Cyclic AMP-dependent phosphorylation of the precursor to beta subunit of mitochondrial F1-ATPase: a physiological mistake?

By using the purified rat liver protein for reference in electrophoresis and peptide mapping experiments, I have identified the beta subunit of mitochondrial F1-ATPase and its cytoplasmic precursor in two-dimensional gel patterns of proteins from S49 mouse lymphoma cells. The beta subunit precursor is a substrate for cAMP-dependent phosphorylation during its synthesis. Normally, both nonphosphorylated and phosphorylated forms of beta subunit precursor are processed rapidly to the smaller, more acidic forms of mature beta subunit. When processing is inhibited with valinomycin, both nonphosphorylated and phosphorylated forms of beta subunit precursor are stabilized. Nonphosphorylated beta subunit is one of the most stable of cellular proteins, but the phosphorylated form is eliminated within minutes of processing. This suggests that phosphorylated beta subunit is recognized as aberrant and excluded from assembly into the ATPase complex. These results argue that cAMP-dependent phosphorylation of the beta subunit precursor is a physiological mistake that is remedied after mitochondrial import and processing.     

Availability to monoclonal antibodies of antigenic sites of the α and β subunits in active,denatured or membrane-bound mitochondrial F1-ATPase

The binding of five monoclonal antibodies to mitochondrial F₁-ATPase has been studied. Competition experiments between monoclonal antibodies demonstrate that these antibodies recognize four different antigenic sites and provide information on the proximity of these sites. The accessibility of the epitopes has been compared for F₁ integrated in the mitochondrial membrane, for purified β-subunit and for purified F₁ maintained in its active form by the presence of nucleotides or inactivated either by dilution in the absence of ATP or by urea treatment. The three anti-β monoclonal antibodies bound more easily to the β-subunit than to active F₁, and recognized equally active F₁ and F₁ integrated in membrane, indicating that their antigenic sites are partly buried similarly in purified or membrane-bound F₁ and better exposed in the isolated β-subunit. In addition, unfolding F₁ by urea strongly increased the binding of one anti-β monoclonal antibody (14 D₅) indicating that this domain is at least partly shielded inside the β-subunit. One anti-α monoclonal antibody (20 D₆) bound poorly to F₁ integrated in the membrane, while the other (7 B₃) had a higher affinity for F₁ integrated in the membrane than for soluble F₁. Therefore, 20 D₆ recognizes an epitope of the α-subunit buried inside F₁ integrated in the membrane, while 7 B₃ binds to a domain of the α-subunit well exposed at the surface of the inner face of the mitochondrial membrane.     

The inhibitor peptide of the mitochondrial F1 · F0-ATPase interacts with calmodulin and stimulates the calmodulin-dependent Ca2+-ATPase of erythrocytes

The binding of calmodulin to the mitochondrial F₁ · F₀-ATPase has been studied. [^125I]Iodoazidocalmodulin binds to the ε-subunit and to the endogeneous ATPase inhibitor peptide in a Ca²⁺-dependent reaction. The effect of the mitochondrial ATPase inhibitor peptide on the purified Ca²⁺-ATPase of erythrocytes has also been analyzed. The inhibitor peptide stimulates the ATPase when pre-incubated with the enzyme. The activation of the Ca²⁺-ATPase by calmodulin is not influenced by the inhibitor peptide, indicating that the two mechanisms of activation are different. These in vitro effects of the two regulatory proteins may reflect a common origin of the two ATPases considered and/or of the regulatory proteins.     

Removal of the ϵ subunit from Escherichia coli F1-ATPase using monoclonal anti-ϵ antibody affinity chromatography

