Contribution of a surface salt bridge to the protein stability of deep-sea Shewanella benthica cytochrome c'.

Two homologous cytochromes c', SBCP and SVCP, from deep-sea Shewanella benthica and Shewanella violacea respectively exhibit only nine surface amino acid substitutions, along with one at the N-terminus. Despite the small sequence difference, SBCP is thermally more stable than SVCP. Here, we examined the thermal stability of SBCP variants, each containing one of the nine substituted residues in SVCP, and found that the SBCP K87V variant was the most destabilized. We then determined the X-ray crystal structure of the SBCP K87V variant at a resolution of 2.1 Å. The variant retains a four-helix bundle structure similar to the wild-type, but notable differences are observed in the hydration structure around the mutation site. Instead of forming of the intrahelical salt bridge between Lys-87 and Asp-91 in the wild-type, a clathrate-like hydration around Val-87 through a hydrogen bond network with the nearby amino acid residues is observed. This network potentially enhances the ordering of surrounding water molecules, leading to an entropic destabilization of the protein. These results suggest that the unfavorable hydrophobic hydration environment around Val-87 and the inability to form the Asp-91-mediated salt bridge contribute to the observed difference in stability between SBCP and SVCP. These findings will be useful in future protein engineering for controlling protein stability through the manipulation of surface intrahelical salt bridges.

Spatiotemporal regulation of PEDF signaling by type I collagen remodeling.

Dynamic remodeling of the extracellular matrix affects many cellular processes, either directly or indirectly, through the regulation of soluble ligands; however, the mechanistic details of this process remain largely unknown. Here we propose that type I collagen remodeling regulates the receptor-binding activity of pigment epithelium-derived factor (PEDF), a widely expressed secreted glycoprotein that has multiple important biological functions in tissue and organ homeostasis. We determined the crystal structure of PEDF in complex with a disulfide cross-linked heterotrimeric collagen peptide, in which the α(I) chain segments-each containing the respective PEDF-binding region (residues 930 to 938)-are assembled with an α2α1α1 staggered configuration. The complex structure revealed that PEDF specifically interacts with a unique amphiphilic sequence, KGHRGFSGL, of the type I collagen α1 chain, with its proposed receptor-binding sites buried extensively. Molecular docking demonstrated that the PEDF-binding surface of type I collagen contains the cross-link-susceptible Lys930 residue of the α1 chain and provides a good foothold for stable docking with the α1(I) N-telopeptide of an adjacent triple helix in the fibril. Therefore, the binding surface is completely inaccessible if intermolecular crosslinking between two crosslink-susceptible lysyl residues, Lys9 in the N-telopeptide and Lys930, is present. These structural analyses demonstrate that PEDF molecules, once sequestered around newly synthesized pericellular collagen fibrils, are gradually liberated as collagen crosslinking increases, making them accessible for interaction with their target cell surface receptors in a spatiotemporally regulated manner.

Analysis of the complete genome sequences of Clostridium perfringens strains harbouring the binary enterotoxin BEC gene and comparative genomics of pCP13-like family plasmids.

