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.

Dysbiosis of Gut Microbiome Is Associated With Rupture of Cerebral Aneurysms.

BACKGROUND AND PURPOSE: Environmental factors are important with respect to the rupture of cerebral aneurysms. However, the relationship between the gut microbiome, an environmental factor, and aneurysm rupture is unclear. Therefore, we compared the gut microbiome in patients with unruptured intracranial aneurysms (UIAs) and ruptured aneurysms (RAs) to identify the specific bacteria causing the rupture of cerebral aneurysms. METHODS: A multicenter, prospective case-control study was conducted over one year from 2019 to 2020. The fecal samples of patients with stable UIAs and RAs immediately after onset were collected. Their gut microbiomes were analyzed using 16S rRNA sequencing. Subsequently, a phylogenetic tree was constructed, and polymerase chain reaction was performed to identify the specific species. RESULTS: A total of 28 RAs and 33 UIAs were included in this study. There was no difference in patient characteristics between RAs and UIAs: age, sex, hypertension, dyslipidemia, diabetes status, body mass index, and smoking. No difference was observed in alpha diversity; however, beta diversity was significantly different in the unweighted UniFrac distances. At the phylum level, the relative abundance of Campylobacter in the RA group was larger than that in the UIA group. Furthermore, the gut microbiome in the RA and UIA groups exhibited significantly different taxonomies. However, Campylobacter was focused on because it is widely known as pathogenic among these bacteria. Then, a phylogenetic tree of operational taxonomic units related to Campylobacter was constructed and 4 species were identified. Polymerase chain reaction for these species identified that the abundance of the genus Campylobacter and Campylobacter ureolyticus was significantly higher in the RA group. CONCLUSIONS: The gut microbiome profile of patients with stable UIAs and RAs were significantly different. The genus Campylobacter and Campylobacter ureolyticus may be associated with the rupture of cerebral aneurysms.

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.

Longitudinal alterations of the gut mycobiota and microbiota on COVID-19 severity.

BACKGROUND: The impact of SARS-CoV-2 infection on the gut fungal (mycobiota) and bacterial (microbiota) communities has been elucidated individually. This study analyzed both gut mycobiota and microbiota and their correlation in the COVID-19 patients with severe and mild conditions and follow-up to monitor their alterations after recovery. METHODS: We analyzed the gut mycobiota and microbiota by bacterial 16S and fungal ITS1 metagenomic sequencing of 40 severe patients, 38 mild patients, and 30 healthy individuals and reanalyzed those of 10 patients with severe COVID-19 approximately 6 months after discharge. RESULTS: The mycobiota of the severe and mild groups showed lower diversity than the healthy group, and in some, characteristic patterns dominated by a single fungal species, Candida albicans, were detected. Lower microbial diversity in the severe group was observed, but no differences in its diversity or community structure were detected between the mild and healthy groups. The microbiota of the severe group was characterized by an increase in Enterococcus and Lactobacillus, and a decrease in Faecalibacterium and Bacteroides. The abundance of Candida was positively correlated with that of Enterococcus in patients with COVID-19. After the recovery of severe patients, alteration of the microbiota remained, but the mycobiota recovered its diversity comparable to that of mild and healthy groups. CONCLUSION: In mild cases, the microbiota is stable during SARS-CoV-2 infection, but in severe cases, alterations persist for 6 months after recovery.

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.

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.

Alteration of microbial composition in the skin and blood in vasculitis.

Vasculitis is a systemic autoimmune disease characterized by leukocyte infiltration into blood vessels. Various microorganisms have been associated with the pathogenesis of vasculitis; however, the causal microbial agents and underlying mechanisms are not fully understood, possibly because of the technical limitations of pathogen detection. In the present study, we characterized the microbiome profile of patients with cutaneous vasculitis using comprehensive metagenome shotgun sequencing. We found that the abundance of the SEN virus was increased in the affected skin and serum of patients with vasculitis compared to healthy donors. In particular, the abundance of SEN virus reads was increased in the sera of patients with cutaneous arteritis. Among the bacteria identified, Corynebacteriales was the most differentially associated with vasculitis. Linear discriminant analysis effect size also indicated differences in the microbial taxa between patients with vasculitis and healthy donors. These findings demonstrate that vasculitis is associated with considerable alteration of the microbiome in the blood and skin and suggest a role for the infectious trigger in vasculitis.

