Photosynthetic ATPases: purification, properties, subunit isolation and function

Identification of a novel isoform of the chloroplast-coupling factor alpha-subunit

Studies were conducted to identify a 64-kD thylakoid membrane protein of unknown function. The protein was extracted from chloroplast thylakoids under low ionic strength conditions and purified to homogeneity by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Four peptides generated from the proteolytic cleavage of the wheat 64-kD protein were sequenced and found to be identical to internal sequences of the chloroplast-coupling factor (CF1) alpha-subunit. Antibodies for the 64-kD protein also recognized the alpha-subunit of CF1. Both the 64-kD protein and the 61-kD CF1 alpha-subunit were present in the monocots barley (Hordeum vulgare), maize (Zea mays), oat (Avena sativa), and wheat (Triticum aestivum); but the dicots pea (Pisum sativum), soybean (Glycine max Merr.), and spinach (Spinacia oleracea) contained only a single polypeptide corresponding to the CF1 alpha-subunit. The 64-kD protein accumulated in response to high irradiance (1000 mumol photons m-2 s-1) and declined in response to low irradiance (80 mumol photons m-2 s-1) treatments. Thus, the 64-kD protein was identified as an irradiance-dependent isoform of the CF1 alpha-subunit found only in monocots. Analysis of purified CF1 complexes showed that the 64-kD protein represented up to 15% of the total CF1 alpha-subunit.

Role of the Chlamydomonas reinhardtii coupling factor 1 gamma-subunit cysteine bridge in the regulation of ATP synthase

The gamma-subunit of coupling factor 1 (CF1) contains a cysteine bridge that is thought to be involved in the redox control of enzymatic activity. In order to test the regulatory significance of this disulfide bond, genetic transformation experiments with Chlamydomonas reinhardtii were performed. C. reinhardtii strain atpC1 (nit1-305, cw 15, mt-), which is null for the gamma-subunit, was transformed and complemented with gamma-subunit constructs containing amino acid substitutions localized to the cysteine bridge between Cys198 and Cys204. Successful complementation was confirmed by phenotypic selection, Northern blot analysis, reverse transcription polymerase chain reaction, and cDNA sequencing. CF1 ATPase activities of the soluble enzymes were measured in the presence and absence of dithiothreitol (DTT). Mutant CF1 enzymes showed no effect of DTT although increased activity was observed for the wild-type enzyme. In vitro, phenazine methosulfate-dependent photophosphorylation assays revealed that wild-type CF1 exhibits a 2-fold stimulation in the presence of 25 mM DTT, whereas each of the mutant enzymes has activities that are DTT-independent. Growth measurements indicated that despite the absence of a regulatory disulfide/dithiol, the mutant strains grew with the same kinetics as wild type. This study provides evidence to illustrate the involvement of the gamma-subunit in the redox regulation of ATP synthesis in vivo. This work is also the first demonstration in C. reinhardtii of stable nuclear transformation using mutated genes to complement a known defect.

Complementation of a Chlamydomonas reinhardtii mutant defective in the nuclear gene encoding the chloroplast coupling factor 1 (CF1) gamma-subunit (atpC)

Chlamydomonas reinhardtii strain atpC1 is a mutant defective in the nuclear gene that encodes the CF1 ATP synthase gamma-subunit polypeptide. Photoautotrophic growth was restored to atpC1 after it was transformed with wild-type DNA. Transformed strains were acetate-independent and arsenate-sensitive, similar in phenotype to the progenitor wild-type strain from which atpC1 was generated. Three transformed strains were examined in detail. Southern blot analyses demonstrated that the transformants were complements and not revertants. The transforming DNA integrated into the nuclear genome in a nonhomologous manner and at a low copy number. Northern blot analyses showed that the gamma-subunit mRNA in the complemented strains was expressed at the same relative level as that of wild-type. Western blots of total protein showed that whereas atpC1 was unable to synthesize any CF1 gamma-subunit, all three complemented strains could. Furthermore, the Western blot analyses demonstrated that the mutation in atpC1 had a pleiotropic effect on the accumulation of the CF1 beta-subunit which was relieved upon complementation. Cell extracts from atpC1 did not have any CF1-dependent catalytic activity, whereas extracts from all of the complemented strains and the wild-type strain had identical activities.

