form and its proposed role in receptor signalling.

S100A12 is a member of the S100 subfamily of EF-hand calcium-binding proteins; it has been shown to be one of the ligands of the 'receptor for advanced glycation end products' (RAGE) that belongs to the immunoglobulin superfamily and is involved in diabetes, Alzheimer's disease, inflammation and tumour invasion. The structure of the dimeric form of native S100A12 from human granulocytes in the presence of calcium in space group R3 has previously been reported. Here, the structure of a second crystal form in space group P2(1) (unit-cell parameters a = 53.9, b = 100.5, c = 112.7A, beta = 94.6 degrees) solved at 2.7A resolution by molecular replacement using the R3 structure as a search model is reported. Like most S100 proteins, S100A12 is a dimer. However, in the P2(1) crystal form dimers of S100A12 are arranged in a spherical hexameric assembly with an external diameter of about 55 A stabilized by calcium ions bound between adjacent dimers. The putative target-binding sites of S100A12 are located at the outer surface of the hexamer, making it possible for the hexamer to bind several targets. It is proposed that the S100A12 hexameric assembly might interact with three extracellular domains of the receptor, bringing them together into large trimeric assemblies.

Crystal structures of penicillin acylase enzyme-substrate complexes: structural insights into the catalytic mechanism.

The crystal structure of penicillin G acylase from Escherichia coli has been determined to a resolution of 1.3 A from a crystal form grown in the presence of ethylene glycol. To study aspects of the substrate specificity and catalytic mechanism of this key biotechnological enzyme, mutants were made to generate inactive protein useful for producing enzyme-substrate complexes. Owing to the intimate association of enzyme activity and precursor processing in this protein family (the Ntn hydrolases), most attempts to alter active-site residues lead to processing defects. Mutation of the invariant residue Arg B263 results in the accumulation of a protein precursor form. However, the mutation of Asn B241, a residue implicated in stabilisation of the tetrahedral intermediate during catalysis, inactivates the enzyme but does not prevent autocatalytic processing or the ability to bind substrates. The crystal structure of the Asn B241 Ala oxyanion hole mutant enzyme has been determined in its native form and in complex with penicillin G and penicillin G sulphoxide. We show that Asn B241 has an important role in maintaining the active site geometry and in productive substrate binding, hence the structure of the mutant protein is a poor model for the Michaelis complex. For this reason, we subsequently solved the structure of the wild-type protein in complex with the slowly processed substrate penicillin G sulphoxide. Analysis of this structure suggests that the reaction mechanism proceeds via direct nucleophilic attack of Ser B1 on the scissile amide and not as previously proposed via a tightly H-bonded water molecule acting as a "virtual" base.

Inhibition of human alpha-thrombin by a phosphonate tripeptide proceeds via a metastable pentacoordinated phosphorus intermediate.

X-ray crystallographic studies of human alpha-thrombin with a novel synthetic inhibitor, an acyl (alpha-aminoalkyl)phosphonate, reveal the existence of a pentacovalent phosphorus intermediate state. Crystal structures of the complex of alpha-thrombin with the phosphonate compound were determined independently using crystals of different ages. The first structure, solved from a crystal less than seven days old, showed a pentacoordinated phosphorus moiety. The second structure, determined from a crystal that was 12 weeks old, showed a tetracoordinated phosphorus moiety. In the first structure, a water molecule, made nucleophilic by coordination to His57 of alpha-thrombin, is bonded to the pentacoordinated phosphorus atom. Its position is approximately equivalent to that occupied by the water molecule responsible for hydrolytic deacylation during normal hydrolysis. The pentacoordinated phosphorus adduct collapses to give the expected pseudo tetrahedral complex, where the phosphorus atom is covalently bonded to Ser195 O(gamma). The crystallographic data presented here therefore suggest that the covalent bond formed between the inhibitor's phosphorus atom and O(gamma) of Ser195 proceeds via an addition-elimination mechanism, which involves the formation of a pentacoordinate intermediate.

The dimerization interface of the metastasis-associated protein S100A4 (Mts1): in vivo and in vitro studies.

