A Low Concentration of Citreoviridin Prevents Both Intracellular Calcium Deposition in Vascular Smooth Muscle Cell and Osteoclast Activation In Vitro.

Vascular calcification (VC) and osteoporosis are age-related diseases and significant risk factors for the mortality of elderly. VC and osteoporosis may share common risk factors such as renin-angiotensin system (RAS)-related hypertension. In fact, inhibitors of RAS pathway, such as angiotensin type 1 receptor blockers (ARBs), improved both vascular calcification and hip fracture in elderly. However, a sex-dependent discrepancy in the responsiveness to ARB treatment in hip fracture was observed, possibly due to the estrogen deficiency in older women, suggesting that blocking the angiotensin signaling pathway may not be effective to suppress bone resorption, especially if an individual has underlying osteoclast activating conditions such as estrogen deficiency. Therefore, it has its own significance to find alternative modality for inhibiting both vascular calcification and osteoporosis by directly targeting osteoclast activation to circumvent the shortcoming of ARBs in preventing bone resorption in estrogen deficient individuals. In the present study, a natural compound library was screened to find chemical agents that are effective in preventing both calcium deposition in vascular smooth muscle cells (vSMCs) and activation of osteoclast using experimental methods such as Alizarin red staining and Tartrate-resistant acid phosphatase staining. According to our data, citreoviridin (CIT) has both an anti-VC effect and anti-osteoclastic effect in vSMCs and in Raw 264.7 cells, respectively, suggesting its potential as an effective therapeutic agent for both VC and osteoporosis.

[Electrophysiological Effect of Citreoviridin on Human InducedPluripotent Stem Cell-derived Cardiomyocytes].

Citreoviridin (CTV) is a mycotoxin produced by various fungi, including Penicillium citreonigrum. One of the toxicities reportedly associated with CTV is neurotoxicity. CTV is also suspected to be associated with acute cardiac beriberi (also known as "Shoshin-kakke") and Keshan disease, which can have adverse effects on the heart, so the in vivo and in vitro toxicity of CTV on the heart or cardiomyocytes in experimental animal models have been reported. However, the toxicity of CTV for the human heart, especially its electrophysiological effect, remains poorly understood. Therefore, to investigate the electrophysiological effect of CTV on the human cardiomyocytes, we conducted a multi-electrode array (MEA) using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). The MEA revealed that 30 µmol/L of CTV stopped the beating of hiPSC-CMs, and the field potential duration and first peak amplitude were shortened at 10 µmol/L. Before the hiPSC-CMs stopped beating, the length of the inter-spike interval varied two- to four-fold. These results demonstrated that CTV induced an electrophysiological disturbance on human cardiomyocytes. This is first paper to elucidate the electrophysiological effect of CTV on human heart directly and may aid in analyzing the risk associated with CTV to ensure food safety.

Aurovertin B exerts potent antitumor activity against triple-negative breast cancer in vivo and in vitro via regulating ATP synthase activity and DUSP1 expression.

Aurovertin B, a natural compound from Calcarisporium arbuscular, exhibits potent antiproliferative activity particularly against triple-negative breast cancer cells (TNBC), while having less cytotoxicity on normal breast cell MCF10A. However, very little is known about the in vivo antitumor activity of aurovertin B and the possible mechanism of the selective effect on triple-negative breast cancer cells. In this study, flow cytometry and DAPI staining analysis showed that aurovertin B treatment in human triple-negative breast cancer cell MDA-MB-231 could induce more apoptotic cells than taxol treatment group. Furthermore, the present study also revealed that aurovertin B induced apoptosis was due to regulation of ATP synthase activity rather than changes in gene expression. Interestingly, the cancer genome atlas (TCGA) data analysis implied that the expression level of DUSP1, a member of the dual-specificity phosphatases, was highly downregulated in breast tissue of TNBC patients compared with their adjacent normal tissues. Real-time PCR and western blot analyses further demonstrated that aurovertin B could dramatically increase mRNA and protein expression levels of DUSP1 in MDA-MB-231 cells but not in MCF10A cells. The potent anti-tumor activity of aurovertin B was further verified in a human MDA-MB-231 xenograft mouse model.

Porphyromonas gingivalis is highly sensitive to inhibitors of a proton-pumping ATPase.

