Interaction between delta and epsilon subunits of F1-ATPase from pig heart mitochondria. Circular dichroism and intrinsic fluorescence of purified and reconstituted delta epsilon complex

Hsp90 and mitochondrial proteases Yme1 and Yta10/12 participate in ATP synthase assembly in Saccharomyces cerevisiae

Hsc82 and Hsp82, the Hsp90 family proteins of yeast, are both required for fermentative growth at 37°C. Inactivation of either of the mitochondrial AAA proteases, Yme1 or Yta10/12, allows fermentative growth of hsc82∆ or hsp82∆ strains at 37°C. Genetic evidence indicates interaction of Hsc82/Hsp82 with the Yme1 and Yta10/Yta12 complexes in promoting F(1)F(o)-ATPase activity, with Hsc82 specifically required for F(1)-ATPase assembly. A previously reported mutation in Rpt3, one of the six ATPases of the proteasome, suppresses yme1∆ phenotypes and increases transcription of HSC82 but not HSP82. These genetic interactions describe a functional role for Hsp90 proteins in mitochondrial biogenesis.

Mitochondrial ATP synthase deficiency due to a mutation in the ATP5E gene for the F1 epsilon subunit

F1Fo-ATP synthase is a key enzyme of mitochondrial energy provision producing most of cellular ATP. So far, mitochondrial diseases caused by isolated disorders of the ATP synthase have been shown to result from mutations in mtDNA genes for the subunits ATP6 and ATP8 or in nuclear genes encoding the biogenesis factors TMEM70 and ATPAF2. Here, we describe a patient with a homozygous p.Tyr12Cys mutation in the epsilon subunit encoded by the nuclear gene ATP5E. The 22-year-old woman presented with neonatal onset, lactic acidosis, 3-methylglutaconic aciduria, mild mental retardation and developed peripheral neuropathy. Patient fibroblasts showed 60-70% decrease in both oligomycin-sensitive ATPase activity and mitochondrial ATP synthesis. The mitochondrial content of the ATP synthase complex was equally reduced, but its size was normal and it contained the mutated epsilon subunit. A similar reduction was found in all investigated F1 and Fo subunits with the exception of Fo subunit c, which was found to accumulate in a detergent-insoluble form. This is the first case of a mitochondrial disease due to a mutation in a nuclear encoded structural subunit of the ATP synthase. Our results indicate an essential role of the epsilon subunit in the biosynthesis and assembly of the F1 part of the ATP synthase. Furthermore, the epsilon subunit seems to be involved in the incorporation of subunit c to the rotor structure of the mammalian enzyme.

Knockdown of F1 epsilon subunit decreases mitochondrial content of ATP synthase and leads to accumulation of subunit c

The subunit epsilon of mitochondrial ATP synthase is the only F1 subunit without a homolog in bacteria and chloroplasts and represents the least characterized F1 subunit of the mammalian enzyme. Silencing of the ATP5E gene in HEK293 cells resulted in downregulation of the activity and content of the mitochondrial ATP synthase complex and of ADP-stimulated respiration to approximately 40% of the control. The decreased content of the epsilon subunit was paralleled by a decrease in the F1 subunits alpha and beta and in the Fo subunits a and d while the content of the subunit c was not affected. The subunit c was present in the full-size ATP synthase complex and in subcomplexes of 200-400 kDa that neither contained the F1 subunits, nor the Fo subunits. The results indicate that the epsilon subunit is essential for the assembly of F1 and plays an important role in the incorporation of the hydrophobic subunit c into the F1-c oligomer rotor of the mitochondrial ATP synthase complex.

Study of the yeast Saccharomyces cerevisiae F1F0-ATPase epsilon-subunit

The yeast Saccharomyces cerevisiae F1F0-ATPase epsilon-subunit (61 residues) was synthesized by the solid-phase peptide approach under both acidic and basic strategies. Only the latter strategy allowed us to obtain a pure epsilon-subunit. The strong propensity of the protein to produce few soluble dimeric species depending on pH has been proved by size-exclusion chromatography, electrophoresis and mass spectrometry. A circular dichroism study showed that an aqueous solution containing 30% trifluoroethanol or 200 mM sodium dodecyl sulphate is required for helical folding. In both solvents at acidic pH, the epsilon-subunit is soluble and monomeric.