BACKGROUND: BEC-producing Clostridium perfringens is a causative agent of foodborne gastroenteritis. It was first reported in 2014, and since then, several isolates have been identified in Japan and the United Kingdom. The novel binary ADP-ribosylating toxin BEC, which consists of two components (BECa and BECb), is encoded on a plasmid that is similar to pCP13 and harbours a conjugation locus, called Pcp, encoding homologous proteins of the type 4 secretion system. Despite the high in vitro conjugation frequency of pCP13, its dissemination and that of related plasmids, including bec-harbouring plasmids, in the natural environment have not been characterised. This lack of knowledge has limited our understanding of the genomic epidemiology of bec-harbouring C. perfringens strains. RESULTS: In this study, we determined the complete genome sequences of five bec-harbouring C. perfringens strains isolated from 2009 to 2019. Each isolate contains a ~ 3.36 Mbp chromosome and 1-3 plasmids of either the pCW3-like family, pCP13-like family, or an unknown family, and the bec-encoding region in all five isolates was located on a ~ 54 kbp pCP13-like plasmid. Phylogenetic and SNP analyses of these complete genome sequences and the 211 assembled C. perfringens genomes in GenBank showed that although these bec-harbouring strains were split into two phylogenetic clades, the sequences of the bec-encoding plasmids were nearly identical (>99.81%), with a significantly smaller SNP accumulation rate than that of their chromosomes. Given that the Pcp locus is conserved in these pCP13-like plasmids, we propose a mechanism in which the plasmids were disseminated by horizontal gene transfer. Data mining showed that strains carrying pCP13-like family plasmids were unexpectedly common (58/216 strains) and widely disseminated among the various C. perfringens clades. Although these plasmids possess a conserved Pcp locus, their 'accessory regions' can accommodate a wide variety of genes, including virulence-associated genes, such as becA/becB and cbp2. These results suggest that this family of plasmids can integrate various foreign genes and is transmissible among C. perfringens strains. CONCLUSION: This study demonstrates the potential significance of pCP13-like plasmids, including bec-encoding plasmids, for the characterisation and monitoring of the dissemination of pathogenic C. perfringens strains.

Substrate-induced product-release mechanism of lipocalin-type prostaglandin D synthase.

Prostaglandin D2 (PGD2), an endogenous somnogen, is a unique PG that is secreted into the cerebrospinal fluid. PGD2 is a relatively fragile molecule and should be transported to receptors localized in the basal forebrain without degradation. However, it remains unclear how PGD2 is stably carried to such remote receptors. Here, we demonstrate that the PGD2-synthesizing enzyme, Lipocalin-type prostaglandin D synthase (L-PGDS), binds not only its substrate PGH2 but also its product PGD2 at two distinct binding sites for both ligands. This behaviour implys its PGD2 carrier function. Nevertheless, since the high affinity (Kd = âˆ¼0.6 µM) of PGD2 in the catalytic binding site is comparable to that of PGH2, it may act as a competitive inhibitor, while our binding assay exhibits only weak inhibition (Ki = 189 µM) of the catalytic reaction. To clarify this enigmatic behavior, we determined the solution structure of L-PGDS bound to one substrate analog by NMR and compared it with the two structures: one in the apo form and the other in substrate analogue complex with 1:2 stoichiometry. The structural comparisons showed clearly that open or closed forms of loops at the entrance of ligand binding cavity are regulated by substrate binding to two sites, and that the binding to a second non-catalytic binding site, which apparently substrate concentration dependent, induces opening of the cavity that releases the product. From these results, we propose that L-PGDS is a unique enzyme having a carrier function and a substrate-induced product-release mechanism.

Interplay of a secreted protein with type IVb pilus for efficient enterotoxigenic Escherichia coli colonization.

Initial attachment and subsequent colonization of the intestinal epithelium comprise critical events allowing enteric pathogens to survive and express their pathogenesis. In enterotoxigenic Escherichia coli (ETEC), these are mediated by a long proteinaceous fiber termed type IVb pilus (T4bP). We have reported that the colonization factor antigen/III (CFA/III), an operon-encoded T4bP of ETEC, possesses a minor pilin, CofB, that carries an H-type lectin domain at its tip. Although CofB is critical for pilus assembly by forming a trimeric initiator complex, its importance for bacterial attachment remains undefined. Here, we show that T4bP is not sufficient for bacterial attachment, which also requires a secreted protein CofJ, encoded within the same CFA/III operon. The crystal structure of CofB complexed with a peptide encompassing the binding region of CofJ showed that CofJ interacts with CofB by anchoring its flexible N-terminal extension to be embedded deeply into the expected carbohydrate recognition site of the CofB H-type lectin domain. By combining this structure and physicochemical data in solution, we built a plausible model of the CofJ-CFA/III pilus complex, which suggested that CofJ acts as a molecular bridge by binding both T4bP and the host cell membrane. The Fab fragments of a polyclonal antibody against CofJ significantly inhibited bacterial attachment by preventing the adherence of secreted CofJ proteins. These findings signify the interplay between T4bP and a secreted protein for attaching to and colonizing the host cell surface, potentially constituting a therapeutic target against ETEC infection.