The usefulness of two monoclonal antibodies, ϵ-1 and ϵ-4, which recognize the ϵ subunit of Escherichia coli F₁-ATPase, for removing that subunit from ATPase was assessed. The ϵ subunit is a tightly bound, but dissociable, inhibitor of the ATPase. ϵ-1 binds ϵ with 10-fold higher affinity than ϵ-4. ϵ-1 recognizes a site on ϵ which is hidden by the quaternary structure of ATPase, while ϵ-4 can recognize ϵ when it is part of ATPase. Each antibody was purified and coupled to Sepharose to generate affinity columns. Solutions of ATPase in a buffer which was designed to reduce the affinity of ϵ for the enzyme were pumped through the columns and the degree of ϵ depletion was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by Western blotting. Neither column retained ATPase significantly. At low ATPase concentrations and low flow rates, the ϵ-1 column was more efficient than the ϵ-4 column, removing in excess of 95% of the ϵ in a single passage compared with 93% removal by the ϵ-4 column. At higher protein concentrations or flow rates, however, the performance of the ϵ-1 column was substantially poorer, while that of the ϵ-4 column was much less affected. Very little ϵ emerged from the ϵ-4 column before most of the measured ϵ-binding capacity was filled. A second passage through the ϵ-4 column reduced residual ϵ to less than 2% of that which was originally present. Pure, active ϵ was eluted from either column by 1 m NH₄OH, pH 11. The relatively poor performance of ϵ-1 is discussed in terms of the low availability of the epitope and the tendency of the ϵ-depleted complex to compete with ϵ-1 for residual ϵ subunit. From consideration of these factors it appears likely that antibodies which recognize exposed epitopes will generally be more effective than antibodies which recognize cryptic epitopes in removing spontaneously dissociable subunits from protein complexes.     

Putative ammo-terminal presequence for β-subunit of plant mitochondrial F1ATPase deduced from the amino-terminal sequence of the mature subunit

The α- and β-subunits of sweet potato mitochondrial F₁ATPase were purified from the F₁ complex by gel filtration and ion-exchange high-performance liquid chromatography. Isoelectric focussing and N-terminal amino acid sequencing indicated that the purified β-subunit contains at least two polypeptides similar to each other. The N-terminal 18 amino acid sequence of the β-subunit showed homology to the amino acid sequence of the tobacco mitochondrial F₁ATPase β-subunit precursor deduced from the nucleotide sequence [(1985) EMBO J. 4, 2159-2165] between residues 56 and 73, suggesting that the N-terminal 55 amino acids of the tobacco precursor constitute the presequence required for mitochondrial targetting.     

Immunological detection of the mitochondrial F1-ATPase alpha subunit in the matrix of rat liver peroxisomes. A protein involved in organelle biogenesis?

Rat liver peroxisomes contain in their matrix the alpha-subunit of the mitochondrial F1-ATPase complex. The identification of this protein in liver peroxisomes has been achieved by immunoelectron microscopy and subcellular fractionation. No beta-subunit of the mitochondrial F1-ATPase complex was detected in the peroxisomal fractions obtained in sucrose gradients or in Nycodenz pelletted peroxisomes. The consensus peroxisomal targeting sequence (Ala-Lys-Leu) is found at the carboxy terminus of the mature alpha-subunit from bovine heart and rat liver mitochondria. Due to the dual subcellular localization of the alpha-subunit and to the structural homologies that exist between this protein and molecular chaperones [(1990) Biol. Chem. 265, 7713-7716] it is suggested that the protein should perform another functional role(s) in both organelles, plus to its characteristic involvement in the regulation of mitochondrial ATPase activity.     

The Role of the Amino-Terminal β-Barrel Domain of the α and β Subunits in the Yeast F1-ATPase

The crystal structure of mitochondrial F₁-ATPase indicatesthat the and subunits fold into a structure defined by threedomains: the top -barrel domain, the middle nucleotide-binding domain,and the C-terminal -helix bundle domain (Abraham et al.1994); Bianchet et al., 1998). The -barrel domains of the and subunits form a crown structure at the top ofF₁, which was suggested to stabilize it (Abraham et al.1994). In this study. the role of the -barrel domain in the and subunits of the yeast Saccharomyces cerevisiae F₁,with regard to its folding and assembly, was investigated. The -barreldomains of yeast F₁ and subunits were expressedindividually and together in Escherichia coli. When expressedseperately, the -barrel domain of the subunit formed a largeaggregate structure, while the domain of the subunit waspredominately a monomer or dimer. However, coexpression of the -barreldomain of subunit domain. Furthermore, the two domains copurified incomplexes with the major portion of the complex found in a small molecularweight form. These results indicate that the -barrel domain of the and subunits interact specifically with each other and thatthese interactions prevent the aggregation of the -barrel domain of the subunit. These results mimic in vivo results and suggest thatthe interactions of the -barrel domains may be critical during thefolding and assembly of F₁.     