Mitochondrial F(0)F(1) ATP synthase. Subunit regions on the F1 motor shielded by F(0), Functional significance, and evidence for an involvement of the unique F(0) subunit F(6)

Studies reported here were undertaken to gain greater molecular insight into the complex structure of mitochondrial ATP synthase (F(0)F(1)) and its relationship to the enzyme's function and motor-related properties. Significantly, these studies, which employed N-terminal sequence, mass spectral, proteolytic, immunological, and functional analyses, led to the following novel findings. First, at the top of F(1) within F(0)F(1), all six N-terminal regions derived from alpha + beta subunits are shielded, indicating that one or more F(0) subunits forms a "cap." Second, at the bottom of F(1) within F(0)F(1), the N-terminal region of the single delta subunit and the C-terminal regions of all three alpha subunits are shielded also by F(0). Third, and in contrast, part of the gamma subunit located at the bottom of F(1) is already shielded in F(1), indicating that there is a preferential propensity for interaction with other F(1) subunits, most likely delta and epsilon. Fourth, and consistent with the first two conclusions above that specific regions at the top and bottom of F(1) are shielded by F(0), further proteolytic shaving of alpha and beta subunits at these locations eliminates the capacity of F(1) to couple a proton gradient to ATP synthesis. Finally, evidence was obtained that the F(0) subunit called "F(6)," unique to animal ATP synthases, is involved in shielding F(1). The significance of the studies reported here, in relation to current views about ATP synthase structure and function in animal mitochondria, is discussed.

Partial assembly of the yeast mitochondrial ATP synthase

The mitochondrial ATP synthase is a molecular motor that drives the phosphorylation of ADP to ATP. The yeast mitochondrial ATP synthase is composed of at least 19 different peptides, which comprise the F1 catalytic domain, the F0 proton pore, and two stalks, one of which is thought to act as a stator to link and hold F1 to F0, and the other as a rotor. Genetic studies using yeast Saccharomyces cerevisiae have suggested the hypothesis that the yeast mitochondrial ATP synthase can be assembled in the absence of 1, and even 2, of the polypeptides that are thought to comprise the rotor. However, the enzyme complex assembled in the absence of the rotor is thought to be uncoupled, allowing protons to freely flow through F0 into the mitochondrial matrix. Left uncontrolled, this is a lethal process and the cell must eliminate this leak if it is to survive. In yeast, the cell is thought to lose or delete its mitochondrial DNA (the petite mutation) thereby eliminating the genes encoding essential components of F0. Recent biochemical studies in yeast, and prior studies in E. coli, have provided support for the assembly of a partial ATP synthase in which the ATP synthase is no longer coupled to proton translocation.

Complementation of deletion mutants in the genes encoding the F1-ATPase by expression of the corresponding bovine subunits in yeast S. cerevisiae

The F1F0 ATP synthase is composed of the F1-ATPase which is bound to F0, in the inner membrane of the mitochondrion. Assembly and function of the enzyme is a complicated task requiring the interactions of many proteins for the folding, import, assembly, and function of the enzyme. The F1-ATPase is a multimeric enzyme composed of five subunits in the stoichiometry of alpha3beta3gammadeltaepsilon. This study demonstrates that four of the five bovine subunits of the F1-ATPase can be imported and function in an otherwise yeast enzyme effectively complementing mutations in the genes encoding the corresponding yeast ATPase subunits. In order to demonstrate this, the coding regions of each of the five genes were separately deleted in yeast providing five null mutant strains. All of the strains displayed negative or a slow growth phenotype on medium containing glycerol as the carbon source and strains with a null mutation in the gene encoding the gamma-, delta- or epsilon-gene became completely, or at a high frequency, cytoplasmically petite. The subunits of bovine F1 were expressed individually in the yeast strains with the corresponding null mutations and targeted to the mitochondrion using a yeast mitochondrial leader peptide. Expression of the bovine alpha-, beta-, gamma-, and epsilon-, but not the delta-, subunit complemented the corresponding null mutations in yeast correcting the corresponding negative phenotypes. These results indicate that yeast is able to import, assemble subunits of bovine F1-ATPase in mitochondria and form a functional chimeric yeast/bovine enzyme complex.

