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Mass photometry (MP) is a versatile, fast and low sample-consuming biophysical technique that gained interest in structural biology to study noncovalent assemblies in native conditions. We report here on a novel method to perform MP analysis in denaturing conditions (dMP) and its application for fast, accurate and straightforward optimization of chemical reactions in cross-linking mass spectrometry (XL-MS) workflows. dMP consists in a robust 2-step protocol that ensures 95% of irreversible denaturation within only 5 min. The proposed single-molecule method clearly overcomes the limitations and outperforms gold standard SDS-PAGE, as illustrated on several biological complexes. dMP provides an unprecedented and unmatched in-solution quantification of all coexisting XL species, including sub-complexes and non-specific XL aggregates, along with identification of significantly higher numbers of XL dipeptides in MS. We anticipate single-molecule dMP to be a high-impact game-changer for the XL-MS community with the potential to leverage the quality and reliability of XL-MS datasets.

International audience; DPCD is a protein that may play a role in cilia formation and whose absence leads to primary ciliary dyskinesia (PCD), a rare disease caused by impairment of ciliated cells. Except for high-throughput studies that identified DPCD as a possible RUVBL1 (R1) and RUVBL2 (R2) partner, no in-depth cellular, biochemical, and structural investigation involving DPCD have been reported so far. R1 and R2 proteins are ubiquitous highly conserved AAA + family ATPases that assemble and mature a plethora of macromolecular complexes and are pivotal in numerous cellular processes, especially by guaranteeing a co-chaperoning function within R2TP or R2TP-like machineries. In the present study, we identified DPCD as a new R1R2 partner in vivo. We show that DPCD interacts directly with R1 and R2 in vitro and in cells. We characterized the physico-chemical properties of DPCD in solution and built a 3D model of DPCD. In addition, we used a variety of orthogonal biophysical techniques including small-angle X-ray scattering, structural mass spectrometry and electron microscopy to assess the molecular determinants of DPCD interaction with R1R2. Interestingly, DPCD disrupts the dodecameric state of R1R2 complex upon binding and this interaction occurs mainly via the DII domains of R1R2.

Serological assays are valuable tools to study SARS‐CoV‐2 spread and, importantly, to identify individuals that were already infected and would be potentially immune to a virus reinfection. SARS‐CoV‐2 Spike protein and its receptor binding domain (RBD) are the antigens with higher potential to develop SARS‐CoV‐2 serological assays. Moreover, structural studies of these antigens are key to understand the molecular basis for Spike interaction with angiotensin converting enzyme 2 receptor, hopefully enabling the development of COVID‐19 therapeutics. Thus, it is urgent that significant amounts of this protein became available at the highest quality. In this study, we produced Spike and RBD in two human derived cell hosts: HEK293‐E6 and Expi293F™. We evaluated the impact of different and scalable bioprocessing approaches on Spike and RBD production yields and, more importantly, on these antigens' quality attributes. Using negative and positive sera collected from human donors, we show an excellent performance of the produced antigens, assessed in serologic enzyme‐linked immunosorbent assay (ELISA) tests, as denoted by the high specificity and sensitivity of the test. We show robust Spike productions with final yields of approx. 2 mg/L of culture that were maintained independently of the production scale or cell culture strategy. To the best of our knowledge, the final yield of 90 mg/L of culture obtained for RBD production, was the highest reported to date. An in‐depth characterization of SARS‐CoV‐2 Spike and RBD proteins was performed, namely the antigen's oligomeric state, glycosylation profiles, and thermal stability during storage. The correlation of these quality attributes with ELISA performance show equivalent reactivity to SARS‐CoV‐2 positive serum, for all Spike and RBD produced, and for all storage conditions tested. Overall, we provide straightforward protocols to produce high‐quality SARS‐CoV‐2 Spike and RBD antigens, that can be easily adapted to both academic and industrial settings; and integrate, for the first time, studies on the impact of bioprocess with an in‐depth characterization of these proteins, correlating antigen's glycosylation and biophysical attributes to performance of COVID‐19 serologic tests., Spike and RBD were produced in human derived cells hosts. Shake flasks, stirred tank and wave bioreactors were compared. The impact of these different scalable bioprocessing approaches on Spike and RBD production yields and quality attributes was evaluated. An in‐depth characterization of SARS‐CoV‐2 Spike and RBD proteins was performed, namely the antigen s oligomeric state, glycosylation profiles and thermal stability during storage. The correlation of these quality attributes with ELISA performance is discussed.

The PAQosome is a large complex composed of the HSP90/R2TP chaperone and a prefoldin-like module. It promotes the biogenesis of cellular machineries but it is unclear how it discriminates closely related client proteins. Among the main PAQosome clients are C/D snoRNPs and in particular their core protein NOP58. Using NOP58 mutants and proteomic experiments, we identify different assembly intermediates and show that C12ORF45, which we rename NOPCHAP1, acts as a bridge between NOP58 and PAQosome. NOPCHAP1 makes direct physical interactions with the CC-NOP domain of NOP58 and domain II of RUVBL1/2 AAA+ ATPases. Interestingly, NOPCHAP1 interaction with RUVBL1/2 is disrupted upon ATP binding. Moreover, while it robustly binds both yeast and human NOP58, it makes little interactions with NOP56 and PRPF31, two other closely related CC-NOP proteins. Expression of NOP58, but not NOP56 or PRPF31, is decreased in NOPCHAP1 KO cells. We propose that NOPCHAP1 is a client-loading PAQosome cofactor that selects NOP58 to promote box C/D snoRNP assembly.

The Notch-signaling ligand DLL1 has emerged as an important player and promising therapeutic target in breast cancer (BC). DLL1-induced Notch activation promotes tumor cell proliferation, survival, migration, angiogenesis and BC stem cell maintenance. In BC, DLL1 overexpression is associated with poor prognosis, particularly in estrogen receptor-positive (ER+) subtypes. Directed therapy in early and advanced BC has dramatically changed the natural course of ER+ BC, however, relapse is a major clinical issue, and new therapeutic strategies are needed. Here, we report the development and characterization of a novel monoclonal antibody specific to DLL1. Using phage display technology, we selected an anti-DLL1 antibody fragment, which was converted into a full human IgG1 (Dl1.72). The Dl1.72 antibody exhibited DLL1 specificity and affinity in the low nanomolar range and significantly impaired DLL1-Notch signaling and expression of Notch target genes in ER+ BC cells. Functionally, in vitro treatment with Dl1.72 reduced MCF-7 cell proliferation, migration, mammosphere formation and endothelial tube formation. In vivo, Dl1.72 significantly inhibited tumor growth, reducing both tumor cell proliferation and liver metastases in a xenograft mouse model, without apparent toxicity. These findings suggest that anti-DLL1 Dl1.72 could be an attractive agent against ER+ BC, warranting further preclinical investigation.