L and D presequence peptides derived from the precursor of F1β subunit of the ATP synthase inhibit mitochondrial protein import by interaction with import machinery

We investigated the effect of L and D enantiomers of a 25-residue peptide derived from the N-terminal region of the presequence of Nicotiana plumbaginifolia F₁ subunit of the ATP synthase, pF₁(1, 25), on import into spinach leaf mitochondria. Three in vitro synthesized precursor proteins using different import pathways were used. Import of the precursor proteins of F₁ subunit of the ATP synthase, pre-F₁, and the alternative oxidase, pre-AOX, required addition of external ATP, whereas the chimeric precursor containing the N-terminal 84 amino acids of the cytochrome b ₂ precursor protein linked to dihydrofolate reductase, pre-b ₂(1, 84)-DHFR was not dependent on ATP. Import of pre-F₁, and pre-AOX was inhibited already at 1 M and 3 M concentration of the L and D enantiomers, whereas inhibition of import of pre-b ₂(1, 84)-DHFR, occurred at concentrations >10 M of both enantiomers. Binding efficiency of the precursor proteins was not affected by addition of the L and D enantiomers. There was no correlation between inhibition of import of pre-F₁ and pre-AOX and dissipation of membrane potential measured as a decrease of Rhodamine 123 fluorescence quenching. The inhibitory effect of the L and D presequence enantiomers on import of pre-F₁ and pre-AOX was concluded to occur within the outer membrane translocase machinery beyond the initial precursor receptor interaction. Furthermore, the fact that the D enantiomer had the same effect as the natural peptide showed that interaction of the presequence with the import machinery was not dependent on chiral properties of the presequence.     

Binding of phytopolyphenol piceatannol disrupts β/γ subunit interactions and rate-limiting step of steady-state rotational catalysis in Escherichia coli F1-ATPase

In observations of single molecule behavior under V(max) conditions with minimal load, the F(1) sector of the ATP synthase (F-ATPase) rotates through continuous cycles of catalytic dwells (～0.2 ms) and 120° rotation steps (～0.6 ms). We previously established that the rate-limiting transition step occurs during the catalytic dwell at the initiation of the 120° rotation. Here, we use the phytopolyphenol, piceatannol, which binds to a pocket formed by contributions from α and β stator subunits and the carboxyl-terminal region of the rotor γ subunit. Piceatannol did not interfere with the movement through the 120° rotation step, but caused increased duration of the catalytic dwell. The duration time of the intrinsic inhibited state of F(1) also became significantly longer with piceatannol. All of the beads rotated at a lower rate in the presence of saturating piceatannol, indicating that the inhibitor stays bound throughout the rotational catalytic cycle. The Arrhenius plot of the temperature dependence of the reciprocal of the duration of the catalytic dwell (catalytic rate) indicated significantly increased activation energy of the rate-limiting step to trigger the 120° rotation. The activation energy was further increased by combination of piceatannol and substitution of γ subunit Met(23) with Lys, indicating that the inhibitor and the β/γ interface mutation affect the same transition step, even though they perturb physically separated rotor-stator interactions.     

Thermodynamic Analyses of Nucleotide Binding to an Isolated Monomeric β Subunit and the α3β3γ Subcomplex of F1-ATPase

Rotation of the γ subunit of the F₁-ATPase plays an essential role in energy transduction by F₁-ATPase. Hydrolysis of an ATP molecule induces a 120° step rotation that consists of an 80° substep and 40° substep. ATP binding together with ADP release causes the first 80° step rotation. Thus, nucleotide binding is very important for rotation and energy transduction by F₁-ATPase. In this study, we introduced a βY341W mutation as an optical probe for nucleotide binding to catalytic sites, and a βE190Q mutation that suppresses the hydrolysis of nucleoside triphosphate (NTP). Using a mutant monomeric βY341W subunit and a mutant α₃β₃γ subcomplex containing the βY341W mutation with or without an additional βE190Q mutation, we examined the binding of various NTPs (i.e., ATP, GTP, and ITP) and nucleoside diphosphates (NDPs, i.e., ADP, GDP, and IDP). The affinity (1/K_d) of the nucleotides for the isolated β subunit and third catalytic site in the subcomplex was in the order ATP/ADP > GTP/GDP > ITP/IDP. We performed van’t Hoff analyses to obtain the thermodynamic parameters of nucleotide binding. For the isolated β subunit, NDPs and NTPs with the same base moiety exhibited similar ΔH⁰ and ΔG⁰ values at 25°C. The binding of nucleotides with different bases to the isolated β subunit resulted in different entropy changes. Interestingly, NDP binding to the α₃β(Y341W)₃γ subcomplex had similar K_d and ΔG⁰ values as binding to the isolated β(Y341W) subunit, but the contributions of the enthalpy term and the entropy term were very different. We discuss these results in terms of the change in the tightness of the subunit packing, which reduces the excluded volume between subunits and increases water entropy.     