Porphyromonas gingivalis is a well-known Gram-negative bacterium that causes periodontal disease. The bacterium metabolizes amino acids and peptides to obtain energy. An ion gradient across its plasma membrane is thought to be essential for nutrient import. However, it is unclear whether an ion-pumping ATPase responsible for the gradient is required for bacterial growth. Here, we report the inhibitory effect of protonophores and inhibitors of a proton-pumping ATPase on the growth of P. gingivalis. Among the compounds examined, curcumin and citreoviridin appreciably reduced the bacterial growth. Furthermore, these compounds inhibited the ATPase activity in the bacterial membrane, where the A-type proton-pumping ATPase (A-ATPase) is located. This study suggests that curcumin and citreoviridin inhibit the bacterial growth by inhibiting the A-ATPase in the P. gingivalis membrane.

Aurovertin B sensitizes colorectal cancer cells to NK cell recognition and lysis.

The natural killer group 2D (NKG2D) receptor on natural killer (NK) cells play an important role in immunosurveillance to cancer cells, which could mediate the eradication of tumor cells through specific interactions with NKG2D ligands on tumor cells. Here we report one natural compound aurovertin B from basidiomycete Albatrellus confluens significantly stimulates the expression of NKG2D ligands on tumor cells, which greatly sensitizes its recognition and lysis by NK cell. It is completely a novel role for aurovertin B to target tumor cells to death mediated by NK cells and our findings indicate aurovertin B may deserve further development as sensitizing agent in NK cell mediated cancer immunotherapy.

Temporal Phosphoproteome Dynamics Induced by an ATP Synthase Inhibitor Citreoviridin.

Citreoviridin, one of toxic mycotoxins derived from fungal species, can suppress lung cancer cell growth by inhibiting the activity of ectopic ATP synthase, but has limited effect on normal cells. However, the mechanism of citreoviridin triggering dynamic molecular responses in cancer cells remains unclear. Here, we performed temporal phosphoproteomics to elucidate the dynamic changes after citreoviridin treatment in cells and xenograft model. We identified a total of 829 phosphoproteins and demonstrated that citreoviridin treatment affects protein folding, cell cycle, and cytoskeleton function. Furthermore, response network constructed by mathematical modeling shows the relationship between the phosphorylated heat shock protein 90 ß and mitogen-activated protein kinase signaling pathway. This work describes that citreoviridin suppresses cancer cell growth and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling by site-specific dephosphorylation of HSP90AB1 on Serine 255 and provides perspectives in cancer therapeutic strategies.

Bioprospecting Chemical Diversity and Bioactivity in a Marine Derived Aspergillus terreus.

A comparative metabolomic study of a marine derived fungus (Aspergillus terreus) grown under various culture conditions is presented. The fungus was grown in eleven different culture conditions using solid agar, broth cultures, or grain based media (OSMAC). Multivariate analysis of LC/MS data from the organic extracts revealed drastic differences in the metabolic profiles and guided our subsequent isolation efforts. The compound 7-desmethylcitreoviridin was isolated and identified, and is fully described for the first time. In addition, 16 known fungal metabolites were also isolated and identified. All compounds were elucidated by detailed spectroscopic analysis and tested for antibacterial activities against five human pathogens and tested for cytotoxicity. This study demonstrates that LC/MS based multivariate analysis provides a simple yet powerful tool to analyze the metabolome of a single fungal strain grown under various conditions. This approach allows environmentally-induced changes in metabolite expression to be rapidly visualized, and uses these differences to guide the discovery of new bioactive molecules.

[Effects of citreoviridin on the expression of MCP-1 and ILs induced by TNF-alpha in vein endothelial cells].