BMX is pursued as a drug target because of its role in various pathophysiological processes. We designed BMX covalent inhibitors with single-digit nanomolar potency with unexploited topological pharmacophore patterns. Importantly, we reveal the first X-ray crystal structure of covalently inhibited BMX at Cys496, which displays key interactions with Lys445, responsible for hampering ATP catalysis and the DFG-out-like motif, typical of an inactive conformation. Molecular dynamic simulations also showed this interaction for two ligand/BMX complexes. Kinome selectivity profiling showed that the most potent compound JS25 is the strongest binder and displays intracellular target engagement in BMX-transfected cells with two-digit nanomolar inhibitory potency. The new inhibitors displayed anti-proliferative effects in androgen-receptor positive prostate cancer cells that where further increased when combined with known inhibitors of related signaling pathways, such as PI3K, AKT and Androgen Receptor. We expect these findings to guide development of new selective BMX therapeutic approaches.

This journal is © The Royal Society of Chemistry 2020., The bone marrow tyrosine kinase in chromosome X (BMX) is pursued as a drug target because of its role in various pathophysiological processes. We designed BMX covalent inhibitors with single-digit nanomolar potency with unexploited topological pharmacophore patterns. Importantly, we reveal the first X-ray crystal structure of covalently inhibited BMX at Cys496, which displays key interactions with Lys445, responsible for hampering ATP catalysis and the DFG-out-like motif, typical of an inactive conformation. Molecular dynamic simulations also showed this interaction for two ligand/BMX complexes. Kinome selectivity profiling showed that the most potent compound is the strongest binder, displays intracellular target engagement in BMX-transfected cells with two-digit nanomolar inhibitory potency, and leads to BMX degradation PC3 in cells. The new inhibitors displayed anti-proliferative effects in androgen-receptor positive prostate cancer cells that where further increased when combined with known inhibitors of related signaling pathways, such as PI3K, AKT and Androgen Receptor. We expect these findings to guide development of new selective BMX therapeutic approaches., Funded under the Royal Society (URF to G. J. L. B., URF\R\180019), FCT Portugal (iFCT to G. J. L. B., IF/00624/2015, Postdoctoral Fellowship SFRH/BPD/95253/2013 to J. D. S., project PTDC/MED-QUI/28764/2017 to J. D. S., 02/SAICT/2017 grant 28333 to T. R., doctoral studentship SFRH/BD/143583/2019 to B. B. S., and DL 57/2016/CP1451/CT0025 to J. D. S.). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreements No. 807281 and 702428. The authors acknowledge the Wellcome Trust (104633/Z/14/Z, to D. W. and C. V. R.), Ministerio de Economía y Competitividad (project RTI2018-099592-B-C21 to F. C.), the Royal G. and Mae H. Westaway Family Memorial Fund (A. N. K.), National Science Foundation Graduate Research Fellowship (Grant No 1122374 to S. K. D.) and Ludwig Center at MIT's Koch Institute (S. K. D.). We also thank the High-Throughput Science Core at MIT supported by the Koch Institute Cancer Center Support Core Grant (P30-CA14051 to A. N. K.). The NMR spectrometers are part of the National NMR Facility, supported by Fundação para a Ciência e Technologic (RECI/BBB-BQB/0230/2012). The authors also thank ESRF and the ID30A-3 beamline staff for support with the X-ray data collection. iNOVA4Health – UID/Multi/04462/2013, a program financially supported by Fundação para a Ciência e Technologic/Ministério da Educação e Ciência, through national funds and co-funded by FEDER under the PT2020 Partnership Agreement, and MOSTMICRO (project LISBOA-01-0145-FEDER-007660) co-funded by FCT/MCTES and FEDER funds through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI) are also acknowledged.

Human carboxylesterase 2 (hCES2) is a glycoprotein involved in the metabolism of drugs and several environmental xenobiotics, whose crystallization has been proved to be a challenging task. This limitation could partly be due to glycosylation heterogeneity and has delayed the disclosure of the 3D structure of hCES2 which would be of upmost relevance for the development of new substrates and inhibitors. The present work evaluated the involvement of glycans in hCES2 activity and thermo stability in an attempt to find alternative active forms of the enzyme that might be adequate for structure elucidation. Partial or non-glycosylated forms of a secreted form of hCES2 have been obtained by three approaches: (i) enzymatic deglycosylation with peptide N-glycosidase F; (ii) incubation with the inhibitor tunicamycin; ii) site directed mutagenesis of each or both N-glycosylation sites. Deglycosylated protein did not show a detectable decrease in enzyme activity. On the other hand, tunicamycin led to decreased levels of secreted hCES2 but the enzyme was still active. In agreement, mutation of each and both N-glycosylation sites led to decreased levels of secreted active hCES2. However, the thermostability of the glycosylation mutants was decreased. The results indicated that glycans are involved, to some extent in protein folding in vivo, however, removal of glycans does not abrogate the activity of secreted hCES2., Highlights • Partial or non-glycosylated forms of Human carboxylesterase 2 (hCES2) were produced. • N-glycosylation is not essential for hCES2 activity. • Partial or non-glycosylated forms are less thermostable than the glycosylated form.

Superoxide reductases (SORs) are enzymes that detoxify the superoxide anion through its reduction to hydrogen peroxide and exist in both prokaryotes and eukaryotes. The substrate is transformed at an iron catalytic center, pentacoordinated in the ferrous state by four histidines and one cysteine. SORs have a highly conserved motif, (E)(K)HxP-, in which the glutamate is associated with a redox-driven structural change, completing the octahedral coordination of the iron in the ferric state, whereas the lysine may be responsible for stabilization and donation of a proton to catalytic intermediates. We aimed to understand at the structural level the role of these two residues, by determining the X-ray structures of the SORs from the hyperthermophilic archaea Ignicoccus hospitalis and Nanoarchaeum equitans that lack the quasi-conserved lysine and glutamate, respectively, but have catalytic rate constants similar to those of the canonical enzymes, as we previously demonstrated. Furthermore, we have determined the crystal structure of the E23A mutant of I. hospitalis SOR, which mimics several enzymes that lack both residues. The structures revealed distinct structural arrangements of the catalytic center that simulate several catalytic cycle intermediates, namely, the reduced and the oxidized forms, and the glutamate-free and deprotonated ferric forms. Moreover, the structure of the I. hospitalis SOR provides evidence for the presence of an alternative lysine close to the iron center in the reduced state that may be a functional substitute for the "canonical" lysine.