N-terminal HCV core protein fragment decreases 20S proteasome activity in the presence of PA28γ.

The Hepatitis C virus (HCV) core protein plays a crucial role in the development of chronic liver diseases such as chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). Its involvement in these diseases is reportedly abolished by a knockout of the proteasome activator PA28γ gene in transgenic mice, suggesting an interaction between the core protein and the PA28γ-proteasome system. This study found a direct interaction between the N-terminal 1-71 fragment of HCV core protein (Core71) and PA28γ in vitro, and that this interaction was found to enhance PA28γ-20S proteasome complex formation. While 20S proteasome activity was increased by PA28γ, it was significantly reduced by Core71 attachment in a dose-dependent manner. These results suggest that the Core-PA28γ interaction has an important role in regulating 20S proteasome activity and furthers our understanding of the pathogenesis of HCV.

Stability of cytochromes c' from psychrophilic and piezophilic Shewanella species: implications for complex multiple adaptation to low temperature and high hydrostatic pressure.

The stability of dimeric cytochrome c' from a thermophile, as compared with that of a homologous mesophilic counterpart, is attributed to strengthened interactions around the heme and at the subunit-subunit interface, both of which are molecular interior regions. Here, we showed that interactions in the equivalent interior regions of homologous cytochromes c' from two psychrophiles, Shewanella benthica and Shewanella violacea (SBCP and SVCP, respectively) were similarly weakened as compared with those of the counterparts of psychrophilic Shewanella livingstonensis and mesophilic Shewanella amazonensis (SLCP and SACP, respectively), and consistently the stability of SVCP, SLCP, and SACP increased in that order. Therefore, the stability of cytochromes c' from the psychrophile, mesophile, and thermophile is systematically regulated in their molecular interior regions. Unexpectedly, however, the stability of SBCP was significantly higher than that of SVCP, and the former had additional molecular surface interactions. Collectively, SBCP had weakened interior interactions like SVCP did, but the former was stabilized at the molecular surface as compared with the latter, implying complex multiple adaptation of the proteins because the psychrophilic sources of SBCP and SVCP are also piezophilic, thriving in deep-sea extreme environments of low temperature and high hydrostatic pressure.

Structural Basis for Dimer Formation of Human Condensin Structural Maintenance of Chromosome Proteins and Its Implications for Single-stranded DNA Recognition.

Eukaryotic structural maintenance of chromosome proteins (SMC) are major components of cohesin and condensins that regulate chromosome structure and dynamics during cell cycle. We here determine the crystal structure of human condensin SMC hinge heterodimer with ~30 residues of coiled coils. The structure, in conjunction with the hydrogen exchange mass spectrometry analyses, revealed the structural basis for the specific heterodimer formation of eukaryotic SMC and that the coiled coils from two different hinges protrude in the same direction, providing a unique binding surface conducive for binding to single-stranded DNA. The characteristic hydrogen exchange profiles of peptides constituted regions especially across the hinge-hinge dimerization interface, further suggesting the structural alterations upon single-stranded DNA binding and the presence of a half-opened state of hinge heterodimer. This structural change potentially relates to the DNA loading mechanism of SMC, in which the hinge domain functions as an entrance gate as previously proposed for cohesin. Our results, however, indicated that this is not the case for condensins based on the fact that the coiled coils are still interacting with each other, even when DNA binding induces structural changes in the hinge region, suggesting the functional differences of SMC hinge domain between condensins and cohesin in DNA recognition.

Crystal structure of the ADP-ribosylating component of BEC, the binary enterotoxin of Clostridium perfringens.