The absence of the mitochondrial ATP synthase delta subunit promotes a slow growth phenotype of rho- yeast cells by a lack of assembly of the catalytic sector F1

In the yeast Saccharomyces cerevisiae, inactivation of the gene encoding the delta subunit of the ATP synthase led to a lack of assembly of the catalytic sector. In addition a slow-growth phenotype was observed on fermentable medium. This alteration appears in strains lacking intact mitochondrial DNA and showing a defect in the assembly of the catalytic sector, such as the yeast strain inactivated in the gene encoding the epsilon subunit. In rho mitochondria having an intact F1, the ion movement resulting from the exchange of ADP formed in the organelle and ATP entering the mitochondrial compartment led to a mitochondrial transmembranous potential delta psi that was sensitive to carboxyactractyloside. This ion movement was dramatically decreased in rho mitochondria lacking the delta subunit and thus the F1 sector, whereas a cell devoid of delta subunit and complemented with a plasmid harboring the ATPdelta gene displayed an assembled F1, a normal generation time and a fully restored mitochondrial potential. This result could be linked to the involvement of the membrane potential delta psi which is indispensible for mitochondrial biogenesis.

Structural mapping of the epsilon-subunit of mitochondrial H(+)-ATPase complex (F1)

Phosphorescence and fluorescence energy transfer measurements have been used to locate the epsilon-subunit within the know structural frame of the mitochondrial soluble part of F-type H(+)-ATPase complex (F1). The fluorescence probe 2'-O-(trinitrophenyl)adenosine-5'-triphosphate was bound to the nucleotide binding sites of the enzyme, whereas the probe 7-diethylamino-3'-(4'-maleimidylphenyl)-4-methylcoumarin was attached to the single sulfhydryl residue of isolated oligomycin sensitivity-conferring protein (OSCP), which was then reconstituted with F1. Fluorescence and phosphorescence resonance energy transfer yields from the lone tryptophan residue of F1 present in the epsilon-polypeptide and the fluorescence labels attached to the F1 complex established that tryptophan is separated by 3.7 nm from Cys-118 of OSCP in the reconstituted OSCP-F1 complex, by 4.9 nm from its closest catalytic site and by more than 6.4 nm from the two other catalytic sites, including the lowest affinity ATP site. These separations together with the crystallographic coordinates of the F1 complex (Abrahams, J.P., A. G. W. Leslie, R. Lutter, and J.E. Walker. 1994. Structure at 2.8 A resolution of F1-ATPase from bovine heart mitochondria. Nature. 370:621-628) place the epsilon-subunit in the stem region of the F1 molecule in a unique asymmetrical position relative to the catalytic sites of the enzyme.

Catalytic mechanism of F1-ATPase

The structure of the core catalytic unit of ATP synthase, alpha 3 beta 3 gamma, has been determined by X-ray crystallography, revealing a roughly symmetrical arrangement of alternating alpha and beta subunits around a central cavity in which helical portions of gamma are found. A low-resolution structural model of F0, based on electron spectroscopic imaging, locates subunit a and the two copies of subunit b outside of a subunit c oligomer. The structures of individual subunits epsilon and c (largely) have been solved by NMR spectroscopy, but the oligomeric structure of c is still unknown. The structures of subunits a and delta remain undefined, that of b has not yet been defined but biochemical evidence indicates a credible model. Subunits gamma, epsilon, b, and delta are at the interface between F1 and F0; gamma epsilon complex forms one element of the stalk, interacting with c at the base and alpha and beta at the top. The locations of b and delta are less clear. Elucidation of the structure F0, of the stalk, and of the entire F1F0 remains a challenging goal.