OBJECTIVE: To investigate the effects of citreoviridin (CIT) on the expression of MCP-1, IL-1beta, IL-6 and IL-8 induced by TNF-alpha in human umbilical vein endothelial cells (HUVECs). METHODS: HUVECs isolated from the umbilical cord of neonates within 1 hour after birth (informed with consent form) were cultured in DMEM/ F12 media. After 80% of HUVECs were confluent, the cells were divided into four groups and treated with CIT (2 micromol/L) and/or TNF-alpha (10 microg/L). The levels of MCP-1, IL-1beta, IL-6 and IL-8 in the supernatant of cell culture media were measured by ELISA, the activation of NF-kappaB in HUVECs was detected by immunofluorescence staining, and the expression of MCP-1 mRNA was determinated by RT-PCR assay. RESULT: The levels of MCP-1, IL-1beta, IL-6 and IL-8 in the supernatant and the expression of NF-kappaB P65 and MCP-1 mRNA of HUVECs were higher in TNF-alpha group and TNF-alpha + CIT group than those in the control group (P < 0.05), and those of TNF-alpha + CIT group was higher than TNF-alpha group (P < 0.05). CONCLUSION: The expression of NF-kappaB, MCP-1, IL-1beta, IL-6 and IL-8 in HUVECs up-regulated by TNF-alpha was promoted by CIT.

Exosomal miR-181a-2-3p derived from citreoviridin-treated hepatocytes activates hepatic stellate cells trough inducing mitochondrial calcium overload.

Increasing evidences indicate the vital role of exosomes-mediated intercellular communication in the pathogenesis of liver fibrosis. However, the underlying mechanisms are still not clearly defined. In this study, we found that citreoviridin (CIT), a mycotoxin and ectopic ATP synthase (e-ATPS) inhibitor, induced liver fibrosis in mice. The exosomes derived from CIT-treated L-02 hepatocytes activated hepatic stellate cells (HSC) LX-2. With exosomal small RNA sequencing, we found 156 differentially expressed miRNAs in the exosomes from CIT-treated L-02 cells, and the predicted target genes of exosomal miRNAs were enriched in calcium signaling pathway. The exosomes from CIT-treated L-02 cells induced mitochondrial calcium accumulation in LX-2 cells. And pharmacological inhibition of mitochondrial calcium uptake relieved exosomes-activated fibrogenic response in LX-2 cells. The miR-181a-2-3p that was predicted to target-regulate mitochondrial calcium uptake 1 (MICU1) was significantly increased in the exosomes from CIT-treated L-02 cells. Exosomes-induced reduction of MICU1, mitochondrial calcium overload and activation of LX-2 cells were reversed by AntagomiR-181a-2-3p. In this study, we pointed out that exosomal miR-181a-2-3p from CIT-treated hepatocytes induced mitochondrial calcium accumulation and activated HSC subsequently through inhibiting the expression of MICU1, shedding new light on the mechanism underlying liver fibrosis and CIT hepatotoxicity.

Citreoviridin induces myocardial apoptosis through PPAR-γ-mTORC2-mediated autophagic pathway and the protective effect of thiamine and selenium.

Citreoviridin (CIT), a mycotoxin and ATP synthase inhibitor, is regarded as one of aetiology factors of cardiac beriberi and Keshan disease. Thiamine (VB1) and selenium (Se) improve the recovery of these two diseases respectively. The underlying mechanisms of cardiotoxic effect of CIT and cardioprotective effect of VB1 and Se have not been fully elucidated. In this study, we found that ectopic ATP synthase was more sensitive to CIT treatment than mitochondrial ATP synthase in H9c2 cardiomyocytes. CIT inhibited the transcriptional activity of peroxisome proliferator activated receptor gamma (PPAR-γ) in mice hearts and H9c2 cells. PPAR-γ agonist attenuated the inhibitory effect of CIT on mechanistic target of rapamycin complex 2 (mTORC2) and stimulatory effect of CIT on autophagy in cardiomyocytes. CIT induced apoptosis through lysosomal-mitochondrial axis in cardiomyocytes. PPAR-γ agonist and autophagy inhibitor alleviated CIT-induced apoptosis and accelerated cardiac biomarker. VB1 and Se accelerated the basal transcriptional activity of PPAR-γ in mice hearts and H9c2 cells. Furthermore, VB1 and Se reversed the effect of CIT on PPAR-γ, autophagy and apoptosis. Our findings defined PPAR-γ-mTORC2-autophagy pathway as the key link between CIT cardiotoxicity and cardioprotective effect of VB1 and Se. The present study would shed new light on the pathogenesis of cardiomyopathy and the cardioprotective mechanism of micronutrients.

Energetic signalling in the control of mitochondrial F1F0 ATP synthase activity in health and disease.