Tissue-specific differences of the mitochondrial protein import machinery: in vitro import,processing and degradation of the pre-F1β subunit of the ATP synthase in spinach leaf and root mitochondria

In this study we report the first comparison of the mitochondrial protein import and processing events in two different tissues from the same organism. Both spinach leaf and root mitochondria were able to import and process the in vitro transcribed and translated Neurospora crassa F₁ subunit of ATP synthase to the mature size product. Temperature optimum for protein import, 20 °C, was considerably lower than that found in other systems. In spinach leaf mitochondria, the processing peptidase has been shown to constitute an integral part of the bc₁ complex of the respiratory chain. In accordance with these results, the majority of the processing activity in root mitochondria was also localized in the membrane. However, although the same amount of the processing peptidase was present per mg of membrane protein in both leaf and root mitochondria, as determined immunologically, the specific processing activity was several-fold higher in roots. Furthermore, in contrast to the processing enzyme in leaf, a portion of the processing activity could be disassociated from the root membrane with relatively weak salt treatment. The processing event in both the leaf and root membranes was always accompanied by a degradation of the F₁ precursor. The degradation activity was found to be several-fold higher in roots than in leaves and was also partially dissociated from the membrane after salt treatment. Both the processing and degradation activities were inhibited by orthophenanthroline, a known metalloprotease inhibitor. These results show tissue-specific differencies of the processing event catalyzed by the bc₁ complex and indicate the presence of two populations of the processing peptidase in root mitochondria.     

1H, 13C, and 15N resonance assignments of subunit F of the A1AO ATP synthase from Methanosarcina mazei Gö1

Energy coupling between the A₁ ATPase of archaea type A₁A_O ATP synthase and its integral membrane sub-complex A_O occurs via the stalk part, formed by the subunits C, D and F. To provide a molecular basis of the energy coupling, we performed NMR studies. Here, we report the assignment of the subunit F. Shovanlal Gayen and Subramanian Vivekanandan contributed equally to this work.     

Novel approaches towards characterization of the high-affinity nucleotide binding sites on mitochondrial F1-ATPase by the fluorescence probes 3′-O-(1-naphthoyl)adenosine di- and triphosphate

The fluorescence properties of 3′-O(1-naphthoyl)adenosine di- and triphosphates (termed N-ADP and N-ATP, respectively) were investigated in detail. Of special importance for the use of these analogues as environmental probes is their high quantum yield (0.58 in water) and the polarity dependence of shape and wavelength position of the emission spectrum. Upon binding of N-ADP and N-ATP to mitochondrial F₁-ATPase, the fluorescence intensity is markedly decreased, due to polarity changes and ‘ground-state’ quenching. Using this signal for equilibrium binding studies, three (at least a priori) equivalent nucleotide-binding sites were detected on the enzyme. The perspective intrinsic dissociation constants are as follows: N-ADP/Mg²⁺ 120 nM; N-ATP/Mg²⁺ 160 nM; N-ADP/EDTA 560 nM; N-ATP/EDTA 3500 nM. For bound ligand the environment was found to be rather unpolar; the rotational mobility of the fluorophore is restricted, its accessibility for iodide anions (as a quencher) is hindered. These facts show a location of the binding sites quite deeply embedded in the protein. The conformation of the binding domains is strongly dependent on the absence or presence of Mg²⁺, as can be seen from the relative efficiencies of the singlet-singlet energy transfer from tyrosine residues in the protein to bound naphthoyl moieties. Investigation of the binding kinetics revealed this process as biphasic (in presence of Mg²⁺). After the first fast step (k_on > 1 · 10⁶ M⁻¹ · s⁻¹), in which the analogue is bound to the enzyme, a slow local conformational rearrangement occurs.     