Impact of G29179T mutation on two commercial PCR assays for SARS-CoV-2 detection.

Nucleic acid amplification test (NAAT) is the gold standard for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection. However, genetic mutations in the virus can affect the result. Cycle threshold (Ct) values of N genes and their association with mutations using SARS-CoV-2 positive specimens diagnosed by the Xpert Xpress SARS-CoV-2 were examined in this study. In total, 196 nasopharyngeal swab specimens were tested for SARS-CoV-2 infection using the Xpert Xpress SARS-CoV-2, and 34 were positive. WGS was performed for four outlier samples with increased ΔCt identified by Scatterplot analysis as well as seven control samples without increased ΔCt in the Xpert Xpress SARS-CoV-2. The presence of the G29179T mutation was identified as a cause of increased ΔCt. PCR using the Allplex™ SARS-CoV-2 Assay did not show a similar increase in ΔCt. Previous reports focusing on N-gene mutations and their effects on SARS-CoV-2 testing including the Xpert Xpress SARS-CoV-2 were also summarized. While a single mutation that impacts one target of a multiplex NAAT is not a true detection failure, mutation compromising NAAT target region can cause confusion of the results and render the assay susceptible to diagnostic failure.

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.

Rapid and simultaneous identification of three mutations by the Novaplex™ SARS-CoV-2 variants I assay kit.

BACKGROUND: . The emergence of SARS-CoV-2 variants has caused an unexpected rebound globally. The World Health Organization has listed three variants (B.1.1.7, B.1.351, and P.1) as variants of concern. To understand the epidemiology and thereby plan appropriate safety measures, differential identification of the variants is indeed critical. OBJECTIVES: . Although whole-genome sequencing is the gold standard for variant identification, it is time-consuming and relatively expensive. Therefore, a rapid, easy, and cost-effective platform targeting multiple regions of the genome is required. Here, we assessed the usefulness of the Novaplex™ SARS-CoV-2 Variants I Assay kit in identifying mutations in the variants. STUDY DESIGN: . We retrospectively examined 30 stored nasal swabs from COVID-19-positive patients tested between November 2020 and March 2021. RNA extracted from these swabs was subjected to the commercial kit and real-time reverse transcription-PCR was performed. To determine the genome sequences of SARS-CoV-2 in the collected samples and deduce the consensus sequences among the identified variants, genome sequencing libraries were prepared and mapped to the reference genome. RESULTS: . Four of the tested samples were determined as variants. Of them, two harbored both H69/V70 deletion and N501Y substitution, whereas two harbored E484K substitution alone. CONCLUSIONS: . The variant with E484K substitution alone ("R.1") has been now categorized as a variant of interest in Japan. Additionally, the kit-based assay was found to be feasible, convenient, and user-friendly in identifying the abovementioned mutations with a turnaround time of only 2 h.

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.

Structural and functional insights into thermally stable cytochrome c' from a thermophile.

