Proton-Motive Force :: physiology
Relationship between rates of respiratory proton extrusion and ATP synthesis in obligately alkaliphilic Bacillus clarkii DSM 8720(T).
Toshikazu Hirabayashi, Toshitaka Goto, Hajime Morimoto, Kazuaki Yoshimune, Hidetoshi Matsuyama, Isao Yumoto
Department of Bioscience and Technology, School of Biological Science and Engineering, Tokai University, Minaminosawa, Minami-ku, Sapporo, Hokkaido 005-8601, Japan.
To elucidate the energy production mechanism of alkaliphiles, the relationship between the rate of proton extrusion via the respiratory chain and the corresponding ATP synthesis rate was examined in obligately alkaliphilic Bacillus clarkii DSM 8720(T) and neutralophilic Bacillus subtilis IAM 1026. The oxygen consumption rate of B. subtilis IAM 1026 cells at pH 7 was approximately 2.5 times higher than that of B. clarkii DSM 8720(T) cells at pH 10. The H⁺/O ratio of B. clarkii DSM 8720(T) cells was approximately 1.8 times higher than that of B. subtilis IAM 1026 cells. On the basis of oxygen consumption rate and H⁺/O ratio, the rate of proton translocation via the respiratory chain in B. subtilis IAM 1026 is expected to be approximately 1.4 times higher than that in B. clarkii DSM 8720(T). Conversely, the rate of ATP synthesis in B. clarkii DSM 8720(T) at pH 10 was approximately 7.5 times higher than that in B. subtilis IAM 1026 at pH 7. It can be predicted that the difference in rate of ATP synthesis is due to the effect of transmembrane electrical potential (Δψ) on protons translocated via the respiratory chain. The Δψ values of B. clarkii DSM 8720(T) and B. subtilis IAM 1026 were estimated as -192 mV (pH 10) and -122 mV (pH 7), respectively. It is considered that the discrepancy between the rates of proton translocation and ATP synthesis between the strains used in this study is due to the difference in ATP production efficiency per translocated proton between the two strains caused by the difference in Δψ.
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Department of Microbiology and Immunology, University of South Alabama College of Medicine, Mobile, Alabama 36695, USA. firstname.lastname@example.org
Gastrointestinal pathogens are faced with an extremely acidic environment. Within moments, a pathogen such as Escherichia coli O157:H7 can move from the nurturing pH 7 environment of a hamburger to the harsh pH 2 milieu of the stomach. Surprisingly, certain microorganisms that grow at neutral pH have elegantly regulated systems that enable survival during excursions into acidic environments. The best-characterized acid-resistance system is found in E. coli.
Department of Microbiology, Washington State University, Pullman 99164-4233, USA.
TonB couples the cytoplasmic membrane protonmotive force (pmf) to active transport across the outer membrane, potentially through a series of conformational changes. Previous studies of a TonB transmembrane domain mutant (TonB-delta V17) and its phenotypical suppressor (ExbB-A39E) suggested that TonB is conformationally sensitive. Here, two new mutations of the conserved TonB transmembrane domain SHLS motif were isolated, TonB-S16L and -H20Y, as were two new suppressors, ExbB-V35E and -V36D. Each suppressor ExbB restored at least partial function to the TonB mutants, although TonB-delta V17, for which both the conserved motif and the register of the predicted transmembrane domain alpha-helix are affected, was the most refractory. As demonstrated previously, TonB can undergo at least one conformational change, provided both ExbB and a functional TonB transmembrane domain are present. Here, we show that this conformational change reflects the ability of TonB to respond to the cytoplasmic membrane proton gradient, and occurs in proportion to the level of TonB activity attained by mutant-suppressor pairs. The phenotype of TonB-delta V17 was more complex than the -S16L and -H20Y mutations, in that, beyond the inability to be energized efficiently, it was also conditionally unstable. This second defect was evident only after suppression by the ExbB mutants, which allow transmembrane domain mutants to be energized, and presented as the rapid turnover of TonB-delta V17. Importantly, this degradation was dependent upon the presence of a TonB-dependent ligand, suggesting that TonB conformation also changes following the energy transduction event. Together, these observations support a dynamic model of energy transduction in which TonB cycles through a set of conformations that differ in potential energy, with a transition to a higher energy state driven by pmf and a transition to a lower energy state accompanying release of stored potential energy to an outer membrane receptor.