The mitochondrial F1F0 ATP synthase is a critical enzyme that works by coupling the proton motive force generated by the electron transport chain via proton transfer through the F0 or proton-pore forming domain of this enzyme to release ATP from the catalytic F1 domain. This enzyme is regulated by calcium, ADP, and inorganic phosphate as well as increased transcription through several pathways. This enzyme is also an ATP hydrolase under ischemic conditions. This "inefficient" hydrolysis of ATP consumes 90% of ATP consumed during ischemia as shown with non-selective ATPase inhibitors oligomycin and Aurovertin B. A benzopyran analog, BMS-199264, selectively inhibits F1F0 ATP hydrolase activity with no effect on ATP synthase activity. BMS-199264 had no effect on ATP before ischemia, but reduced the decline in ATP during ischemia. Selective hydrolase inhibition seen with the small molecule BMS-199264 suggests a conformational change in the F1F0 ATPase enzyme when switching from synthase to hydrolase activity.

Down-regulation of mitochondrial F1F0-ATP synthase in human colon cancer cells with induced 5-fluorouracil resistance.

5-Fluorouracil (5-FU) is widely used for treatment of advanced colorectal cancer. However, it is common for such patients to develop resistance to 5-FU, and this drug resistance becomes a critical problem for chemotherapy. The mechanisms underlying this resistance are largely unknown. To screen for proteins possibly responsible for 5-FU resistance, cells resistant to 5-FU were derived from human colon cancer cell lines and two-dimensional gel electrophoresis-based comparative proteomics was done. Two-dimensional gel electrophoresis data showed there was lower expression of the alpha subunit of mitochondrial F(1)F(0)-ATP synthase (ATP synthase) in 5-FU-resistant cells compared with parent cells. Western blotting showed that expression of other ATP synthase complex subunits was also lower in 5-FU-resistant cell lines and that these resistant cells also showed decreased ATP synthase activity and reduced intracellular ATP content. The ATP synthase inhibitor, oligomycin A, strongly antagonized 5-FU-induced suppression of cell proliferation. When 5-FU sensitivity was compared with ATP synthase activity in six different human colon cancer cell lines, a positive correlation has been found. Furthermore, suppressed ATP synthase d-subunit expression by siRNA transfection increased cell viability in the presence of 5-FU. Bioenergetic dysfunction of mitochondria has been reported as a hallmark of many types of cancers (i.e., down-regulation of ATP synthase beta-subunit expression in liver, kidney, colon, squamous oesophageal, and lung carcinomas, as well as in breast and gastric adenocarcinomas). Our findings show that ATP synthase down-regulation may not only be a bioenergetic signature of colorectal carcinomas but may also lead to cellular events responsible for 5-FU resistance.

Citreoviridin induces triglyceride accumulation in hepatocytes through inhibiting PPAR-α in vivo and in vitro.

Citreoviridin (CIT) is a mycotoxin produced by Penicillum citreonigrum, Aspergillus terreus and Eupenicillium ochrosalmoneum. CIT occurs naturally in moldy rice and corn. CIT is associated with the development of atherosclerosis in the general population. Alteration in hepatic lipid metabolism is a pathogenic factor in atherosclerosis. However the effect and the underlying mechanism of CIT on hepatic lipid metabolism are largely unknown. In this study, we reported that CIT induced triglyceride accumulation in mice liver and human liver HepG2 cells as shown in oil red O staining. CIT (0.1 mg/kg-0.3 mg/kg) for 6 weeks elevated liver triglyceride contents in mice. CIT inhibited the transactivation activity of peroxisome proliferator-activated receptor-α (PPAR-α) in hepatocyte in vivo and in vitro, as shown by the reduced mRNA levels of PPAR-α target genes which play key roles in lipid metabolism in various aspects. PPAR-α agonist fenofibrate attenuated CIT-induced triglyceride accumulation in HepG2 cells. Furthermore, CIT increased serum total cholesterol/high-density lipoprotein cholesterol ratio, a strong risk factor for cardiovascular disease. In summary, we reported that CIT induced PPAR-α-dependent hepatic triglyceride accumulation and dyslipidemia. Our data will provide new mechanistic insights into CIT-induced lipid alterations.

Role of α/ß interface in F1 ATPase rotational catalysis probed by inhibitors and mutations.