The S100 calcium-binding proteins are implicated in signal transduction, motility, and cytoskeletal dynamics. The three-dimensional structure of several S100 proteins revealed that the proteins form non-covalent dimers. However, the mechanism of the S100 dimerization is still obscure. In this study we characterized the dimerization of S100A4 (also named Mts1) in vitro and in vivo. Analytical ultracentrifugation revealed that apoS100A4 was present in solution as a mixture of monomers and dimers in a rapidly reversible equilibrium (K(d) = 4 +/- 2 microm). The binding of calcium promoted dimerization. Replacement of Tyr-75 by Phe resulted in the stabilization of the dimer. Helix IV is known to form the major part of the dimerization interface in homologous S100 proteins. By using the yeast two-hybrid system we showed that only a few residues of helix IV, namely Phe-72, Tyr-75, Phe-78, and Leu-79, are essential for dimerization in vivo. A homology model demonstrated that these residues form a hydrophobic cluster on helix IV. Their role is to stabilize the structure of individual subunits rather than provide specific interactions across the dimerization surface. Our mutation data showed that the specificity at the dimerization surface is not particularly stringent, which is consistent with recent data indicating that S100 proteins can form heterodimers.

The three-dimensional structure of human S100A12.

The crystal structure of human EF-hand calcium-binding protein S100A12 in its calcium-bound form has been determined to 1.95 A resolution by molecular replacement using the structure of the S100B protein. The S100 family members are homologous to calmodulin and other related EF-hand calcium-binding proteins. Like the majority of S100 proteins, S100A12 is a dimer, with the interface between the two subunits being composed mostly of hydrophobic residues. The fold of S100A12 is similar to the other known crystal and solution structures of S100 proteins, except for the linker region between the two EF-hand motifs. Sequence and structure comparison between members of the S100 family suggests that the target-binding region in S100A12 is formed by the linker region and C-terminal residues of one subunit and the N-terminal residues of another subunit of the dimer. The N-terminal region of the target-binding site includes two glutamates that are conserved in most of the S100 sequences. The comparison also provided a better understanding of the role of the residues important for intra- and inter-subunit hydrophobic interactions. The precise role of S100A12 in cell behaviour is yet undefined, as is the case for the whole family, although it has been shown that the interaction of S100A12 with the RAGE receptor is implicated in inflammatory response.

Structure of a slow processing precursor penicillin acylase from Escherichia coli reveals the linker peptide blocking the active-site cleft.

Penicillin G acylase is a periplasmic protein, cytoplasmically expressed as a precursor polypeptide comprising a signal sequence, the A and B chains of the mature enzyme (209 and 557 residues respectively) joined by a spacer peptide of 54 amino acid residues. The wild-type AB heterodimer is produced by proteolytic removal of this spacer in the periplasm. The first step in processing is believed to be autocatalytic hydrolysis of the peptide bond between the C-terminal residue of the spacer and the active-site serine residue at the N terminus of the B chain. We have determined the crystal structure of a slowly processing precursor mutant (Thr263Gly) of penicillin G acylase from Escherichia coli, which reveals that the spacer peptide blocks the entrance to the active-site cleft consistent with an autocatalytic mechanism of maturation. In this mutant precursor there is, however, an unexpected cleavage at a site four residues from the active-site serine residue. Analyses of the stereochemistry of the 260-261 bond seen to be cleaved in this precursor structure and of the 263-264 peptide bond have suggested factors that may govern the autocatalytic mechanism.

Heterocomplex formation between metastasis-related protein S100A4 (Mts1) and S100A1 as revealed by the yeast two-hybrid system.

S100A4 (Mts1) is a Ca(2+)-binding protein of the S100 family. This protein plays an important role in promoting tumor metastasis. In order to identify S100A4 interacting proteins, we have applied the yeast two-hybrid system as an in vivo approach. By screening a mouse mammary adenocarcinoma library, we have demonstrated that S100A4 forms a heterocomplex with S100A1, another member of the S100 family. The non-covalent heterodimerization was confirmed by fluorescence spectroscopy and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Mutational analysis revealed that replacement of Cys(76) and/or Cys(81) of S100A4 by Ser abolishes the S100A4/S100A1 heterodimerization, but does not affect the S100A4 homodimerization in vivo.

Structure of the intact transactivation domain of the human papillomavirus E2 protein.