Binary enterotoxin of Clostridium perfringens (BEC), consisting of the components BECa and BECb, was recently identified as a novel enterotoxin produced by C. perfringens that causes acute gastroenteritis in humans. Although the detailed mechanism of cell intoxication by BEC remains to be defined, BECa shows both NAD+-glycohydrolase and actin ADP-ribosyltransferase activities in the presence of NAD+. In this study, we determined the first crystal structure of BECa in its apo-state and in complex with NADH. The structure of BECa shows striking resemblance with other binary actin ADP-ribosylating toxins (ADPRTs), especially in terms of its overall protein fold and mechanisms of substrate recognition. We present a detailed picture of interactions between BECa and NADH, including bound water molecules located near the C1'-N glycosidic bond of NADH and the catalytically important ADP-ribosylating turn-turn (ARTT) loop. We observed that the conformational rearrangement of the ARTT loop, possibly triggered by a conformational change involving a conserved tyrosine residue coupled with substrate binding, plays a crucial role in catalysis by properly positioning a catalytic glutamate residue in the E-X-E motif of the ARTT loop in contact with the nucleophile. Our results for BECa provide insight into the common catalytic mechanism of the family of binary actin ADPRTs.

Comparative study on stabilization mechanism of monomeric cytochrome c5 from deep-sea piezophilic Shewanella violacea.

Monomeric cytochrome c5 from deep-sea piezophilic Shewanella violacea (SVcytc5) was stable against heat and denaturant compared with the homologous protein from shallow-sea piezo-sensitive Shewanella livingstonensis (SLcytc5). Here, the SVcytc5 crystal structure revealed that the Lys-50 side chain on the flexible loop formed a hydrogen bond with heme whereas that of corresponding hydrophobic Leu-50 could not form such a bond in SLcytc5, which appeared to be one of possible factors responsible for the difference in stability between the two proteins. This structural insight was confirmed by a reciprocal mutagenesis study on the thermal stability of these two proteins. As SVcytc5 was isolated from a deep-sea piezophilic bacterium, the present comparative study indicates that adaptation of monomeric SVcytc5 to high pressure environments results in stabilization against heat.

Thermodynamic and NMR analyses of NADPH binding to lipocalin-type prostaglandin D synthase.

Lipocalin-type prostaglandin D synthase (L-PGDS) is one of the most abundant proteins in human cerebrospinal fluid (CSF) with dual functions as a prostaglandin D2 (PGD2) synthase and a transporter of lipophilic ligands. Recent studies revealed that L-PGDS plays important roles in protecting against various neuronal diseases induced by reactive oxygen species (ROS). However, the molecular mechanisms of such protective actions of L-PGDS remain unknown. In this study, we conducted thermodynamic and nuclear magnetic resonance (NMR) analyses, and demonstrated that L-PGDS binds to nicotinamide coenzymes, including NADPH, NADP(+), and NADH. Although a hydrophilic ligand is not common for L-PGDS, these ligands, especially NADPH showed specific interaction with L-PGDS at the upper pocket of its ligand-binding cavity with an unusually bifurcated shape. The binding affinity of L-PGDS for NADPH was comparable to that previously reported for NADPH oxidases and NADPH in vitro. These results suggested that L-PGDS potentially attenuates the activities of NADPH oxidases through interaction with NADPH. Given that NADPH is the substrate for NADPH oxidases that play key roles in neuronal cell death by generating excessive ROS, these results imply a novel linkage between L-PGDS and ROS.

Fast reconstruction of scanning transmission electron microscopy images using Markov random field model.

In this study, we proposed a fast method of reconstruction for scanning transmission electron microscopy images. The proposed method is based on the Markov random field model and Bayesian inference, and we found that the method can reconstruct such images of sizes 512 × 512 and 264 × 240 in less than 200 ms and 100 ms, respectively. Furthermore, we showed that the method of reconstruction from multiple images without averaging them has better reconstruction performance than that from the averaged image.

Structure of the CFA/III major pilin subunit CofA from human enterotoxigenic Escherichia coli determined at 0.90 Å resolution by sulfur-SAD phasing.