RuvB-Like transcription factors function in cell cycle regulation, development and human disease, such as cancer and heart hyperplasia. The mechanisms that regulate adenosine triphosphate (ATP)-dependent activity, oligomerization and post-translational modifications in this family of enzymes are yet unknown. We present the first crystallographic structure of full-length human RuvBL2 which provides novel insights into its mechanistic action and biology. The ring-shaped hexameric RuvBL2 structure presented here resolves for the first time the mobile domain II of the human protein, which is responsible for protein-protein interactions and ATPase activity regulation. Structural analysis suggests how ATP binding may lead to domain II motion through interactions with conserved N-terminal loop histidine residues. Furthermore, a comparison between hsRuvBL1 and 2 shows differences in surface charge distribution that may account for previously described differences in regulation. Analytical ultracentrifugation and cryo electron microscopy analyses performed on hsRuvBL2 highlight an oligomer plasticity that possibly reflects different physiological conformations of the protein in the cell, as well as that single-stranded DNA (ssDNA) can promote the oligomerization of monomeric hsRuvBL2. Based on these findings, we propose a mechanism for ATP binding and domain II conformational change coupling.

The solute pool of the actinobacterium Rubrobacter xylanophilus has been investigated as a function of the growth temperature and concentration of NaCl in the medium (Empadinhas et al. Extremophiles 11: 667-673, 2007). Changing the carbon source from glucose to maltose in a minimal growth medium led to the accumulation of an unknown organic compound whose structure was investigated by NMR and confirmed by chemical synthesis in the present study as: (2R)-2-(1-O-α-D-mannopyranosyl)-3-(1-O-α-D-glucopyranosyl)-D-glycerate (MGlyG). In addition to this newly identified diglycoside, the solute pool of R. xylanophilus included trehalose, mannosylglycerate, di-myo-inositol phosphate and di-N-acetyl-glucosamine phosphate. The structure of MGlyG was established by NMR and confirmed by chemical synthesis. The availability of g-amounts of the synthetic material allowed us to perform stabilization tests on three model enzymes (malate dehydrogenase, staphylococcal nuclease, and lysozyme), and compare the efficacy of MGlyG with other natural glyceryl glycosides, such as α-D-mannosyl-D-glycerate, α-D-glucosyl-D-glycerate and α-D-glucosyl-(1 → 6)-α-D-glucosyl-(1 → 2)-D-glycerate.

Notch signalling is an evolutionary conserved cell-to-cell communication pathway crucial for development and tissue homeostasis. Abnormal Notch signalling by mutations or deregulated expression of its receptors and/or ligands can lead to cancer making it a potential therapeutic target. Delta-like1 (DLL1) is a ligand of the Notch pathway implicated in different types of cancer, including breast cancer. Herein, we produced rhDLL1-DE3, a novel soluble form of DLL1 protein, which contains the DSL domain and EGF1-3 repeats critical for Notch pathway activation. cDNA fragments of human DLL1, encoding truncated versions of DLL1 with regions required to activate Notch receptors, were cloned and expressed as histidine-fused proteins in bacterial and mammalian cells. Expression tests in mammalian cells showed almost exclusively expression of the rhDLL1-DE3 protein form comprising the minimal binding regions DSL to EGF3 to Notch receptors. The highest yield of rhDLL1-DE3 was obtained from E. coli inclusion bodies. The produced protein, with purity higher than 95% bound to human Notch1 recombinant protein, by both Biolayer interferometry and ELISA assays. Cellular assays revealed rhDLL1-DE3 was biologically active as it increased expression of Notch-dependent genes in inducible pluripotent and breast cancer cells. Moreover, rhDLL1-DE3 allowed the generation of polyclonal antibodies by immunization that efficiently recognized DLL1 proteins by immunoblot, and caused a significant decrease of Notch1 expression in MCF7 breast cancer cells. The rhDLL1-DE3 protein might thus be used for Notch pathway activation and to generate anti-DLL1 monoclonal antibodies by immunization or phage display technology to unveil the effect of DLL1 in breast cancer.

Summary Protein post-translation modification plays an important role in regulating DNA repair; however, the role of arginine methylation in this process is poorly understood. Here we identify the arginine methyltransferase PRMT5 as a key regulator of homologous recombination (HR)-mediated double-strand break (DSB) repair, which is mediated through its ability to methylate RUVBL1, a cofactor of the TIP60 complex. We show that PRMT5 targets RUVBL1 for methylation at position R205, which facilitates TIP60-dependent mobilization of 53BP1 from DNA breaks, promoting HR. Mechanistically, we demonstrate that PRMT5-directed methylation of RUVBL1 is critically required for the acetyltransferase activity of TIP60, promoting histone H4K16 acetylation, which facilities 53BP1 displacement from DSBs. Interestingly, RUVBL1 methylation did not affect the ability of TIP60 to facilitate ATM activation. Taken together, our findings reveal the importance of PRMT5-mediated arginine methylation during DSB repair pathway choice through its ability to regulate acetylation-dependent control of 53BP1 localization., Graphical Abstract, Highlights • PRMT5 is a regulator of homologous recombination-mediated double-strand break repair • PRMT5 methylates RUVBL1 at R205, regulating TIP60-mediated histone acetylation • Loss of RUVBL1 methylation leads to 53BP1 retention at break ends • Arginine methylation crosstalks with histone acetylation to regulate repair pathway choice, Clarke et al. show that methylation of RUVBL1 by the arginine methyltransferase PRMT5 is required for homologous recombination-mediated double-strand break repair by promoting TIP60-mediated histone H4K16 acetylation. Loss of PRMT5 activity and defective RUVBL1 methylation leads to 53BP1 retention, increased sensitivity to DNA damaging agents, and genome instability.