The F1 sector of ATP synthase (FOF1) synthesizes or hydrolyses ATP via a rotational catalysis mechanism that couples chemical reaction with subunit rotation. Phytopolyphenols such as curcumin can inhibit bulk phase F1 ATPase activity by extending the catalytic dwell time during subunit rotation (Sekiya, M., Hisasaka, R., Iwamoto-Kihara, A., Futai, M., Nakanishi-Matsui, M., Biochem. Biophys. Res. Commun. 452 (2014) 940-944). Citreoviridin, a polyene α-pyrone mycotoxin isolated from Penicillium sp, also inhibits ATPase activity. Molecular docking and mutational analysis indicated that these compounds interact with a region near the ß-subunit Arg398 residue that lies at the interface with the α-subunit. Binding of these inhibitors lowered the rotation rate and increased the duration of the catalytic dwell synergistically with substitution of ß-subunit Ser174 to Phe (ßS174F), which rendered the enzyme defective for conformational transmission between ß-subunits of different catalytic stages. Furthermore, substitution of α-subunit Glu402 to Ala (αE402A) in the α/ß-interface also decreased the rotation rate by increasing the duration of the catalytic dwell. Interestingly, this mutation restored the catalytic dwell of the ßS174F variant to that of the wild-type enzyme. These results suggest that the α/ß-interface is involved in conformational changes of the ß-subunit during rotational catalysis.

Inhibition sites in F1-ATPase from bovine heart mitochondria.

High-resolution crystallographic studies of a number of inhibited forms of bovine F1-ATPase have identified four independent types of inhibitory site: the catalytic site, the aurovertin B-binding site, the efrapeptin-binding site and the site to which the natural inhibitor protein IF1 binds. Hitherto, the binding sites for other inhibitors, such as polyphenolic phytochemicals, non-peptidyl lipophilic cations and amphiphilic peptides, have remained undefined. By employing multiple inhibition analysis, we have identified the binding sites for these compounds. Several of them bind to the known inhibitory sites. The amphiphilic peptides melittin and synthetic analogues of the mitochondrial import pre-sequence of yeast cytochrome oxidase subunit IV appear to mimic the natural inhibitor protein, and the polyphenolic phytochemical inhibitors resveratrol and piceatannol compete for the aurovertin B-binding site (or sites). The non-peptidyl lipophilic cation rhodamine 6G acts at a separate unidentified site, indicating that there are at least five inhibitory sites in the F1-ATPase. Each of the above inhibitors has significantly different activity against the bacterial Bacillus PS3 alpha3beta3gamma subcomplex compared with that observed with bovine F1-ATPase. IF1 does not inhibit the bacterial enzyme, even in the absence of the epsilon-subunit. An understanding of these inhibitors may enable rational development of therapeutic agents to act as novel antibiotics against bacterial ATP synthases or for the treatment of several disorders linked to the regulation of the ATP synthase, including ischaemia-reperfusion injury and some cancers.

Biosynthetic Pathway of the Reduced Polyketide Product Citreoviridin in Aspergillus terreus var. aureus Revealed by Heterologous Expression in Aspergillus nidulans.

Citreoviridin (1) belongs to a class of F1-ATPase ß-subunit inhibitors that are synthesized by highly reducing polyketide synthases. These potent mycotoxins share an α-pyrone polyene structure, and they include aurovertin, verrucosidin, and asteltoxin. The identification of the citreoviridin biosynthetic gene cluster in Aspergillus terreus var. aureus and its reconstitution using heterologous expression in Aspergillus nidulans are reported. Two intermediates were isolated that allowed the proposal of the biosynthetic pathway of citreoviridin.

Oligomycin, inhibitor of the F0 part of H+-ATP-synthase, suppresses the TNF-induced apoptosis.

The release of cytochrome c from the intermembrane space of mitochondria into the cytosol is one of the critical events in apoptotic cell death. In the present study, it is shown that release of cytochrome c and apoptosis induced by tumor necrosis factor alpha (TNF) in HeLa cells can be inhibited by (i) overexpression of an oncoprotein Bcl-2, (ii) Cyclosporin A, an inhibitor of the mitochondrial permeability transition pore (PTP) or (iii) oligomycin, an inhibitor of H+- ATP-synthase. Staurosporine-induced apoptosis is sensitive to Bcl-2 but insensitive to Cyclosporin A and oligomycin. The effect of oligomycin is not due to changes in mitochondrial membrane potential or to inhibition of ATP synthesis/hydrolysis since (a) uncouplers (CCCP, DNP) which discharge the membrane potential fail to abolish the protective action of oligomycin and (b) aurovertin B (another inhibitor of H+-ATP-synthase, affecting its F1 component) do not affect apoptosis. A role of oligomycin-sensitive F0 component of H+-ATP-synthase in the TNF-induced PTP opening and apoptosis is suggested.

