EPR spectroscopy of 5-DOXYL-stearic acid bound to the mitochondrial uncoupling protein reveals its competitive displacement by alkylsulfonates in the channel and allosteric displacement by ATP

Mitochondrial Uncoupling Proteins: Subtle Regulators of Cellular Redox Signaling

SIGNIFICANCE

Mitochondria are the energetic, metabolic, redox, and information signaling centers of the cell. Substrate pressure, mitochondrial network dynamics, and cristae morphology state are integrated by the protonmotive force Δp or its potential component, ΔΨ, which are attenuated by proton backflux into the matrix, termed uncoupling. The mitochondrial uncoupling proteins (UCP1-5) play an eminent role in the regulation of each of the mentioned aspects, being involved in numerous physiological events including redox signaling. Recent Advances: UCP2 structure, including purine nucleotide and fatty acid (FA) binding sites, strongly support the FA cycling mechanism: UCP2 expels FA anions, whereas uncoupling is achieved by the membrane backflux of protonated FA. Nascent FAs, cleaved by phospholipases, are preferential. The resulting Δp dissipation decreases superoxide formation dependent on Δp. UCP-mediated antioxidant protection and its impairment are expected to play a major role in cell physiology and pathology. Moreover, UCP2-mediated aspartate, oxaloacetate, and malate antiport with phosphate is expected to alter metabolism of cancer cells.

CRITICAL ISSUES

A wide range of UCP antioxidant effects and participations in redox signaling have been reported; however, mechanisms of UCP activation are still debated. Switching off/on the UCP2 protonophoretic function might serve as redox signaling either by employing/releasing the extra capacity of cell antioxidant systems or by directly increasing/decreasing mitochondrial superoxide sources. Rapid UCP2 degradation, FA levels, elevation of purine nucleotides, decreased Mg²⁺, or increased pyruvate accumulation may initiate UCP-mediated redox signaling.

FUTURE DIRECTIONS

Issues such as UCP2 participation in glucose sensing, neuronal (synaptic) function, and immune cell activation should be elucidated. Antioxid. Redox Signal. 29, 667-714.

Functionally Active Membrane Proteins Incorporated in Mesostructured Silica Films

A versatile synthetic protocol is reported that allows high concentrations of functionally active membrane proteins to be incorporated in mesostructured silica materials. Judicious selections of solvent, surfactant, silica precursor species, and synthesis conditions enable membrane proteins to be stabilized in solution and during subsequent coassembly into silica-surfactant composites with nano- and mesoscale order. This was demonstrated by using a combination of nonionic ( n-dodecyl-β-d-maltoside or Pluronic P123), lipid-like (1,2-diheptanoyl- s n-glycero-3-phosphocholine), and perfluoro-octanoate surfactants under mild acidic conditions to coassemble the light-responsive transmembrane protein proteorhodopsin at concentrations up to 15 wt % into the hydrophobic regions of worm-like mesostructured silica materials in films. Small-angle X-ray scattering, electron paramagnetic resonance spectroscopy, and transient UV-visible spectroscopy analyses established that proteorhodopsin molecules in mesostructured silica films exhibited native-like function, as well as enhanced thermal stability compared to surfactant or lipid environments. The light absorbance properties and light-activated conformational changes of proteorhodopsin guests in mesostructured silica films are consistent with those associated with the native H+-pumping mechanism of these biomolecules. The synthetic protocol is expected to be general, as demonstrated also for the incorporation of functionally active cytochrome c, a peripheral membrane protein enzyme involved in electron transport, into mesostructured silica-cationic surfactant films.