Modification of domains of α and β subunits of F1-ATPase from the thermophylic bacterium PS3, in their isolated and associated forms,by 3′-O-(4-benzoyl)benzoyl adenosine 5′-triphosphate (BzATP)

Photoaffinity labeling by 3-O-(4-benzoyl)benzoyl adenosine 5-triphosphate (BzATP) of the adenine nucleotide binding site(s) on isolated and complexed and subunits of F₁-ATPase from the thermophilic bacterium PS3 (TF₁) is described. BzATP binds to both isolated and subunits, to complexed subunit but not to complexed subunit. Amino acid sequence determination of radiolabeled peptides obtained by proteolytic digestion of [-³²P]BzATP-labeled subunit indicates that residues on both the amino-terminal (residues A41-E67) and carboxy-terminal (residues Q422–Q476) were modified by BzATP. One of the residues in the carboxy-terminal modified by BzATP is most probably Q422. Although the binding stoichiometry of 1 mol of BzATP incorporated by either isolated or complexed subunit was maintained, the spatial conformation of the polypeptide determines which amino acid residue(s) is more accessible to the reactive radical. CNBr derived fragments G10-M64, E75-M233, and D390-M469 were labeled with the isolated subunit. With complexed subunit the label was found only in CNBr fragments: E75-M233 and G339-M389. The locations where the covalently bound BzATP was found, in the soluble and assembled subunits, indicate that different conformational states exist. In the isolated form of the and subunits the amino- and carboxy-termini can fold and reach the central domain of the polypeptide, the domain containing the adenine nucleotide binding site. When combines with to form the ₃₃ core complex the new conformation of the subunits is such that covalent labeling by BzATP of and of the amino terminal of subunit is excluded.     

Nucleotide sequence of cDNA clones encoding the β subunit of mitochondrial ATP synthase from the green alga Chlamydomonas reinhardtii: The precursor protein encoded by the cDNA contains both an N-terminal presequence and a C-terminal extension

cDNA and genomic clones encoding the subunit of mitochondrial ATP synthase from Chlamydomonas reinhardtii have been isolated using heterologous DNA probes from the photosynthetic bacterium Rhodospirillum rubrum. The protein encoded by the cDNA is 79–83% identical to corresponding proteins from higher-plant and mammalian mitochondria, and 75% identical to the R. rubrum protein. It contains both an N-terminal presequence and a unique C-terminal extension. The presequence, which is the first mitochondrial presequence determined in C. reinhardtii, is similar in structure to mitochondrial presequences from other organisms. As chloroplast presequences from C. reinhardtii also share features with mitochondrial presequences from other organisms (L.-G. Franzén et al., FEBS Lett 260 (1990) 165–168), this raises interesting questions about protein targeting to chloroplasts and mitochondria in C. reinhardtii. The possibility that the C-terminal extension is involved in targeting the protein to the mitochondrion is discussed. Southern blot analysis indicates that the protein is encoded by a single-copy gene.     

Apoplastic expression of the xylanase and β(1–3, 1–4) glucanase domains of the xyn D gene from Ruminococcus flavefaciens leads to functional polypeptides in transgenic tobacco plants

We are investigating the possibilities of transgenic plants as bioreactors for the production of industrial enzymes using cell wall-hydrolysing enzymes as first examples. Within the frame work of this work two distinct domains of the xynD gene from Ruminococcus flavefaciens encoding a xylanase (XYLD-A) and a (1–3, 1–4)glucanase (XYLD-C) were separately cloned into a plant expression vector which would target the proteins into the apoplast. Transgenic tobacco plants were obtained expressing xylan-hydrolysing as well as lichenan-hydrolysing activities. Despite similar steady-state levels of the respective mRNAs xylan hydrolysis rates were between 40 and 170 mol min–1 m^–2 leaf area depending on the transgenic plant while (1–3, 1–4)glucan degradation was much more effective ranging between 200 and 2000 mol min^–1 m^–2. The high activity levels of the XYLD-C expressing plants were reflected on the protein level. XYLD-C accumulated in the intercellular space and was one of the most prominent bands in protein gels. Despite their apoplastic location as confirmed by activity measurements using intercellular fluids the transgenic plants had not undergone any phenotypic alteration.