The deposited article is a post-print version and has been submitted to peer review. The deposited article is a pre-print version This deposit is composed by the main article and the supplementary materials are present in the publisher's page in the following link: https://ars.els-cdn.com/content/image/1-s2.0-S1534580712002511-mmc1.pdf Cilia and flagella are involved in a variety of processes and human diseases, including ciliopathies and sterility. Their motility is often controlled by a central microtubule (MT) pair localized within the ciliary MT-based skeleton, the axoneme. We characterized the formation of the motility apparatus in detail in Drosophila spermatogenesis. We show that assembly of the central MT pair starts prior to the meiotic divisions, with nucleation of a singlet MT within the basal body of a small cilium, and that the second MT of the pair only assembles much later, upon flagella formation. BLD10/CEP135, a conserved player in centriole and flagella biogenesis, can bind and stabilize MTs and is required for the early steps of central MT pair formation. This work describes a genetically tractable system to study motile cilia formation and provides an explanation for BLD10/CEP135's role in assembling highly stable MT-based structures, such as motile axonemes and centrioles. Fundação para a Ciência e Tecnologia grants: (PTDC/BIA-BCM/105602/2008); EMBO Installation Grant; Instituto Gulbenkian de Ciência; EMBO YIP Program; European Research Council grant: ([FP7/2010]/ERC Grant “261344-CentrioleStructNumber.”); Ciência 2007; EMBO, Marie Curie Actions. info:eu-repo/semantics/publishedVersion

Cystathionine β-synthase (CBS) plays a central role in hydrogen sulfide (H2S) metabolism in human (patho) physiology. Each CBS monomer is composed of a central catalytic domain containing a pyridoxal 5’-phosphate active site flanked by two regulatory domains: an N-terminal domain harboring a b-type heme, and a C-terminal S-adenosyl-L-methionine (AdoMet)-binding domain. CBS is intricately regulated at different levels, its function depending on the redox state and availability of exogenous ligands, such as AdoMet and the gasotransmitters nitric oxide (NO) and carbon monoxide (CO). Herein, we employed static and stopped-flow absorption spectroscopy to investigate the kinetics of CO and NO binding to the CBS heme. NO binds faster and with higher affinity than CO, each gasotransmitter initially binding to opposite sides of the heme plane. AdoMet surprisingly enhances CO inhibition of H2S production, as well as CO and NO binding. In the clinically relevant p.P49L CBS variant, we observed much higher CO affinity with respect to WT CBS, likely resulting from increased flexibility of the heme surroundings. Overall, the binding kinetics attest the in vivo relevance of CO and NO inhibition of CBS-catalyzed H2S production, an underlying mechanism in the crosstalk between gasotransmitters.

A sulfide:quinone oxidoreductase (SQR) was isolated from the membranes of the hyperthermoacidophilic archaeon Acidianus ambivalens, and its X-ray structure, the first reported for an SQR, was determined to 2.6 A resolution. This enzyme was functionally and structurally characterized and was shown to have two redox active sites: a covalently bound FAD and an adjacent pair of cysteine residues. Most interestingly, the X-ray structure revealed the presence of a chain of three sulfur atoms bridging those two cysteine residues. The possible implications of this observation in the catalytic mechanism for sulfide oxidation are discussed, and the role of SQR in the sulfur dependent bioenergetics of A. ambivalens, linked to oxygen reduction, is addressed.

The complex of RuvBL1 and its homologue RuvBL2, two evolutionarily highly conserved eukaryotic proteins belonging to the AAA(+) (ATPase associated with diverse cellular activities) family of ATPases, was co-expressed in Escherichia coli. For crystallization purposes, the flexible domains II of RuvBL1 and RuvBL2 were truncated. The truncated RuvBL1-RuvBL2 complex was crystallized using the hanging-drop vapour-diffusion method at 293 K. The crystals were hexagonal-shaped plates and belonged to either the orthorhombic space group C222(1), with unit-cell parameters a = 111.4, b = 188.0, c = 243.4 A and six monomers in the asymmetric unit, or the monoclinic space group P2(1), with unit-cell parameters a = 109.2, b = 243.4, c = 109.3 A, beta = 118.7 degrees and 12 monomers in the asymmetric unit. The crystal structure could be solved by molecular replacement in both possible space groups and the solutions obtained showed that the complex forms a dodecamer.

As a key regulator of mitosis, the Ser/Thr protein polo-like kinase-1 (Plk-1) is a well validated drug target in cancer therapy. In order to enable structure-guided drug design, determination of the crystal structure of the kinase domain of Plk-1 was attempted. Using a multi-parallel cloning and expression approach, a set of length variants were identified which could be expressed in large amounts from insect cells and which could be purified to high purity. However, all attempts to crystallize these constructs failed. Crystals were ultimately obtained by generating designed ankyrin-repeat proteins (DARPins) selective for Plk-1 and using them for cocrystallization. Here, the first crystal structure of the kinase domain of wild-type apo Plk-1, in complex with DARPin 3H10, is presented, underlining the power of selective DARPins as crystallization tools. The structure was refined to 2.3 A resolution and shows the active conformation of Plk-1. It broadens the basis for modelling and cocrystallization studies for drug design. The binding epitope of 3H10 is rich in arginine, glutamine and lysine residues, suggesting that the DARPin enabled crystallization by masking a surface patch which is unfavourable for crystal contact formation. Based on the packing observed in the crystal, a truncated DARPin variant was designed which showed improved binding characteristics.

Flavodiiron proteins (FDPs) are present in organisms from all domains of life and have been described so far to be involved in the detoxification of oxygen or nitric oxide (NO), acting as O2 and/or NO reductases. The Escherichia coli FDP, named flavorubredoxin (FlRd), is the most extensively studied FDP. Biochemical and in vivo studies revealed that FlRd is involved in NO detoxification as part of the bacterial defense mechanisms against reactive nitrogen species. E. coli FlRd has a clear preference for NO as a substrate in vitro, exhibiting a very low reactivity toward O2. To contribute to the understanding of the structural features defining this substrate selectivity, we determined the crystallographic structure of E. coli FlRd, both in the isolated and reduced states. The overall tetrameric structure revealed a highly conserved flavodiiron core domain, with a metallo-β-lactamase-like domain containing a diiron center, and a flavodoxin domain with a flavin mononucleotide cofactor. The metal center in the oxidized state has a μ-hydroxo bridge coordinating the two irons, while in the reduced state, this moiety is not detected. Since only the flavodiiron domain was observed in these crystal structures, the structure of the rubredoxin domain was determined by NMR. Tunnels for the substrates were identified, and through molecular dynamics simulations, no differences for O2 or NO permeation were found. The present data represent the first structure for a NO-selective FDP.