Attenuating Staphylococcus aureus Virulence by Targeting Flotillin Protein Scaffold Activity

Scaffold proteins are ubiquitous chaperones that bind proteins and facilitate physical interaction of multi-enzyme complexes. Here we used a biochemical approach to dissect the scaffold activity of the flotillin-homolog protein FloA of the multi-drug-resistant human pathogen Staphylococcus aureus. We show that FloA promotes oligomerization of membrane protein complexes, such as the membrane-associated RNase Rny, which forms part of the RNA-degradation machinery called the degradosome. Cells lacking FloA had reduced Rny function and a consequent increase in the targeted sRNA transcripts that negatively regulate S. aureus toxin expression. Small molecules that altered FloA oligomerization also reduced Rny function and decreased the virulence potential of S. aureus in vitro, as well as in vivo, using invertebrate and murine infection models. Our results suggest that flotillin assists in the assembly of protein complexes involved in S. aureus virulence, and could thus be an attractive target for the development of new antimicrobial therapies.

Antioxidant and Regulatory Role of Mitochondrial Uncoupling Protein UCP2 in Pancreatic β-cells

Research on brown adipose tissue and its hallmark protein, mitochondrial uncoupling protein UCP1, has been conducted for half a century and has been traditionally studied in the Institute of Physiology (AS CR, Prague), likewise UCP2 residing in multiple tissues for the last two decades. Our group has significantly contributed to the elucidation of UCP uncoupling mechanism, fully dependent on free fatty acids (FFAs) within the inner mitochondrial membrane. Now we review UCP2 physiological roles emphasizing its roles in pancreatic β-cells, such as antioxidant role, possible tuning of redox homeostasis (consequently UCP2 participation in redox regulations), and fine regulation of glucose-stimulated insulin secretion (GSIS). For example, NADPH has been firmly established as being a modulator of GSIS and since UCP2 may influence redox homeostasis, it likely affects NADPH levels. We also point out the role of phospholipase iPLA2 isoform  in providing FFAs for the UCP2 antioxidant function. Such initiation of mild uncoupling hypothetically precedes lipotoxicity in pancreatic β-cells until it reaches the pathological threshold, after which the antioxidant role of UCP2 can be no more cell-protective, for example due to oxidative stress-accumulated mutations in mtDNA. These mechanisms, together with impaired autocrine insulin function belong to important causes of Type 2 diabetes etiology.

Undecanesulfonate does not allosterically activate H+ uniport mediated by uncoupling protein-1 in brown adipose tissue mitochondria

Undecanesulfonate is transported by uncoupling protein-1. Its inability to induce H+ uniport with reconstituted uncoupling protein-1 supports fatty acid cycling hypothesis. Rial et al. [Rial, E., Aguirregoitia, E., Jimenez-Jimenez, J., & Ledesma, A. (2004). Alkylsulfonates activate the uncoupling protein UCP1: Implications for the transport mechanism. Biochimica et Biophysica Acta, 1608, 122-130], have challenged the fatty acid cycling by observing uncoupling of brown adipose tissue mitochondria due to undecanesulfonate, interpreted as allosteric activation of uncoupling protein-1. We have estimated undecanesulfonate effects after elimination of endogenous fatty acids by carnitine cycle in the presence or absence of bovine serum albumin. We show that the undecanesulfonate effect is partly due to fatty acid release from albumin when undecanesulfonate releases bound fatty acid and partly represents a non-specific uncoupling protein-independent acceleration of respiration, since it proceeds also in rat heart mitochondria lacking uncoupling protein-1 and membrane potential is not decreased upon addition of undecanesulfonate without albumin. When the net fatty acid-induced uncoupling was assayed, the addition of undecanesulfonate even slightly inhibited the uncoupled respiration. We conclude that undecanesulfonate does not allosterically activate uncoupling protein-1 and that fatty acid cycling cannot be excluded on a basis of its non-specific effects.