CofA, a major pilin subunit of colonization factor antigen III (CFA/III), forms pili that mediate small-intestinal colonization by enterotoxigenic Escherichia coli (ETEC). In this study, the crystal structure of an N-terminally truncated version of CofA was determined by single-wavelength anomalous diffraction (SAD) phasing using five sulfurs in the protein. Given the counterbalance between anomalous signal strength and the undesired X-ray absorption of the solvent, diffraction data were collected at 1.5 Å resolution using synchrotron radiation. These data were sufficient to elucidate the sulfur substructure at 1.38 Å resolution. The low solvent content (29%) of the crystal necessitated that density modification be performed with an additional 0.9 Å resolution data set to reduce the phase error caused by the small sulfur anomalous signal. The CofA structure showed the αß-fold typical of type IVb pilins and showed high structural homology to that of TcpA for toxin-coregulated pili of Vibrio cholerae, including spatial distribution of key residues critical for pilin self-assembly. A pilus-filament model of CofA was built by computational docking and molecular-dynamics simulation using the previously reported filament model of TcpA as a structural template. This model revealed that the CofA filament surface was highly negatively charged and that a 23-residue-long loop between the α1 and α2 helices filled the gap between the pilin subunits. These characteristics could provide a unique binding epitope for the CFA/III pili of ETEC compared with other type IVb pili.

Crystal structure of thermally stable homodimeric cytochrome c'-ß from Thermus thermophilus.

Cytochrome c'-ß is a heme protein that belongs to the cytochrome P460 family and consists of homodimeric subunits with a predominantly antiparallel ß-sheet fold. Here, the crystal structure of cytochrome c'-ß from the thermophilic Thermus thermophilus (TTCP-ß) is reported at 1.74 Šresolution. TTCP-ß has a typical antiparallel ß-sheet fold similar to that of cytochrome c'-ß from the moderately thermophilic Methylococcus capsulatus (MCCP-ß). The phenylalanine cap structure around the distal side of the heme is also similar in TTCP-ß and MCCP-ß, indicating that both proteins similarly bind nitric oxide and carbon monoxide, as observed spectroscopically. Notably, TTCP-ß exhibits a denaturation temperature of 117°C, which is higher than that of MCCP-ß. Mutational analysis reveals that the increased homodimeric interface area of TTCP-ß contributes to its high thermal stability. Furthermore, 14 proline residues, which are mostly located in the TTCP-ß loop regions, possibly contribute to the rigid loop structure compared with MCCP-ß, which has only six proline residues. These findings, together with those from phylogenetic analysis, suggest that the structures of Thermus cytochromes c'-ß, including TTCP-ß, are optimized for function under the high-temperature conditions in which the source organisms live.

Structural basis for the toxin-coregulated pilus-dependent secretion of Vibrio cholerae colonization factor.

Colonization of the host intestine is the most important step in Vibrio cholerae infection. The toxin-coregulated pilus (TCP), an operon-encoded type IVb pilus (T4bP), plays a crucial role in this process, which requires an additional secreted protein, TcpF, encoded on the same TCP operon; however, its mechanisms of secretion and function remain elusive. Here, we demonstrated that TcpF interacts with the minor pilin, TcpB, of TCP and elucidated the crystal structures of TcpB alone and in complex with TcpF. The structural analyses reveal how TCP recognizes TcpF and its secretory mechanism via TcpB-dependent pilus elongation and retraction. Upon binding to TCP, TcpF forms a flower-shaped homotrimer with its flexible N terminus hooked onto the trimeric interface of TcpB. Thus, the interaction between the minor pilin and the N terminus of the secreted protein, namely, the T4bP secretion signal, is key for V. cholerae colonization and is a new potential therapeutic target.

Structure-activity relationship of marinostatin, a serine protease inhibitor isolated from a marine organism.

A 12-residue MST isolated from a marine organism is a potent serine protease inhibitor that has a double cyclic structure composed of two ester linkages formed between the beta-hydroxyl and beta-carboxyl groups, Thr(3)-Asp(9) and Ser(8)-Asp(11). MST was synthesized by a regioselective esterification procedure employing two sets of orthogonally removable side-chain protecting groups for the Asp and Ser/Thr residues. In the MST molecule, there were no significant changes observed in yield by changing the order of esterification. SAR study of MST revealed that the minimum required structure for expressing the inhibitory activity is the sequence (1-9) in a monocyclic structure where Pro(7) located in the ring plays a crucial role in keeping the structural rigidity. By applying the structural motif of MST, we rationally designed protease inhibitory specificities that differ from those of the natural product.