Pontin and Reptin are related partner proteins belonging to the AAA+ (ATPases Associated with various cellular Activities) family. They are implicated in multiple and seemingly unrelated processes encompassing the regulation of gene transcription, the remodeling of chromatin, DNA damage sensing and repair, and the assembly of protein and ribonucleoprotein complexes, among others. The 2nd International Workshop on Pontin and Reptin took place at the Instituto de Tecnologia Química e Biológica António Xavier in Oeiras, Portugal on October 10–12, 2014, and reported significant new advances on the mechanisms of action of these two AAA+ ATPases. The major points under discussion were related to the mechanisms through which these proteins regulate gene transcription, their roles as co-chaperones, and their involvement in pathophysiology, especially in cancer and ciliary biology and disease. Finally, they may become anticancer drug targets since small chemical inhibitors were shown to produce anti-tumor effects in animal models., This work was supported by Fundação para a Ciência e Tecnologia (Portugal) grants PTDC/BBB-BEP/1724/2012 to PM and TB, and PEst-OE/EQB/LA0004/2011 to PM; Creative Research Initiatives Program (Research Center for Chromatin Dynamics, 2009-0081563) to SB; Ministry of Economy and Competitiveness of the Spanish Government grants SAF2011-22988 and SAF2014-52301-R to OL; NIH grants R01 GM084465 and CA166054 to AD; The Canadian Institutes of Health Research grant (MOP-93778) to WAH; Equipe Labélisée Ligue Contre le Cancer and Institut National du Cancer grant PLBIO10-155 to JR.

NADH:quinone oxidoreductases (NDHs), constitute one of the electron entry points into membrane-bound respiratory chains, oxidising NADH and reducing quinones. Type-II NDHs (NDH-2) are functionally unable to translocate protons and are typically constituted by a single approximately 50 kDa subunit lacking iron-sulfur clusters and containing one flavin as the sole redox centre. No three dimensional crystal structure is yet available for NDHs. We describe the crystallisation and preliminary structure determination of a NDH-2 that contains a covalently bound FAD, isolated from the membrane fraction of Acidianus ambivalens, a hyperthermoacidophilic archaeon capable of growing at 80 degrees C and pH 2.0. NDH-2 was solubilised with the detergent n-dodecyl-beta-d-maltoside and crystallised using ammonium phosphate as precipitant. The structure was solved by MIRAS using Pt and I derivatives.

The coordination environment of the Cu(B) center of the quinol oxidase from Acidianus ambivalens, a type B heme-copper oxygen reductase, was investigated by Fourier transform (FT) IR and extended X-ray absorption fine structure (EXAFS) spectroscopy. The comparative structural chemistry of dinuclear Fe-Cu sites of the different types of oxygen reductases is of great interest. Fully reduced A. ambivalens quinol oxidase binds CO at the heme a (3) center, with nu(CO)=1,973 cm(-1). On photolysis, the CO migrated to the Cu(B) center, forming a Cu (B) (I) -CO complex with nu(CO)=2,047 cm(-1). Raising the temperature of the samples to 25 degrees C did not result in a total loss of signal in the FTIR difference spectrum although the intensity of these signals was reduced sevenfold. This observation is consistent with a large energy barrier against the geminate rebinding of CO to the heme iron from Cu(B), a restricted limited access at the active-site pocket for a second binding, and a kinetically stable Cu(B)-CO complex in A. ambivalens aa (3). The Cu(B) center was probed in a number of different states using EXAFS spectroscopy. The oxidized state was best simulated by three histidines and a solvent O scatterer. On reduction, the site became three-coordinate, but in contrast to the bo (3) enzyme, there was no evidence for heterogeneity of binding of the coordinated histidines. The Cu(B) centers in both the oxidized and the reduced enzymes also appeared to contain substoichiometric amounts (0.2 mol equiv) of nonlabile chloride ion. EXAFS data of the reduced carbonylated enzyme showed no difference between dark and photolyzed forms. The spectra could be well fit by 2.5 imidazoles, 0.5 Cl(-) and 0.5 CO ligands. This arrangement of scatterers would be consistent with about half the sites remaining as unligated Cu(his)(3) and half being converted to Cu(his)(2)Cl(-)CO, a 50/50 ratio of Cu(his)(2)Cl(-) and Cu(his)(3)CO, or some combination of these formulations.

The aa 3 type B oxygen reductase from the thermophilic archaeon Acidianus ambivalens (QO) was immobilized on silver electrodes and studied by potential-dependent surface-enhanced resonance Raman (SERR) spectroscopy. The immobilized enzyme retains the native structure at the level of the heme pockets and exhibits reversible electrochemistry. From the potential dependence of specific spectral marker bands, the midpoint potentials of hemes a and a 3 were unambiguously determined for the first time, being 320 ± 20 mV for the former and 390 ± 20 mV for the latter. Both hemes could be treated as independent one-electron Nernstian redox couples, indicating that the interaction potential is smaller than 50 mV. The reversed order of the midpoint potentials compared to those of type A (mitochondrial-like) oxidases, as well as the lack of substantial Coulombic interactions, suggests a different mechanism of electroprotonic energy transduction. In contrast to type A enzymes, a-a 3 intraprotein electron transfer in QO is already guaranteed by the order of the midpoint potentials at the onset of enzyme reduction and, therefore, does not require a complex network of cooperativities to ensure exergonicity. In the immobilized state, conformational transitions of the QO a 3 -Cu B active site, which are believed to be essential for proton translocation, are drastically slowed compared to those in solution. We ascribe this finding to the effect of the interfacial electric field, which is of the same order of magnitude as in biological membranes. These results suggest that the membrane potential may play an active role in the regulation of the enzymatic activity of QO.

SUMMARY Type II NAD(P)H:quinone oxidoreductases (NDH-2) catalyze the two-electron transfer from NAD(P)H to quinones, without any energy-transducing site. NDH-2 accomplish the turnover of NAD(P)H, regenerating the NAD(P) + pool, and may contribute to the generation of a membrane potential through complexes III and IV. These enzymes are usually constituted by a nontransmembrane polypeptide chain of ∼50 kDa, containing a flavin moiety. There are a few compounds that can prevent their activity, but so far no general specific inhibitor has been assigned to these enzymes. However, they have the common feature of being resistant to the complex I classical inhibitors rotenone, capsaicin, and piericidin A. NDH-2 have particular relevance in yeasts like Saccharomyces cerevisiae and in several prokaryotes, whose respiratory chains are devoid of complex I, in which NDH-2 keep the [NADH]/[NAD + ] balance and are the main entry point of electrons into the respiratory chains. Our knowledge of these proteins has expanded in the past decade, as a result of contributions at the biochemical level and the sequencing of the genomes from several organisms. The latter showed that most organisms contain genes that potentially encode NDH-2. An overview of this development is presented, with special emphasis on microbial enzymes and on the identification of three subfamilies of NDH-2.