Binding of fatty acids to the uncoupling protein from brown adipose tissue mitochondria

The uncoupling protein 1 (UCP1) is a H(+) carrier which plays a key role in heat generation in brown adipose tissue. The H(+) transport activity of UCP1 is activated by long-chain fatty acids and inhibited by purine nucleotides. While nucleotide binding has been well characterized, the interaction of fatty acid with UCP1 remains unknown. Here I demonstrate the binding of fatty acids by competition with a fluorescent nucleotide probe 2(')-O-dansyl guanosine 5(')-triphosphate (GTP), which has been shown previously to bind at the nucleotide binding site in UCP1. Fatty acids but not their esters competitively inhibit the binding of 2(')-O-dansyl GTP to UCP1. The fatty acid effect was enhanced at higher pH, suggesting the binding of fatty acid anion to UCP1. The inhibition constants K(i) were determined by fluorescence titrations for various fatty acids. Short-chain (C<8) fatty acids display no affinity, whereas medium-chain (C10-14) and unsaturated C18 fatty acids exhibit stronger affinity (K(i)=65 microM, for elaidic acid). This specificity profile agrees with previous functional data obtained in both proteoliposomes and mitochondria, suggesting a possible physiological role of this fatty acid binding site.

The mechanism of proton transport mediated by mitochondrial uncoupling proteins

The effort to understand the mechanism of uncoupling by UCP has devolved into two models - the fatty acid protonophore model and the proton buffering model. Evidence for each hypothesis is summarized and evaluated. We also evaluate the obligatory requirement for fatty acids in UCP1-mediated uncoupling and the question of fatty acid affinity for UCP1. The structural bases of UCP transport function and nucleotide inhibition are discussed in light of recent mutagenesis studies and in relationship to the sequences of newly discovered UCPs.

Mitochondrial uncoupling protein may participate in futile cycling of pyruvate and other monocarboxylates

The physiological role of monocarboxylate transport in brown adipose tissue mitochondria has been reevaluated. We studied pyruvate, alpha-ketoisovalerate, alpha-ketoisocaproate, and phenylpyruvate uniport via the uncoupling protein (UCP1) as a GDP-sensitive swelling in K+ salts induced by valinomycin or by monensin and carbonyl cyanide-p-(trifluoromethoxy)phenylhydrazone in Na+ salts. We have demonstrated that this uniport is inhibited by fatty acids. GDP inhibition in K+ salts was not abolished by an uncoupler, indicating a negligible monocarboxylic acid penetration via the lipid bilayer. In contrast, the electroneutral pyruvate uptake (swelling in ammonium pyruvate or potassium pyruvate induced by change in pH) mediated by the pyruvate carrier was inhibited by its specific inhibitor alpha-cyano-4-hydroxycinnamate but not by fatty acids. Moreover, alpha-cyano-4-hydroxycinnamate enhanced the energization of brown adipose tissue mitochondria, which was monitored fluorometrically by 2-(4-dimethylaminostyryl)-1-methylpyridinium iodide and safranin O. Consequently, we suggest that UCP1 might participate in futile cycling of unipolar ketocarboxylates under certain physiological conditions while expelling these anions from the matrix. The cycle is completed on their return via the pyruvate carrier in an H+ symport mode.

Fatty acid cycling mechanism and mitochondrial uncoupling proteins

We hypothesize that fatty acid-induced uncoupling serves in bioenergetic systems to set the optimum efficiency and tune the degree of coupling of oxidative phosphorylation. Uncoupling results from fatty acid cycling, enabled by several phylogenetically specialized proteins and, to a lesser extent, by other mitochondrial carriers. It is suggested that the regulated uncoupling in mammalian mitochondria is provided by uncoupling proteins UCP-1, UCP-2 and UCP-3, whereas in plant mitochondria by PUMP and StUCP, all belonging to the gene family of mitochondrial carriers. UCP-1, and hypothetically UCP-3, serve mostly to provide nonshivering thermogenesis in brown adipose tissue and skeletal muscle, respectively. Fatty acid cycling was documented for UCP-1, PUMP and ADP/ATP carrier, and is predicted also for UCP-2 and UCP-3. UCP-1 mediates a purine nucleotide-sensitive uniport of monovalent unipolar anions, including anionic fatty acids. The return of protonated fatty acid leads to H+ uniport and uncoupling. UCP-2 is probably involved in the regulation of body weight and energy balance, in fever, and defense against generation of reactive oxygen species. PUMP has been discovered in potato tubers and immunologically detected in fruits and corn, whereas StUCP has been cloned and sequenced froma a potato gene library. PUMP is supposed to act in the termination of synthetic processes in mature fruits and during the climacteric respiratory rise.