Division of Biomedical Sciences, Imperial College London, SW7 2AZ, UK.
The structure of the catalytic and electron-transfer subunits (NarGH) of the integral membrane protein, respiratory nitrate reductase (Nar) has been determined to 2.0 A resolution revealing the molecular architecture of this Mo-bisMGD (molybdopterin-guanine-dinucleotide) containing enzyme which includes a previously undetected FeS cluster. Nar, together with the related enzyme formate dehydrogenase (Fdh-N), is a key enzyme in the generation of proton motive force across the membrane in Escherichia coli nitrate respiration. A comparative study revealed that Nar and Fdh-N employ different approaches for acquiring substrate, reflecting different catalytic mechanisms. Nar uses a very narrow and nonpolar substrate-conducting cavity with a nonspecific substrate binding site, whereas Fdh-N accommodates a wider, positively charged substrate-conducting cavity with a more specific substrate binding site. The Nar structure also demonstrates the first example of an Asp side chain acting as a Mo ligand providing a structural basis for the classification of Mo-bisMGD enzymes.
Department of Biochemistry, University of Cambridge, UK. email@example.com
There is a futile cycle of pump and leak of protons across the mitochondrial inner membrane. The contribution of the proton cycle to standard metabolic rate is significant, particularly in skeletal muscle, and it accounts for 20% or more of the resting respiration of a rat. The mechanism of the proton leak is uncertain: basal proton conductance is not a simple biophysical leak across the unmodified phospholipid bilayer. Equally, the evidence that it is catalysed by homologues of the brown adipose uncoupling protein, UCP1, is weak. The yeast genome contains no clear UCP homologue but yeast mitochondria have normal basal proton conductance. UCP1 catalyses a regulated inducible proton conductance in brown adipose tissue and the possibility remains open that UCP2 and UCP3 have a similar role in other tissues, although this has yet to be demonstrated.
Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, Utah 84112, USA.
Bacterial flagella contain a specialized secretion apparatus that functions to deliver the protein subunits that form the filament and other structures to outside the membrane. This apparatus is related to the injectisome used by many gram-negative pathogens and symbionts to transfer effector proteins into host cells; in both systems this export mechanism is termed 'type III' secretion. The flagellar secretion apparatus comprises a membrane-embedded complex of about five proteins, and soluble factors, which include export-dedicated chaperones and an ATPase, FliI, that was thought to provide the energy for export. Here we show that flagellar secretion in Salmonella enterica requires the proton motive force (PMF) and does not require ATP hydrolysis by FliI. The export of several flagellar export substrates was prevented by treatment with the protonophore CCCP, with no accompanying decrease in cellular ATP levels. Weak swarming motility and rare flagella were observed in a mutant deleted for FliI and for the non-flagellar type-III secretion ATPases InvJ and SsaN. These findings show that the flagellar secretion apparatus functions as a proton-driven protein exporter and that ATP hydrolysis is not essential for type III secretion.
The PrlA and PrlG phenotypes are caused by a loosened association among the translocase SecYEG subunits.
Laboratoire Transports et Signalisation Cellulaires, CNRS UMR 8619, Bâtiment 430, Université de Paris XI, Orsay, 91405, France.