Thiosulphate is one of the products of the initial step of the elemental sulphur oxidation pathway in the thermoacidophilic archaeon Acidianus ambivalens. A novel thiosulphate:quinone oxidoreductase (TQO) activity was found in the membrane extracts of aerobically grown cells of this organism. The enzyme was purified 21-fold from the solubilized membrane fraction. The TQO oxidized thiosulphate with tetrathionate as product and ferricyanide or decyl ubiquinone (DQ) as electron acceptors. The maximum specific activity with ferricyanide was 73.4 U (mg protein)(-1) at 92 degrees C and pH 6, with DQ it was 397 mU (mg protein)(-1) at 80 degrees C. The Km values were 2.6 mM for thiosulphate (k(cat) = 167 s(-1)), 3.4 mM for ferricyanide and 5.87 micro M for DQ. The enzymic activity was inhibited by sulphite (Ki = 5 micro M), metabisulphite, dithionite and TritonX-100, but not by sulphate or tetrathionate. A mixture of caldariella quinone, sulfolobus quinone and menaquinone was non-covalently bound to the protein. No other cofactors were detected. Oxygen consumption was measured in membrane fractions upon thiosulphate addition, thus linking thiosulphate oxidation to dioxygen reduction, in what constitutes a novel activity among Archaea. The holoenzyme was composed of two subunits of apparent molecular masses of 28 and 16 kDa. The larger subunit appeared to be glycosylated and was identical to DoxA, and the smaller was identical to DoxD. Both subunits had been described previously as a part of the terminal quinol:oxygen oxidoreductase complex (cytochrome aa3).

The SOR (sulphur oxygenase reductase) is the initial enzyme in the sulphur-oxidation pathway of Acidianus ambivalens. Expression of the sor gene in Escherichia coli resulted in active, soluble SOR and in inclusion bodies from which active SOR could be refolded as long as ferric ions were present in the refolding solution. Wild-type, recombinant and refolded SOR possessed indistinguishable properties. Conformational stability studies showed that the apparent unfolding free energy in water is approx. 5 kcal·mol−1 (1 kcal=4.184 kJ), at pH 7. The analysis of the quaternary structures showed a ball-shaped assembly with a central hollow core probably consisting of 24 subunits in a 432 symmetry. The subunits form homodimers as the building blocks of the holoenzyme. Iron was found in the wild-type enzyme at a stoichiometry of one iron atom/subunit. EPR spectroscopy of the colourless SOR resulted in a single isotropic signal at g=4.3, characteristic of high-spin ferric iron. The signal disappeared upon reduction with dithionite or incubation with sulphur at elevated temperature. Thus both EPR and chemical analysis indicate the presence of a mononuclear iron centre, which has a reduction potential of −268 mV at pH 6.5. Protein database inspection identified four SOR protein homologues, but no other significant similarities. The spectroscopic data and the sequence comparison led to the proposal that the Acidianus ambivalens SOR typifies a new type of non-haem iron enzyme containing a mononuclear iron centre co-ordinated by carboxylate and/or histidine ligands.

The membrane bound aa(3)-type quinol:oxygen oxidoreductase from the hyperthermophilic archaeon, Acidianus ambivalens, which thrives at a pH of 2.5 and a temperature of 80 degrees C, has several unique structural and functional features as compared to the other members of the heme-copper oxygen reductase superfamily, but shares the common redox-coupled, proton-pumping function. To better understand the properties of the heme a(3)-Cu(B) catalytic site, a resonance Raman spectroscopic study of the enzyme under a variety of conditions and in the presence of various ligands was carried out. Assignments of several heme vibrational modes as well as iron-ligand stretching modes are made to serve as a basis for comparing the structure of the enzyme to that of other oxygen reductases. The CO-bound oxidase has conformations that are similar to those of other oxygen reductases. However, the addition of CO to the resting enzyme does not generate a mixed valence species as in the bovine aa(3) enzyme. The cyanide complex of the oxidized enzyme of A. ambivalens does not display the high stability of its bovine counterpart, and a redox titration demonstrates that there is an extensive heme-heme interaction reflected in the midpoint potentials of the cyanide adduct. The A. ambivalens oxygen reductase is very stable under acidic conditions, but it undergoes an earlier alkaline transition than the bovine enzyme. The A. ambivalens enzyme exhibits a redox-linked reversible conformational transition in the heme a(3)-Cu(B) center. The pH dependence and H/D exchange demonstrate that the conformational transition is associated with proton movements involving a group or groups with a pK(a) of approximately 3.8. The observed reversibility and involvement of protons in the redox-coupled conformational transition support the proton translocation model presented earlier. The implications of such conformational changes are discussed in relation to general redox-coupled proton pumping mechanisms in the heme-copper oxygen reductases.

Thermophiles are organisms that grow optimally above 50 degrees C and up to approximately 120 degrees C. These extreme conditions must have led to specific characteristics of the cellular components. In this paper we extensively analyze the types of respiratory complexes from thermophilic aerobic prokaryotes. The different membrane-bound complexes so far characterized are described, and the genomic data available for thermophilic archaea and bacteria are analyzed. It is observed that no specific characteristics can be associated to thermophilicity as the different types of complexes I-IV are present randomly in thermophilic aerobic organisms, as well as in mesophiles. Rather, the extensive genomic analyses indicate that the differences concerning the several complexes are related to the organism phylogeny, i.e., to evolution and lateral gene transfer events.

The oxidation and reduction of elemental sulfur and reduced inorganic sulfur species are some of the most important energy-yielding reactions for microorganisms living in volcanic hot springs, solfataras, and submarine hydrothermal vents, including both heterotrophic, mixotrophic, and chemolithoautotrophic, carbon dioxide-fixing species. Elemental sulfur is the electron donor in aerobic archaea like Acidianus and Sulfolobus. It is oxidized via sulfite and thiosulfate in a pathway involving both soluble and membrane-bound enzymes. This pathway was recently found to be coupled to the aerobic respiratory chain, eliciting a link between sulfur oxidation and oxygen reduction at the level of the respiratory heme copper oxidase. In contrast, elemental sulfur is the electron acceptor in a short electron transport chain consisting of a membrane-bound hydrogenase and a sulfur reductase in (facultatively) anaerobic chemolithotrophic archaea Acidianus and Pyrodictium species. It is also the electron acceptor in organoheterotrophic anaerobic species like Pyrococcus and Thermococcus, however, an electron transport chain has not been described as yet. The current knowledge on the composition and properties of the aerobic and anaerobic pathways of dissimilatory elemental sulfur metabolism in thermophilic archaea is summarized in this contribution.