Uncoupler-reversible inhibition of mitochondrial ATPase by metal chelates of bathophenanthroline. II. Comparison with other inhibitors.

(1) Trisbathophenanthroline-Fe2+ (BPh3Fe2+)alters the hyperbolic relationship between concentration of ATP and reaction velocity of F1-ATPase to sigmoidal, with a simultaneous decrease in maximal velocity. (2) BPh3Fe2+ binds to the beta-subunit of F1 and competes with the binding of aurovertin. The reversal of this effect uncouplers in enhanced by ADP and diminished by ATP. BPh3Fe2+ also changes the hyperbolic concentration dependence of aurovertin binding to sigmoidal. (3) BPh3Fe2+ stabilizes F1 against the cold inactivation and cold dissociation in an uncoupler-reversible manner. (4) BPh3Fe2+ efficiently protects F1 against the light-induced inactivation occurring in the presence of Rose Bengal, and the effect is reversed by uncouplers. (5) The results are discussed in relation to the reaction mechanism of F1-ATPase and other enzymes catalyzing the reversible hydrolysis of pyrophosphate bonds.

Studies on the ATPase complex from beef-heart mitochondria. I. Isolation and characterization of an oligomycin-sensitive and an olgiomycin-insensitive ATPase complex from beef-heart mitochondria.

1. A new method for the isolation of the oliogomycin-sensitive ATPase from beef-heart mitochondria is described. 2. A Triton-soluble ATPase complex was isolated as a by-product of the standard procedure, or as the main product when the submitochondrial particles were pretreated with 1% Triton. The ATPase activity of this complex is sensitive neither to oligomycin nor to dicyclohexylcarbodiimide. 3. The ATPase activity of the oligomycin-sensitive ATPase complex is nearly completely dependent on added phospholipids. The highest activation was found with asolectin. 4. The oligomycin-sensitive complex can be integrated into phospholipid vesicles resulting in an ATP- and Mg2+-dependent energization of the vesicles as monitored with the fluorescent dye 9-amino-6-chloro-2-methoxyacridine. 5. Aurovertin-binding studies based on fluorescence measurement reveal the presence of 1.5 mumol aurovertin-binding sites per g protein for the oligomycin-sensitive complex and about 2.2 mumol for the oligomycin-insensitive complex. 6. The preparation of the oligomycin-sensitive complex contains at least 6--7 polypeptides in addition to those derived from F1. One of these polypeptides, with an apparent molecular weight of 31 000, is virtually absent from the oligomycin-insensitive complex. 7. Some of these polypeptides have been identified and isolated.

Coupling factor ATPase complex of Rhodospirillum rubrum. Purification and characterization of an oligomycin and N,N'-dicyclohexylcarbodiimide-sensitive (Ca+ + Mg2+)-ATPase.

An ATPase complex sensitive to the energy transfer inhibitors oligomycin, dicyclohexylcarbodiimide and venturicidin has been solubilized from Rhodospirillum rubrum chromatophores with Triton X-100 and further purified by centrifugation on a glycerol gradient. The partially purified RrFo . F1 contains 13 distinct polypeptide subunits, as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, including the subunits of the oligomycin-sensitive, water-soluble RrF1 ATPase. The ATPase activity of RrF0 . F1 as that of the membrane-bound enzyme complex depends on Ca2+ or Mg2+ and from detailed kinetic studies it is concluded that the divalent cation-ATP complex is the substrate for both ATPase complexes. Free ATP and free Mg2+ act as competitive inhibitors, with Ki values of 1 mM and 7 muM, respectively. The subunit composition of the purified RrFo . F1 and its similarity to the membrane-bound ATPase with respect to cation dependence and sensitivity to energy transfer inhibitors suggests that it contains all the subunits of the R. rubrum coupling factor-ATPase complex.