Cloning, expression, crystallization and preliminary X-ray crystallographic analysis of a human condensin SMC2 hinge domain with short coiled coils.

In higher eukaryotes, the condensin complex, which mainly consists of two structural maintenance of chromosomes (SMC) subunits, SMC2 (CAP-E) and SMC4 (CAP-C), plays a critical role in the formation of higher order chromosome structures during mitosis. Biochemical and electron-microscopic studies have revealed that the SMC2 and SMC4 subunits dimerize through the interaction of their hinge domains, forming a characteristic V-shaped heterodimer. However, the details of their function are still not fully understood owing to a lack of structural information at the atomic level. In this study, the human SMC2 hinge domain with short coiled coils was cloned, expressed, purified and crystallized in the orthorhombic space group C222 in native and SeMet-derivatized forms. Because of the poor diffraction properties of these crystals, the mutant Leu68-->SeMet was designed and crystallized in order to obtain the experimental phases. The SeMet-derivatized crystals of the mutant belonged to space group P3(2)12, with unit-cell parameters a=b=128.8, c=91.4 A. The diffraction data obtained from a crystal that diffracted to 2.4 A resolution were suitable for SAD phasing.

Structural Basis for PPARα Activation by 1H-pyrazolo-[3,4-b]pyridine Derivatives.

Small-molecule agonism of peroxisome proliferator-activated receptor α (PPARα), a ligand-activated transcriptional factor involved in regulating fatty acid metabolism, is an important approach for treating dyslipidemia. Here, we determined the structures of the ligand-binding domain (LBD) of PPARα in complex with 1H-pyrazolo[3,4-b]pyridine-4-carboxylic acid derivatives, which were recently identified as PPARα-selective activators with markedly different structures from those of the well-known PPARα agonists fibrates. The crystal structures of the complexes showed that they form a canonical hydrogen-bond network involving helix 12 in the LBD, which is thought to be essential for PPARα activation, as also observed for fibrates. However, the phenyl side chain of the compounds occupies a small cavity between Ile272 and Ile354, which is rarely accessed by fibrates. This unique feature may be essential for subtype selectivity and combine with the well-characterized binding mode of fibrates to improve activity. These findings demonstrate the advantage of using 1H-pyrazolo-[3,4-b]pyridine as a skeleton of PPARα agonists and provide insight into the design of molecules for treating dyslipidemia.

Hyperstability and crystal structure of cytochrome c(555) from hyperthermophilic Aquifex aeolicus.

In order to elucidate the relationship between the stability and the structure of the monohaem cytochrome c(555) (AA c(555)) from the hyperthermophilic bacterium Aquifex aeolicus, chemical denaturation and crystal structure determination were carried out. AA c(555) exhibited higher stability than the thermophilic Hydrogenobacter thermophilus cytochrome c(552) (HT c(552)), which is one of the most stable cytochromes c. The three-dimensional crystal structure of AA c(555), which was determined using the multiple anomalous dispersion technique at 1.15 A resolution, included a unique 14-residue extra helix, while the side-chain interactions of several amino-acid residues responsible for the stability of HT c(552) were conserved in AA c(555). The side chain of the Met61 residue in the extra helix was aligned towards the haem, forming a coordination bond between the Met S and haem Fe atoms. In other cytochromes c the corresponding regions always form Omega loops which also include the haem-liganding Met residue and are known to be involved in the initial step in cytochrome c denaturation. The formation of the extra helix in AA c(555) results in the highest helix content, 59.8%, among the monohaem cytochromes c. The extra helix should mainly contribute to the hyperstability of AA c(555) and is presumed to be a novel strategy of cytochromes c for adaptation to a hyperthermophilic environment.