The membranes of the thermoacidophilic archaeon Sulfolobus metallicus exhibit an oxygen consumption activity of 0.5 nmol O(2) min(-1) mg(-1), which is insensitive to rotenone, suggesting the presence of a type-II NADH dehydrogenase. Following this observation, the enzyme was purified from solubilised membranes and characterised. The pure protein is a monomer with an apparent molecular mass of 49 kDa, having a high N-terminal amino acid sequence similarity towards other prokaryotic enzymes of the same type. It contains a covalently attached flavin, which was identified as being FMN by 31P-NMR spectroscopy, a novelty among type-II NADH dehydrogenases. Metal analysis showed the absence of iron, indicating that no FeS clusters are present in the protein. The average reduction potential of the FMN group was determined to be +160 mV, at 25 degrees C and pH 6.5, by redox titrations monitored by visible spectroscopy. Catalytically, the enzyme is a NADH:quinone oxidoreductase, as it is capable of transferring electrons from NADH to several quinones, including ubiquinone-1, ubiquinone-2 and caldariella quinone. Maximal turnover rates of 195 micromol NADH oxidized min(-1) mg(-1) at 60 degrees C were obtained using ubiquinone-2 as electron acceptor, after enzyme dilution and incubation with phospholipids.

Superoxide reductase (SOR), which is commonly found in prokaryotic organisms, affords protection from oxidative stress by reducing the superoxide anion to hydrogen peroxide. The reaction is catalyzed at the iron centre, which is highly conserved among the prokaryotic SORs structurally characterized to date. Reported here is the first structure of an SOR from a eukaryotic organism, the protozoan parasiteGiardia intestinalis(GiSOR), which was solved at 2.0 Å resolution. By collecting several diffraction data sets at 100 K from the same flash-cooled protein crystal using synchrotron X-ray radiation, photoreduction of the iron centre was observed. Reduction was monitored using an online UV–visible microspectrophotometer, following the decay of the 647 nm absorption band characteristic of the iron site in the glutamate-bound, oxidized state. Similarly to other 1Fe-SORs structurally characterized to date, the enzyme displays a tetrameric quaternary-structure arrangement. As a distinctive feature, the N-terminal loop of the protein, containing the characteristic EKHxP motif, revealed an unusually high flexibility regardless of the iron redox state. At variance with previous evidence collected by X-ray crystallography and Fourier transform infrared spectroscopy of prokaryotic SORs, iron reduction did not lead to dissociation of glutamate from the catalytic metal or other structural changes; however, the glutamate ligand underwent X-ray-induced chemical changes, revealing high sensitivity of theGiSOR active site to X-ray radiation damage.

A new type-II NADH dehydrogenase (NDH-II) was isolated from the hyperthermoacidophilic archaeon Acidianus ambivalens. This enzyme is a monomer with an apparent molecular mass of 47 kDa, containing a covalently bound flavin, and no iron–sulfur clusters. Upon isolation, NDH-II loses activity, which can, nevertheless, be restored by incubation with phospholipids. Catalytically, it is a proficient NADH:caldariella quinone oxidoreductase (130 mmol NADH oxidized/mg protein-1/min-1) but it can also donate electrons to synthetic quinones, strongly suggesting its involvement in the respiratory chain. The apparent Km for NADH was found to be ∼6 μM, both for the purified and membrane-integrated enzyme, thus showing that detergent solubilization and purification did not affect the substrate binding site. Further, it is the first example of a type-II NADH dehydrogenase that contains the flavin covalently attached, which may be related to the need to stabilize the otherwise labile cofactor in a thermophilic environment. A fully operative minimal version of Acidianus ambivalens respiratory system was successfully reconstituted into artificial liposomes, using three basic components isolated from the organism: the type-II NADH dehydrogenase, caldariella quinone, the organism-specific quinone, and the aa3 type quinol oxidase. This system, which mimics the in vivo chain, is efficiently energized by NADH, driving oxygen consumption by means of the terminal oxidase.

RuvBL1 and its homolog RuvBL2 belong to the AAA+ family of ATPases and play important roles in chromatin remodeling, in transcriptional regulation, in DNA repair and in the c-Myc and Wnt signaling pathways. Proteins involved in these pathways are often mutated in human cancers. Both RuvBL proteins form a complex and act alone or together in diverse cellular processes. The three-dimensional structures of human RuvBL1 refined using diffraction data to 2.2 A resolution and of the human RuvBL1/RuvBL2 complex with a truncated domain II at 3 A resolution are presented. The dodecameric RuvBL1/RuvBL2 complex structure differs from previously described models. It consists of two heterohexameric rings with alternating RuvBL1 and RuvBL2 monomers that interact with each other via domain II. ATPase and helicase activities of RuvBL1 and RuvBL2 were also tested. Interestingly, truncation of domain II resulted not only in a substantial increase of ATP consumption by the RuvBL proteins, but also in stimulation of helicase activity, which was not observed with the full-length proteins.

Desulforubrerythrin from Campylobacter jejuni has recently been biochemical and spectroscopically characterized. It is a member of the rubrerythrin family, and it is composed of three structural domains: the N-terminal desulforedoxin domain with a non-heme iron center, followed by a four-helix bundle domain harboring a binuclear iron center and finally a C-terminal rubredoxin domain. To date, this is the first example of a protein presenting this kind of structural domain organization, and therefore the determination of its crystal structure may unveil unexpected structural features. Several attempts were made in order to obtain protein crystals, but always without success. As part of our strategy the thermofluor method was used to increase protein stability and its propensity to crystallize. This approach has been recently used to optimize protein buffer formulation, thus yielding more stable and homogenous protein samples. Thermofluor has also been used to identify cofactors/ligands or small molecules that may help stabilize native protein states. A successful thermofluor approach was used to select a pH buffer condition that allowed the crystallization of Campylobacter jejuni desulforubrerythrin, by screening both buffer pH and salt concentration. A buffer formulation was obtained which increased the protein melting temperature by 7°C relatively to the initial purification buffer. Desulforubrerythrin was seen to be stabilized by lower pH and high salt concentration, and was dialyzed into the new selected buffer, 100mM MES pH 6.2, 500mM NaCl. This stability study was complemented with a second thermofluor assay in which different additives were screened. A crystallization screening was carried out and protein crystals were rapidly obtained in one condition. Protein crystal optimization was done using the same additive screening. Interestingly, a correlation between the stability studies and crystallization experiments using the additive screening could be established. The work presented here shows an elegant example where thermofluor was shown to be a key biophysical method that allowed the identification of an improved buffer formulation and the applicability of this technique to increase the propensity of a protein to crystallize is discussed.