Thermophilic Hydrogenophilus thermoluteolus cytochrome c' (PHCP) exhibits higher thermal stability than a mesophilic counterpart, Allochromatium vinosum cytochrome c' (AVCP), which has a homo-dimeric structure and ligand-binding ability. To understand the thermal stability mechanism and ligand-binding ability of the thermally stable PHCP protein, the crystal structure of PHCP was first determined. It formed a homo-dimeric structure, the main chain root mean square deviation (rmsd) value between PHCP and AVCP being 0.65 Å. In the PHCP structure, six specific residues appeared to strengthen the heme-related and subunit-subunit interactions, which were not conserved in the AVCP structure. PHCP variants having altered subunit-subunit interactions were more severely destabilized than ones having altered heme-related interactions. The PHCP structure further revealed a ligand-binding channel and a penta-coordinated heme, as observed in the AVCP protein. A spectroscopic study clearly showed that some ligands were bound to the PHCP protein. It is concluded that the dimeric PHCP from the thermophile is effectively stabilized through heme-related and subunit-subunit interactions with conservation of the ligand-binding ability. BRIEF SUMMARY: We report the X-ray crystal structure of cytochrome c' (PHCP) from thermophilic Hydrogenophilus thermoluteolus. The high thermal stability of PHCP was attributed to heme-related and subunit-subunit interactions, which were confirmed by a mutagenesis study. The ligand-binding ability of PHCP was examined by spectrophotometry. PHCP acquired the thermal stability with conservation of the ligand-binding ability. This study furthers the understanding of the stability and function of cytochromes c.

Homo-trimeric Structure of the Type IVb Minor Pilin CofB Suggests Mechanism of CFA/III Pilus Assembly in Human Enterotoxigenic Escherichia coli.

In gram-negative bacteria, the assembly of type IV pilus (T4P) and the evolutionally related pseudopilus of type II secretion system involves specialized structural proteins called pilins and pseudopilins, respectively, and is dynamically regulated to promote bacterial pathogenesis. Previous studies have suggested that a structural "tip"-like hetero-complex formed through the interaction of at least three minor (pseudo) pilins plays an important role in this process, while some members of the pathogenic type IVb subfamily are known to have only one such minor pilin subunit whose function is still unknown. Here, we determined the crystal structure of the type IVb minor pilin CofB of colonization factor antigen/III from human enterotoxigenic Escherichia coli at 1.88-Å resolution. The crystal structure, in conjunction with physicochemical analysis in solution, reveals a symmetrical homo-trimeric arrangement distinct from the hetero-complexes of minor (pseudo) pilins observed in other T4P and type II secretion systems. Each CofB monomer adopts a unique three-domain architecture, in which the C-terminal ß-sheet-rich lectin domain can effectively initiate trimer association of its pilin-like N-terminal domain through extensive hydrophobic interactions followed by domain swapping at the central hinge-like domain. Deletion of cofB produces a phenotype with no detectable pili formation on the cell surface, while molecular modeling indicates that the characteristic homo-trimeric structure of CofB is well situated at the pilus tip of colonization factor antigen/III formed by the major pilin CofA, suggesting a role for the minor pilin in the efficient initiation of T4P assembly.

Cloning, expression, purification, crystallization and X-ray crystallographic analysis of CofB, the minor pilin subunit of CFA/III from human enterotoxigenic Escherichia coli.

Colonization factor antigen III (CFA/III) is one of the virulence factors of human enterotoxigenic Escherichia coli (ETEC) that forms the long, thin, proteinaceous fibres of type IV pili through assembly of its major and minor subunits CofA and CofB, respectively. The crystal structure of CofA has recently been reported; however, the lack of structural information for CofB, the largest among the known type IV pilin subunits, hampers a comprehensive understanding of CFA/III pili. In this study, constructs of wild-type CofB with an N-terminal truncation and the corresponding SeMet derivative were cloned, expressed, purified and crystallized. The crystals belonged to the rhombohedral space group R32, with unit-cell parameters a = b = 103.97, c = 364.57 Å for the wild-type construct and a = b = 103.47, c = 362.08 Å for the SeMet-derivatized form. Although the diffraction quality of these crystals was initially very poor, dehydration of the crystals substantially improved the resolution limit from ∼ 4.0 to ∼ 2.0 Å. The initial phase was solved by the single-wavelength anomalous dispersion (SAD) method using a dehydrated SeMet CofB crystal, which resulted in an interpretable electron-density map.