Interactions of the F1-ATPase subunits from Escherichia coli detected by the yeast two-hybrid system

Subunit interactions among the F1-ATPase subunits were studied by the yeast two-hybrid system. Various pairwise combinations of genes encoding alpha, beta, gamma, delta and epsilon subunits of Escherichia coli H+-ATPase fused to the DNA-binding or activation domain of the yeast GAL4 gene were introduced into yeast and expression of a reporter gene encoding beta-galactosidase was detected. Combinations of the alpha and beta subunit genes, and of the epsilon and gamma subunit genes showed high levels of reporter gene expression, while those of alpha and delta, beta and delta, gamma and delta, and delta and epsilon demonstrated weak but significant reporter gene expression. However, combinations of alpha and gamma, beta and gamma, alpha and epsilon, and beta and epsilon did not induce reporter expression. None of the fused genes alone induced reporter gene expression. These results suggested that specific and strong interactions between the alpha and beta, gamma and epsilon, and weak interactions between the alpha and delta, beta and delta, and gamma and delta subunits occurred in yeast cells in the two-hybrid system. Effects of previously identified mutant beta subunits with Leu-40 to Pro. Glu-41 to Lys or Pro-332 to Gln substitutions which caused defects in molecular assembly of F1-ATPase were analyzed with regard to alpha-beta interactions. No interaction of the alpha and beta subunits was observed in this system using the beta subunit with mutation of Pro-332 to Gln. However, for the other two mutations, alpha-beta interactions were observed. This system may be useful for isolating mutants which have defects in interaction of F1-ATPase subunits.

Tryptophan phosphorescence as a structural probe of mitochondrial F1-ATPase epsilon-subunit

We report the detection of tryptophan phosphorescence emission from the sole residue in the epsilon-subunit of the bovine heart mitochondrial F1-ATPase complex. The phosphorescence spectrum, intensity and decay kinetics have been measured over the temperature range 160-273 K. The fine structure in the phosphorescence spectrum at low temperature, with the 0-0 vibrational band centered at 411 nm, reveals the hydrophobic nature of the chromophore's environment. Both the large width of the 0-0 vibrational band and the heterogeneous decay kinetics in fluid solution emphasize the existence of multiple conformations of the epsilon-subunit, structures which are rather stable as they do not interconvert in the millisecond time scale. Further, from the relatively long triplet lifetime at 273 K, it is possible to infer the existence of a tight, rigid core in the structure of the epsilon-subunit. Under subunit-dissociating conditions (6 M urea), the spectrum at 160 K undergoes a slight blue shift but since the phosphorescence lifetime, at all temperatures, is similar or longer than in the absence of dissociant, we conclude that dissociation does not lead to solvent exposure of the tryptophanyl side-chain. This conclusion is supported by the results obtained at 273 K by dissociating F1 in the presence of 0.3 M guanidine hydrochloride. Phosphorescence lifetimes indicate that 6 M urea leads to a more compact structure of the epsilon-subunit, whereas the opposite occurs when Mg-ATP is added to nucleotide-depleted F1. These spectroscopic changes establish unequivocally that the binding of the adenine nucleotide to the enzyme is accompanied by conformational changes involving the epsilon-subunit.

Differential nucleotide binding to catalytic and noncatalytic sites and related conformational changes involving alpha/beta-subunit interactions as monitored by sensitive intrinsic fluorescence in Schizosaccharomyces pombe mitochondrial F1