RuvBL1 (RuvB-like 1) and its homolog RuvBL2 are evolutionarily highly conserved AAA + ATPases essential for many cellular activities. They play an important role in chromatin remodeling, transcriptional regu- lation and DNA damage repair. RuvBL1 and RuvBL2 are overexpressed in different types of cancer and interact with major oncogenic factors, such as b-catenin and c-Myc regulating their function. We solved the first three-dimensional crystal structure of the human RuvBL complex with a truncated domain II and show that this complex is competent for helicase activity. The structure reveals a dodecamer consisting of two heterohexameric rings with alternating RuvBL1 and RuvBL2 monomers bound to ADP/ATP, that interact with each other via the retained part of domain II. The dodecameric quaternary structure of the R1DDII/R2DDII complex observed in the crystal structure was confirmed by small-angle X-ray scat- tering analysis. Interestingly, truncation of domain II led to a substantial increase in ATP consumption of RuvBL1, RuvBL2 and their complex. In addition, we present evidence that DNA unwinding of the human RuvBL proteins can be auto-inhibited by domain II, which is not present in the homologous bacterial helicase RuvB. Our data give new insights into the molecular arrangement of RuvBL1 and RuvBL2 and strongly suggest that in vivo activities of these highly interesting therapeutic drug targets are regulated by cofac- tors inducing conformational changes via domain II in order to modulate the enzyme complex into its active state.

Superoxide reductases (SORs) are the most recent oxygen-detoxification system to be identified in anaerobic and microaerobic bacteria and archaea. SORs are metalloproteins that are characterized by their possession of a catalytic nonhaem iron centre in the ferrous form coordinated by four histidine ligands and one cysteine ligand. Ignicoccus hospitalis, a hyperthermophilic crenarchaeon, is the only organism known to date to serve as a host for Nanoarchaeum equitans, a nanosized hyperthermophilic archaeon isolated from a submarine hot vent which completely depends on the presence of and contact with I. hospitalis cells for growth to occur. Similarly to I. hospitalis, N. equitans has a neelaredoxin (a 1Fe-type SOR) that keeps toxic oxygen species under control, catalysing the one-electron reduction of superoxide to hydrogen peroxide. Blue crystals of recombinant N. equitans SOR in the oxidized form (12.7 kDa, 109 residues) were obtained using polyethylene glycol (PEG 2000 MME) as precipitant. These crystals diffracted to 1.9 Å resolution at 100 K and belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 51.88, b = 82.01, c = 91.30 Å. Cell-content analysis suggested the presence of four monomers in the asymmetric unit. The Matthews coefficient (V(M)) was determined to be 1.9 Å(3) Da(-1), corresponding to an estimated solvent content of 36%. Self-rotation function and native Patterson calculations suggested a tetramer with 222 point-group symmetry, similar to other 1Fe-SORs. The three-dimensional structure will be determined by the molecular-replacement method.

Neelaredoxins are a type of superoxide reductase (SOR), which are blue 14 kDa metalloproteins with a catalytic nonhaem iron centre coordinated by four histidines and one cysteine in the ferrous form. Anaerobic organisms such as Archaeoglobus fulgidus, a hyperthermophilic sulfate-reducing archaeon, have developed defence mechanisms against toxic oxygen species in which superoxide reductases play a key role. SOR is responsible for scavenging toxic superoxide anion radicals (O(2)(*-)), catalysing the one-electron reduction of superoxide to hydrogen peroxide. Crystals of recombinant A. fulgidus neelaredoxin in the oxidized form (13.7 kDa, 125 residues) were obtained using polyethylene glycol and ammonium sulfate. These crystals diffracted to 1.9 A resolution and belonged to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 75.72, c = 185.44 A. Cell-content analysis indicated the presence of a tetramer in the asymmetric unit, with a Matthews coefficient (V(M)) of 2.36 A(3) Da(-1) and an estimated solvent content of 48%. The three-dimensional structure was determined by the MAD method and is currently under refinement.

Superoxide reductases (SORs) are metalloproteins which constitute the most recently identified oxygen-detoxification system in anaerobic and microaerobic bacteria and archaea. SORs are involved in scavenging superoxide radicals from the cell by catalyzing the reduction of superoxide ({\rm O}_{2};{\bullet -}) to hydrogen peroxide and are characterized by a catalytic nonhaem iron centre coordinated by four histidine ligands and one cysteine ligand. Ignicoccus hospitalis, a hyperthermophilic crenarchaeon, is known to have a neelaredoxin-type SOR that keeps toxic oxygen species levels under control. Blue crystals of recombinant I. hospitalis oxidized neelaredoxin (14.1 kDa, 124 residues) were obtained. These crystals diffracted to 2.4 A resolution in-house at room temperature and belonged to the hexagonal space group P6(2)22 or P6(4)22, with unit-cell parameters a = b = 108, c = 64 A. Cell-content analysis indicated the presence of one monomer in the asymmetric unit.

Thiosulphate is one of the products of the initial step of the elemental sulphur oxidation pathway in the thermoacidophilic archaeon Acidianus ambivalens. A novel thiosulphate:quinone oxidoreductase (TQO) activity was found in the membrane extracts of aerobically grown cells of this organism. The enzyme was purified 21-fold from the solubilized membrane fraction. The TQO oxidized thiosulphate with tetrathionate as product and ferricyanide or decyl ubiquinone (DQ) as electron acceptors. The maximum specific activity with ferricyanide was 73.4 U (mg protein)(-1) at 92 degrees C and pH 6, with DQ it was 397 mU (mg protein)(-1) at 80 degrees C. The Km values were 2.6 mM for thiosulphate (k(cat) = 167 s(-1)), 3.4 mM for ferricyanide and 5.87 micro M for DQ. The enzymic activity was inhibited by sulphite (Ki = 5 micro M), metabisulphite, dithionite and TritonX-100, but not by sulphate or tetrathionate. A mixture of caldariella quinone, sulfolobus quinone and menaquinone was non-covalently bound to the protein. No other cofactors were detected. Oxygen consumption was measured in membrane fractions upon thiosulphate addition, thus linking thiosulphate oxidation to dioxygen reduction, in what constitutes a novel activity among Archaea. The holoenzyme was composed of two subunits of apparent molecular masses of 28 and 16 kDa. The larger subunit appeared to be glycosylated and was identical to DoxA, and the smaller was identical to DoxD. Both subunits had been described previously as a part of the terminal quinol:oxygen oxidoreductase complex (cytochrome aa3).