Reconstituted plant uncoupling mitochondrial protein allows for proton translocation via fatty acid cycling mechanism

Potato and tomato plant uncoupling mitochondrial protein (PUMP) was reconstituted into liposomes, and K+ or H+ fluxes associated with fatty acid (FA)-induced ion movement were measured using fluorescent ion indicators potassium binding benzofuraneisophthalate and 6-methoxy-N-(3-sulfopropyl)-quinolinium. We suggest that PUMP, like its mammalian counterpart, the uncoupling protein of brown adipose tissue mitochondria (Garlid, K. D., Orosz, D. E., Modrianský, M., Vassanelli, S., and Jeek, P. (1996), J. Biol. Chem. 271, 2615-2702), allows for H+ translocation via a FA cycling mechanism. Reconstituted PUMP translocated anionic linoleic and heptylbenzoic acids, undecanesulfonate, and hexanesulfonate, but not phenylvaleric and abscisic acids or Cl-. Transport was inhibited by ATP and GDP. Internal acidification of protein-free liposomes by linoleic or heptylbenzoic acid indicated that H+ translocation occurs by FA flip-flopping across the lipid bilayer. However, addition of valinomycin after FA-initiated GDP-sensitive H+ efflux solely in proteoliposomes, indicating that influx of anionic FA via PUMP precedes a return of protonated FA carrying H+. Phenylvaleric acid, unable to flip-flop, was without effect. Kinetics of FA and undecanesulfonate uniport suggested the existence of an internal anion binding site. Exponential flux-voltage characteristics were also studied. We suggest that regulated uncoupling in plant mitochondria may be important during fruit ripening, senescence, and seed dormancy.

A structure-activity study of fatty acid interaction with mitochondrial uncoupling protein

Fatty acid (FA) uniport via mitochondrial uncoupling protein (UcP) was detected fluorometrically with PBFI, potassium-binding benzofuran phthalate and SPQ, 6-methoxy-N-(3-sulfopropyl)-quinolinium, indicating K+ and H+, respectively. The FA structural patterns required for FA flip-flop, UcP-mediated FA uniport, activation of UcP-mediated H+ transport in proteoliposomes, and inhibition of UcP-mediated Cl- uniport by FA, were identical. Positive responses were found exclusively with FA which were able to flip-flop in a protonated form across the membrane and no responses were found with 'inactive' FA lacking the flip-flop ability. The findings support the existence of FA cycling mechanism.

Evidence for anion-translocating plant uncoupling mitochondrial protein in potato mitochondria

Transport properties of plant mitochondria from potato tubers were investigated using the swelling technique and membrane potential measurements. Proton-dependent swelling of fatty acid-depleted mitochondria in potassium acetate with valinomycin was possible only in the presence of fatty acids (linoleic acid and 12-(4-azido-2-nitrophenylamino)dodecanoic acid) and was inhibited by various purine nucleotides including ATP, GDP, and GTP. Swelling representing uptake of hexanesulfonate was also inhibited by purine nucleotides. Also, the membrane potential of fatty acid-depleted potato mitochondria energized by succinate declined upon the addition of linoleic acid or 12-(4-azido-2-nitrophenylamino)dodecanoic acid, and this decrease was prevented by ATP and other purine nucleotides. These transport activities are identical to those reported for brown adipose tissue mitochondria and related to the uncoupling protein; therefore, we ascribed them to the plant mitochondrial uncoupling protein (PUMP). A major difference between plant and mammalian uncoupling protein is that PUMP transports small hydrophilic anions such as Cl- very slowly, if at all. We suggest that PUMP may play an important role in plant physiology, where a regulated uncoupling and thermogenesis can proceed during fruit and seed development.