Papillomaviruses cause warts and proliferative lesions in skin and other epithelia. In a minority of papillomavirus types ('high risk, including human papillomaviruses 16, 18, 31, 33, 45 and 56), further transformation of the wart lesions can produce tumours. The papillomavirus E2 protein controls primary transcription and replication of the viral genome. Both activities are governed by a approximately 200 amino-acid amino-terminal module (E2NT) which is connected to a DNA-binding carboxy-terminal module by a flexible linker. Here we describe the crystal structure of the complete E2NT module from human papillomavirus 16. The E2NT module forms a dimer both in the crystal and in solution. Amino acids that are necessary for transactivation are located at the dimer interface, indicating that the dimer structure may be important in the interactions of E2NT with viral and cellular transcription factors. We propose that dimer formation may contribute to the stabilization of DNA loops which may serve to relocate distal DNA-binding transcription factors to the site of human papillomavirus transcription initiation.

Crystallization and preliminary X-ray diffraction analysis of human calcium-binding protein S100A12.

S100A12, a member of the calgranulin family, isolated from human blood, has been crystallized by vapour diffusion in the presence of Ca(2+). Crystals belong to the space group R3 with unit-cell dimensions a = b = 99.6 c = 64.2 A. There are two monomers per asymmetric unit, with a solvent content of 57.9%. The crystals diffract to at least 2.2 A resolution and complete X-ray data have been collected to 2.5 A on a conventional laboratory source.

Crystallization and preliminary X-ray diffraction studies on the N-utilizing substance-B (NusB) from Mycobacterium tuberculosis.

N-utilizing substance B (NusB) is a protein which forms part of a complex assembly in transcriptional antitermination in Mycobacterium tuberculosis. It forms a heterodimer with the product of the NusE gene (identical to the ribosomal protein S10) and mediates the process of transcriptional antitermination by forming the core complex with the nut site of the ribosomal RNA along with other protein factors. NusB has been cloned and overexpressed in Escherichia coli and crystallized using the hanging-drop vapour-diffusion method. The space group is P2(1)2(1)2(1), with unit-cell parameters a = 46.6, b = 64.2, c = 90.1 A. A native data set complete to 1.6 A resolution has been collected from a single crystal.

Structural consequences of the B5 histidine --> tyrosine mutation in human insulin characterized by X-ray crystallography and conformational analysis.

The addition of phenols to hexameric insulin solutions produces a particularly stable hexamer, resulting from a rearrangement in which residues B1-B8 change from an extended conformation (T-state) to form an alpha-helix (R-state). The R-state is, in part, stabilized by nonpolar interactions between the phenolic molecule and residue B5 His at the dimer-dimer interface. The B5 His --> Tyr mutant human insulin was constructed to see if the tyrosine side chain would mimic the effect of phenol binding in the hexamer and induce the R-state. In partial support of this hypothesis, the molecule crystallized as a half-helical hexamer (T(3)R(3)) in conditions that conventionally promote the fully nonhelical (T6) form. As expected, in the presence of phenol or resorcinol, the B5 Tyr hexamers adopt the fully helical (R6) conformation. Molecular modeling calculations were performed to investigate the conformational preference of the T-state B5 Tyr side chain in the T(3)R(3) form, this side chain being associated with structural perturbations of the A7-A10 loop in an adjacent hexamer. For an isolated dimer, several different orientations of the side chain were found, which were close in energy and readily interconvertible. In the crystal environment only one of these conformations remains low in energy; this conformation corresponds to that observed in the crystal structure. This suggests that packing constraints around residue B5 Tyr result in the observed structural rearrangements. Thus, rather than promoting the R-state in a manner analogous to phenol, the mutation appears to destabilize the T-state. These studies highlight the role of B5 His in determining hexamer conformation and in mediating crystal packing interactions, properties that are likely be important in vivo.

Regulatory features of the trp operon and the crystal structure of the trp RNA-binding attenuation protein from Bacillus stearothermophilus.