Mitochondrial F1 from the yeast Schizosaccharomyces pombe exhibits an intrinsic tryptophan fluorescence sensitive to adenine nucleotides and inorganic phosphate [Divita, G., Di Pietro, A., Deléage, G., Roux, B., & Gautheron, D.C. (1991) Biochemistry 30, 3256-3262]. The present results indicate that the intrinsic fluorescence is differentially modified by nucleotide binding to either catalytic or noncatalytic sites. Guanine or hypoxanthine nucleotides, which selectively bind to the catalytic site, produce a hyperbolic saturation monitored by fluorescence quenching at 332 nm, the maximal emission wavelength. On the contrary, adenine nucleotides, which bind to both catalytic and noncatalytic sites, exhibit a biphasic saturation. High-affinity ATP binding produces a marked quenching as opposed to the lower-affinity one. In contrast, ADP exhibits a sigmoidal saturation, with high-affinity binding producing no quenching but responsible for positive cooperativity of binding to the lower-affinity site. The catalytic-site affinity for GDP is almost 20-fold higher at pH 5.0 as compared to pH 9.0, and the high sensitivity of the method allows detection of the 10-fold lower-affinity GMP binding. In contrast, high-affinity binding of ADP, or AMP, is not pH-dependent. The selective catalytic-site saturation induces a F1 conformational change decreasing the Stern-Volmer constant for acrylamide and the tryptophan fraction accessible to iodide. ATP saturation of both catalytic and noncatalytic sites produces an additional reduction of the accessible fraction to acrylamide.

Large-scale purification and characterization of the five subunits of F1-ATPase from pig heart mitochondria

A large-scale purification procedure was developed to isolate the five subunits of F1-ATPase from pig heart mitochondria. The previously described procedure (Williams, N. and Pedersen, P.L. (1986) Methods Enzymol. 126, 484-489) to dissociate the rat liver F1-ATPase by cold treatment followed by warming at 37 degrees C has been adapted for the pig heart enzyme. Removal of endogenous nucleotides from that enzyme before dissociation led to the efficient separation of the alpha and gamma subunits from beta, delta and epsilon subunits. The beta subunit was purified in the hundred-milligram range by anion-exchange chromatography in the absence of any denaturing agent. This subunit was free from any bound nucleotide and almost no ATPase and adenylate kinase-like activities were detected. The delta and epsilon subunits were purified by reversed-phase chromatography (RP-HPLC) in the milligram range. As recently reported (Penin, F., Deléage, G., Gagliardi, D., Roux, B. and Gautheron, D.C. (1990) Biochemistry 29, 9358-9364), these purified subunits kept biophysical features of folded proteins and their ability to reconstitute the tight delta epsilon complex. The alpha and gamma subunits remained poorly soluble and required dissociation by 8 M guanidinium chloride prior to their purification by RP-HPLC. In addition, characterizations of the five subunits by IEF and SDS-polyacrylamide gel electrophoresis are reported, as well as ultraviolet spectra and solubility properties of the beta, delta and epsilon subunits.

Intrinsic tryptophan fluorescence of Schizosaccharomyces pombe mitochondrial F1-ATPase. A powerful probe for phosphate and nucleotide interactions

Mitochondrial F1 from the yeast Schizosaccharomyces pombe, in contrast to the mammalian enzyme, exhibits a characteristic intrinsic tryptophan fluorescence with a maximal excitation at 291 nm and a maximal emission at 332 nm. Low values of Stern-Volmer quenching constants, 4.0 M-1 or 1.8 M-1, respectively, in the presence of either acrylamide or iodide, indicate that tryptophans are mainly buried inside the native enzyme. Upon subunit dissociation and unfolding by 6 M guanidine hydrochloride (Gdn.HCl), the maximal emission is shifted to 354 nm, a value very similar to that obtained with N-acetyltryptophanamide, a solute-tryptophan model compound. The tryptophan content of each isolated subunit has been estimated by fluorescence titration in the presence of Gdn.HCl with free tryptophan as a standard. Two tryptophans and one tryptophan are found respectively in the alpha and epsilon subunits, whereas none is detected in the beta, gamma, and delta subunits. These subunit contents are consistent with the total of seven tryptophans estimated for native F1 with alpha 3 beta 3 gamma 1 delta 1 epsilon 1 stoichiometry. The maximal emission of the isolated epsilon subunit is markedly blue-shifted to 310-312 nm by interaction with the isolated delta subunit, which suggests that the epsilon subunit tryptophan might be a very minor contributor to the native F1 fluorescence measured at 332 nm. This fluorescence is very sensitive to phosphate, which produces a marked blue shift indicative of tryptophans in a more hydrophobic environment. On the other hand, ADP and ATP quench the maximal emission at 332 nm, lower tryptophan accessibility to acrylamide, and reveal tryptophan heterogeneity.