Photomodification of mitochondrial proteins by azido fatty acids and its effect on mitochondrial energetics. Further evidence for the role of the ADP/ATP carrier in fatty-acid-mediated uncoupling

Azido derivatives of long-chain fatty acids, 12-(4-azido-2-nitrophenylamino)dodecanoic acid (N3-NpNH-Lau) and 16-(4-azido-2-nitrophenylamino)hexadecanoic acid (N3-NpNH-Pam), were used to study the mechanism of the protonophoric function of long-chain fatty acids in mitochondrial membranes. N3-NpNH-Lau was found to increase resting-state respiration and decrease the membrane potential in a dose-dependent way in a manner similar to that of the natural fatty acid, myristate. Both effects of N3-NpNH-Lau as well as of the myristate were reversed or prevented by the inhibitor of the mitochondrial ADP/ATP carrier (AAC), carboxyatractyloside. This protective effect of carboxyatractyloside was well expressed in rat heart mitochondria and less so in mitochondria within digitonin-permeabilized Ehrlich ascites tumour cells. Photomodification of Ehrlich ascites tumour mitochondria by ultraviolet irradiation in the presence of N3-NpNH-Lau made them more resistant to the uncoupling effect of myristate, and photomodification of rat heart mitochondria resulted in a strong inhibition of AAC which could not be reversed by serum albumin. Photolabelling of rat heart mitochondria with tritiated N3-NpNH-Pam revealed around 10 labelled bands on SDS/polyacrylamide gel electrophoresis. Based on immunodetection with a specific antibody, one of them, corresponding to 30 kDa, was identified as AAC. Specific interaction of AAC with azido fatty acids was confirmed by a high radiolabelling of this band. The role of fatty acids in fine control of the efficiency of oxidative phosphorylation is discussed.

Inner membrane anion channel and dicarboxylate carrier in brown adipose tissue mitochondria

In brown adipose tissue mitochondria, the anion transport proteins should respond to regulatory mechanisms controlling the thermogenic or resting state. We re-evaluated the role of transport of organic/metabolite anions in these mitochondria, namely with regards to delta pH-regulation and substrate specificity. Valinomycin-induced osmotic swelling in potassium salts indicated by light scattering either directly on a fluorometer, or as the reciprocal absorbance, was used to characterize the anion uniport. A delta pH "jump" was thus created in respiring mitochondria and the delta pH-driven transport was studied. The two major features are reported: (1) existence of the inner membrane anion channel exhibiting the same full spectrum of anion and inhibitor specificity as in liver; and (2) existence of dicarboxylate carrier, so far disputed in brown adipose tissue mitochondria. The inner membrane anion channel was activated either by elevating delta pH in respiring mitochondria or by depleting matrix Mg2+ at alkaline pH. Dicarboxylate carrier was activated by elevated delta pH under conditions when the channel was blocked. A specific delta pH regulation could explain this activation and silence of the carrier in early studies. In conclusion, wide substrate specificity makes the inner membrane anion channel suitable for the regulation of volume homeostasis and a feed-back control between the delta psi-driven and the delta pH-driven transport. The delta pH-activated dicarboxylate carrier is essential in the coupled state for malate uptake which enables fatty acid synthesis, while, in the uncoupled state, inaccessibility of dicarboxylates favors oxidation of fatty acids or pyruvate.

Photoaffinity labelling of the mitochondrial uncoupling protein by [3H]azido fatty acid affects the anion channel

Brown adipose tissue (BAT) mitochondria were incubated with the azido derivative of fatty acid (hexadecanoic) containing four tritium atoms, [3H]AzHA, and among all mitochondrial proteins only a few proteins were photolabelled after irradiation with UV. It suggests the existence of specific fatty acid binding sites on mitochondrial proteins. It was also possible to label with [3H]AzHA the isolated uncoupling protein (UcP) of BAT mitochondria with a low stoichiometry--lower than one AzHA per dimeric UcP. These results together with the observed competition (i.e. prevention of photolabelling) of various UcP anionic substrates with [3H]AzHA and its dodecanoic acid analogue, suggest the existence of the specific fatty acid binding site on UcP identical with the anion channel or anion translocating site.