Characterization of both the cis and trans -acting regulatory elements indicates that the Bacillus stearothermophilustrp operon is regulated by an attenuation mechanism similar to that which controls the trp operon in Bacillus subtilis. Secondary structure predictions indicate that the leader region of the trp mRNA is capable of folding into terminator and anti- terminator RNA structures. B. stearothermophilus also encodes an RNA-binding protein with 77% sequence identity with the RNA-binding protein (TRAP) that regulates attenuation in B. subtilis. The X-ray structure of this protein has been determined in complex with L-tryptophan at 2.5 A resolution. Like the B. subtilis protein, B. stearothermophilus TRAP has 11 subunits arranged in a ring-like structure. The central cavities in these two structures have different sizes and opposite charge distributions, and packing within the B. stearothermophilus TRAP crystal form does not generate the head-to-head dimers seen in the B. subtilis protein, suggesting that neither of these properties is functionally important. However, the mode of L-tryptophan binding and the proposed RNA binding surfaces are similar, indicating that both proteins are activated by l -tryptophan and bind RNA in essentially the same way. As expected, the TRAP:RNA complex from B. stearothermophilus is significantly more thermostable than that from B. subtilis, with optimal binding occurring at 70 degrees C.

Expression, purification and DNA-cleavage activity of recombinant 68-kDa human topoisomerase I-target for antitumor drugs.

The gene encoding human DNA topoisomerase (topo) I, the target of numerous anticancer drugs, has been subcloned into bacterial, yeast and baculovirus-based expression systems in attempts to overexpress the enzyme for extensive structural and functional characterisation. Expression in E.coli produced a protein which was not suitable for structural studies. Expression in the yeast system was more successful enabling the enzyme to be purified and characterised. However, the resulting yield was modest for our requirements and the full-length protein was found to be susceptible to proteolysis when expressed in this system. As it is known that topo I from human placental tissue contains significant quantities of a 68kDa proteolytic fragment which retains both DNA relaxation and cleavage activity, we have isolated this fragment and shown by N-terminal sequence analysis that it starts at Lysine-191. This information was used to construct vectors which direct the overexpression of this fragment in baculovirus infected insect cells. The recombinant protein has been purified to homogeneity in a yield of 5-10mg/l of cell culture. The fragment is stable and retains all of the DNA driving activities of the intact enzyme. We have characterised the interactions of the topo I fragment with synthetic DNA substrates and identified oligonucleotides and conditions that allow covalent complexes between 68kDa topo I and DNA to be formed with high efficiency and in large quantity. A flow linear dichroism technique has been further developed and applied for real-time monitoring of supercoiled (sc) DNA relaxation by the enzyme and for comparative analysis of inhibition of 68kDa topo I by camptothecin (CPT).

Raman and CD spectroscopy of recombinant 68-kDa DNA human topoisomerase I and its complex with suicide DNA-substrate.

N-terminally truncated recombinant 68-kDa human topoisomerase (topo) I exhibits the same DNA-driving activities as the wild-type protein. In the present study, Raman and circular dichroism techniques were employed for detailed structural characterization of the 68-kDa human topo I and its transformations induced by the suicide sequence-specific oligonucleotide (solig) binding and cleavage. Spectroscopic data combined with statistical prediction techniques were employed to construct a model of the secondary structure distribution along the primary protein structure in solution. The 68-kDa topo I was found to consist of ca. 59% alpha-helix, 24% beta-strand and/or sheets, and 17% other structures. A secondary structure transition of the 68-kDa topo I was found to accompany solig binding and cleavage. Nearly 15% of the alpha-helix of 68-kDa topo I is transferred within the other structures when in the complex with its DNA substrate. Raman spectroscopy analysis also shows redistribution of the structural rotamers of the 68-kDa topo I disulfide bonds and significant changes in the H-bonding of the Tyr residues and in the microenvironment/conformation of the Trp side chains. No structural modifications of the DNA substrate were detected by spectroscopic techniques. The data presented provide the first direct experimental evidence of the human topo I conformational transition after the cleavage step in the reaction of binding and cleavage of DNA substrate by the enzyme. This evidence supports the model of the enzyme function requiring the protein conformational transition. The most probable location of the enzyme transformations was the core and the C-terminal conservative 68-kDa topo I structural domains. By contrast, the linker domain was found to have an extremely low potential for solig-induced structural transformations. The pattern of redistribution of protein secondary structures induced by solig binding and covalent suicide complex formation supports the model of an intramolecular bipartite mode of topo I/DNA interaction in the substrate binding and cleavage reaction.