Relationship of the membrane ATPase from Halobacterium saccharovorum to vacuolar ATPases

Polyclonal antiserum against subunit A (67 kDa) of the vacuolar ATPase from Neurospora crassa reacted with subunit I (87 kDa) from a membrane ATPase of the extremely halophilic archaebacterium Halobacterium saccharovorum. The halobacterial ATPase was inhibited by nitrate and N-ethylmaleimide; the extent of the latter inhibition was diminished in the presence of adenosine di- or triphosphates. 4-Chloro-7-nitrobenzofurazan inhibited the halobacterial ATPase also in a nucleotide-protectable manner; the bulk of inhibitor was associated with subunit II (60 kDa). The data suggested that this halobacterial ATPase may have conserved structural features from both the vacuolar and the F-type ATPases.

The dithiothreitol-stimulated dissociation of the chloroplast coupling factor 1 epsilon-subunit is reversible

The chloroplast coupling factor 1 complex (CF1) contains an epsilon-subunit which inhibits the CF1 ATPase activity. Chloroform treatment of Chlamydomonas reinhardtii thylakoid membranes solubilizes only forms of the enzyme which apparently lack the delta-subunit. Four interrelated observations are described in this paper. (1) The dithiothreitol- (DTT) induced ATPase activation of CF1(-delta) and the DTT-induced formation of a physically resolvable CF1(-delta,epsilon) from the CF1(-delta) precursor are compared. The similar time-courses of these two phenomena suggest that the dissociation of the epsilon-subunit is an obligatory process in the DTT-induced ATPase activation of soluble CF1. (2) The reversible dissociation of the epsilon-subunit of the CF1 is demonstrated by the exchange of subunits between distinguishable oligomers. 35S-labelled chloroplast coupling factor 1 lacking the delta and epsilon subunits [CF1(-delta,epsilon)] was added to a solution of non-radioactive coupling factor 1 lacking only the delta subunit [CF1(-delta)]. After separation of the two enzyme forms, via high resolution anion-exchange chromatography, radioactivity was detected in the chromatographic fractions containing CF1(-delta). (3) epsilon-deficient CF1 can be resolved from DTT pretreated epsilon-containing CF1 for several days after the removal of DTT. On the other hand, brief incubation of the DTT pretreated epsilon-containing CF1 with low concentrations of o-iodosobenzoate results in chromatographs containing only the peak of epsilon-containing CF1. A simple explanation for this phenomenon is that reduction of CF1 with DTT increases the apparent dissociation constant for the epsilon-subunit to an estimated 3.5 x 10(-8) M (+/- 1.0 x 10(-8) M) from a value of less than or equal to 5 x 10(-11) M for the oxidized enzyme. (4) ATPase activity data show that oxidation of the epsilon-deficient enzyme does not completely inhibit its manifest activity, but oxidation of DTT pre-treated CF1 which contains the epsilon-subunit completely inhibits manifest activity. A simple model is proposed for the influence of the oxidation state of the soluble enzyme on the distribution of ATPase-inactive and ATPase-active subunit configurations.

The F1-type ATPase in anaerobic Lactobacillus casei

An ATPase from anaerobic Lactobacillus casei has been isolated and 100-times purified. The 400 kDa enzyme molecule was found to have a hexagonal structure 10 nm in diameter composed of at least six protein masses. SDS-electrophoresis reveals four or, under certain conditions, five types of subunit, of apparent molecular masses 57 (alpha), 55 (beta), 40 (gamma), 22 (delta) and 14 (epsilon) kDa with stoichiometry of 3 alpha, 3 beta, gamma, delta, epsilon. The following features resembling F1-ATPases from other sources were found to be inherent in the solubilized L. casei ATPase. (i) Detachment from the membrane desensitizes ATPase to low DCCD concentrations and sensitizes it to water-soluble carbodiimide. (ii) Soluble ATPase is inhibited by Nbf chloride and azide, is resistant to SH-modifiers and is activated by sulfite and octyl glucoside, the activating effect being much stronger than in the case of the membrane-bound ATPase. Substrate specificity of the enzyme is also similar to that of other factors F1. Divalent cations strongly activate the soluble enzyme when added at a concentration equal to that of ATP. An excess of Mn2+, Mg2+ or Co2+ inhibits ATPase activity of F1, whereas that of Ca2+ induces its further activation. No other F1-like ATPases are found in L. casei. It is concluded that this anaerobic bacterium possesses a typical F1-ATPase similar to those in mitochondria, chloroplasts, aerobic and photosynthetic eubacteria.