prlA mutations in the gene encoding the SecY subunit of the membrane domain of the Escherichia coli preprotein translocase confer many phenotypes: enhanced translocation rates, increased affinity for SecA, diminished requirement for functional leader sequences, reduced proton-motive force (PMF) dependence of preprotein translocation and facilitated translocation of preproteins with folded domains. We now report that both prlA and prlG mutations weaken the associations between the SecY, SecE and SecG subunits of the translocase. This loosened association increases the initiation of translocation by facilitating the insertion of SecA with its bound preprotein but reduces the stimulatory effect of the PMF during the initial step of translocation. Furthermore, the originally isolated prlA4 mutant, which possesses a particularly labile SecYEG complex, acquired a secondary mutation that restored the stability while conserving the flexibility of the complex. Combinations of certain prlA and prlG mutations, known to cause synthetic lethality in vivo, dramatically loosen subunit association and lead to complete disassembly of SecYEG. These findings underscore the importance of the loosened SecYEG association for the Prl phenotypes. We propose a model in which each of the PrlA and PrlG phenotypes derive from this enhanced SecYEG conformational flexibility.
Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan.
Translocation of many soluble proteins across cell membranes occurs in an ATPase-driven manner. For construction of the bacterial flagellum responsible for motility, most of the components are exported by the flagellar protein export apparatus. The FliI ATPase is required for this export, and its ATPase activity is regulated by FliH; however, it is unclear how the chemical energy derived from ATP hydrolysis is used for the export process. Here we report that flagellar proteins of Salmonella enterica serovar Typhimurium are exported even in the absence of FliI. A fliH fliI double null mutant was weakly motile. Certain mutations in FlhA or FlhB, which form the core of the export gate, substantially improved protein export and motility of the double null mutant. Furthermore, proton motive force was essential for the export process. These results suggest that the FliH-FliI complex facilitates only the initial entry of export substrates into the gate, with the energy of ATP hydrolysis being used to disassemble and release the FliH-FliI complex from the protein about to be exported. The rest of the successive unfolding/translocation process of the substrates is driven by proton motive force.
Energy-dependent conformational change in the TolA protein of Escherichia coli involves its N-terminal domain, TolQ, and TolR.
Unité de Microbiologie et Génétique, ERS2009 (CNRS-INSA-Université Lyon 1), F-69622 Villeurbanne Cedex, France.
TolQ, TolR, and TolA inner membrane proteins of Escherichia coli are involved in maintaining the stability of the outer membrane. They share homology with the ExbB, ExbD, and TonB proteins, respectively. The last is involved in energy transduction between the inner and the outer membrane, and its conformation has been shown to depend on the presence of the proton motive force (PMF), ExbB, and ExbD. Using limited proteolysis experiments, we investigated whether the conformation of TolA was also affected by the PMF. We found that dissipation of the PMF by uncouplers led to the formation of a proteinase K digestion fragment of TolA not seen when uncouplers are omitted. This fragment was also detected in Delta tolQ, Delta tolR, and tolA(H22P) mutants but, in contrast to the parental strain, was also seen in the absence of uncouplers. We repeated those experiments in outer membrane mutants such as lpp, pal, and Delta rfa mutants: the behavior of TolA in lpp mutants was similar to that observed with the parental strain. However, the proteinase K-resistant fragment was never detected in the Delta rfa mutant. Altogether, these results suggest that TolA is able to undergo a PMF-dependent change of conformation. This change requires TolQ, TolR, and a functional TolA N-terminal domain. The potential role of this energy-dependent process in the stability of the outer membrane is discussed.
Department of Molecular Physiology and Biological Physics, University of Virginia, P.O. Box 800736, Charlottesville, VA 22908-0736, USA. firstname.lastname@example.org
The F0F1 ATP synthase is a large complex of at least 22 subunits, more than half of which are in the membranous F0 sector. This nearly ubiquitous transporter is responsible for the majority of ATP synthesis in oxidative and photo-phosphorylation, and its overall structure and mechanism have remained conserved throughout evolution. Most examples utilize the proton motive force to drive ATP synthesis except for a few bacteria, which use a sodium motive force. A remarkable feature of the complex is the rotary movement of an assembly of subunits that plays essential roles in both transport and catalytic mechanisms. This review addresses the role of rotation in catalysis of ATP synthesis/hydrolysis and the transport of protons or sodium.
Plant Biochemistry, Center for Chemistry and Chemical Engineering, Box 124, Lund University, SE-221 00 Lund, Sweden. email@example.com