Cotranscription of the wild-type chloroplast atpE gene encoding the CF1/CF0 epsilon subunit with the 3' half of the rps7 gene in Chlamydomonas reinhardtii and characterization of frameshift mutations in atpE

We have characterized two independently isolated point mutants in Chlamydomonas reinhardtii, ac-u-a-1-15 and FUD 17, mapping to the chloroplast ac-u-a locus which corresponds to the atpE gene. Both mutants have a single A:T base pair deletion in a sequence of 6 A:T base pairs at nucleotide positions 102 to 107. This causes a frameshift, altering the coding sequence for the next 8 amino acids and creating a termination codon at amino acid position 44, 98 amino acids from the C-terminus of the protein. Assembly of the ATP synthase is impaired in the mutants; less than 5% of the wild-type level of alpha and beta subunits and no gamma or epsilon subunits are associated with thylakoid membranes of the mutants. The genes encoding the beta and epsilon subunits of the chloroplast ATP synthase from C. reinhardtii are not cotranscribed, in contrast to all other photosynthetic organisms examined to date. Four transcripts, of approximately 1.7, 2.9, 3.3 and 7.0 x 10(3) nucleotides (nt), are found for the atpE gene. S1 nuclease mapping of the 1.7 x 10(3) nt transcript shows that the atpE gene message is preceded by a leader of about 1250 nt. DNA sequence analysis of this region revealed a 159 bp open reading frame corresponding to the 3' half of the rps7 gene, encoding the S7 protein of the small subunit of the chloroplast ribosome. Only the 5' portion of this gene is located in the opposite unique sequence region of the C. reinhardtii chloroplast genome where the rps7 gene was previously mapped by heterologous hybridization.

The primary structure of the gamma-subunit of the ATPase from Synechocystis 6803

The nucleotide sequence of the gene coding for the F0F1-ATPase gamma-subunit (atpC) from the transformable cyanobacterium Synchocystis 6083 has been determined. The deduced translation product consists of 314 amino acid residues and is highly homologous (72% identical residues) to the sequences of other cyanobacterial gamma-subunits. The Synechocystis 6803 sequence is also homologous to the chloroplast gamma-sequence. Like in the other cyanobacterial subunits, only the first of the 3 cysteine residues, which are involved in energy-linked functions of the gamma-subunit in spinach chloroplasts, is conserved in Synechocystis 6803.

The chloroplast genes encoding subunits of the H(+)-ATP synthase

Three CF1 and three CF0 subunits of the chloroplast H(+)-ATP synthase are encoded on the chloroplast genome. The chloroplast atp genes are organized as two operons in plants but not in the green alga, Chlamydomonas reinhardtii. The atpBE or β operon shows a relatively simple organisation and transcription pattern, while the atpIHFA or α operon is transcribed into a large variety of mRNAs. The atp genes are related to those of cyanobacteria and, more distantly, to those of non-photosynthetic bacteria such as E. coli, suggesting a common origin of most F1F0 ATP synthase subunits. Both the chloroplast and cyanobacterial ATP synthases have four F0 subunits, not three as in the E. coli complex. The proton pore of the CF0 is proposed to be formed by the interaction of subunits III and IV.

Structure, organization and expression of cyanobacterial ATP synthase genes

The genes encoding the nine polypeptides of the ATP synthase from Synechococcus sp. PCC 6301, a unicellular cyanobacterium, and Anabaena sp. PCC 7120, a filamentous cyanobacterium, have recently been isolated and their sequences determined. These represent the first such sequences available from procaryotic organisms that perform oxygenic photosynthesis. Similar to the organization in chloroplasts, the ATP synthase genes of both cyanobacteria are arranged in two gene clusters which are not closely linked in the chromosome. Three of the genes located in one cluster in cyanobacteria, however, are localized in the nuclear rather than the chloroplast genomes of plants. The cyanobacterial ATP synthase genes are ordered in the same manner as those in the single gene cluster of Escherichia coli. Cyanobacteria contain an additional gene denoted atpG which appears to be a duplicated and diverged from of the atpF gene. The larger cyanobacterial cluster, atp 1, is comprised of eight ATP synthase subunit genes arranged in the order atpI-atpH-atpG-atpF-atpD-atpA-atpC. An overlap between the atpF and atpD gene coding regions observed in Anabaena sp. PCC 7120 is absent in both Synechococcus sp. PCC 6301 and E. coli. The second cluster of genes, atp 2, contains the remaining two ATP synthase genes in the order atpB-atpE. Unlike the situation in many chloroplast genomes, this gene pair does not overlap in either cyanobacterial species. In Anabaena sp. PCC 7120, atp 1 and atp 2 each comprise an operon and the transcription initiation sites for each gene cluster have been identified. The cyanobacterial ATP synthase subunits are much more closely related in sequence to the equivalent polypeptides from chloroplasts than they are to those of E. coli. The similarity in chloroplast and cyanobacterial ATP synthase subunit sequences and gene oreganization argue strongly for an endosymbiotic origin for plant chloroplasts.

Genes encoding the alpha, gamma, delta, and four F0 subunits of ATP synthase constitute an operon in the cyanobacterium Anabaena sp. strain PCC 7120

A cluster of genes encoding subunits of ATP synthase of Anabaena sp. strain PCC 7120 was cloned, and the nucleotide sequences of the genes were determined. This cluster, denoted atp1, consists of four F0 genes and three F1 genes encoding the subunits a (atpI), c (atpH), b' (atpG), b (atpF), delta (atpD), alpha (aptA), and gamma (atpC) in that order. Closely linked upstream of the ATP synthase subunit genes is an open reading frame denoted gene 1, which is equivalent to the uncI gene of Escherichia coli. The atp1 gene cluster is at least 10 kilobase pairs distant in the genome from apt2, a cluster of genes encoding the beta (atpB) and epsilon (atpE) subunits of the ATP synthase. This two-clustered ATP synthase gene arrangement is intermediate between those found in chloroplasts and E. coli. A unique feature of the Anabaena atp1 cluster is overlap between the coding regions for atpF and atpD. The atp1 cluster is transcribed as a single 7-kilobase polycistronic mRNA that initiates 140 base pairs upstream of gene 1. The deduced translation products for the Anabaena sp. strain PCC 7120 subunit genes are more similar to chloroplast ATP synthase subunits than to those of E. coli.

The activation and inactivation of the Dunaliella salina chloroplast coupling factor 1 (CF1) in vivo and in situ

Preillumination of intact cells of the eukaryotic, halotolerant, cell-wall-less green alga Dunaliella salina induces a dark ATPase activity the magnitude of which is about 3-5-fold higher than the ATPase activity observed in dark-adapted cells. The light-induced activity arises from the activation and stabilization in vivo of chloroplast coupling factor 1 (CF1). This activity, approximately 150-300 mumol ATP hydrolyzed/mg Chl per h, rapidly decays (with a half-time of about 6 min at room temperature) in intact cells but only slowly decays (with a half-time of about 45 min at room temperature) if the cells are lysed by osmotic shock immediately after illumination. The activated form of the ATPase in lysed cells is inhibited if the membranes are treated with ferri- but not ferrocyanide, suggesting that the stabilization of the activated form of CF1 is due to the reduction of the enzyme in vivo in the light.

Catalytic and noncatalytic nucleotide binding sites of the Escherichia coli F1 ATPase. Amino acid sequences of beta-subunit tryptic peptides labeled with 2-azido-ATP

Under appropriate conditions tight, noncovalent binding of 2-azido-adenine nucleotides to either catalytic or noncatalytic binding sites on the E. coli F1-ATPase occurs. After removal of unbound ligands, UV-irradiation results primarily in the covalent incorporation of nucleotide moieties into the beta-subunit in both catalytic and noncatalytic site labeling experiments. Minor labeling of the alpha-subunit was also observed. After trypsin digestion and purification of the labeled peptides, microsequencing studies identified two adjacent beta-subunit tryptic peptides labeled by 2-azido-ADP or -ATP. These beta-subunit peptides were labeled on tyrosine-331 (catalytic sites) and tyrosine-354 (noncatalytic sites) in homology with the labeling patterns of the mitochondrial and chloroplast enzymes.

Effects of nitrogen starvation on the function and organization of the photosynthetic membranes in Cryptomonas maculata (Cryptophyceae)

Nitrogen deficiency affects both photosystems and the antennae pigment systems in the photosynthetic apparatus of the marine alga, Cryptomonas maculata. Under increasing energy fluence rates, O2 evolution in nitrogen-deficient (-N) cell suspensions never reached a positive value; in control cultures (+N), O2 evolution increased and was saturated at about 6.4 W·m(-2) with about 100 μmol O2·mg chlorophyll(-1)·h(-1). During fluorescence-induction experiments at room temperature, Fo and Fmax were significantly increased in-N cells whereas the Fvar/Fmax ratio decreased from 0.6 to 0.1. These observations can be correlated with a significantly decreased population of 12.5-nm-size particles in the exoplasmic-fracture (EF) faces of freeze-cleaved thylakoid membranes in-N cells (Rhiel et al., 1985, Protoplasma 129, 62-73). The EF particles are suggested to represent photosystem II associated with chlorophyll a/c-protein complexes (LHCP). The banding pattern of isolated and Triton X-100-solubilized thylakoid membranes of both +N and-N cells in sucrose gradients showed that the LHCP is still present in-N cells. The same applies to sodium dodecyl sulfate-polyacrylamide gel electrophoresis of these membrane fractions. The reduced number of the 12.5-nm particles in the EF faces of-N cells may be a result of decoupling of the LHCP constituents of the photosystem-II complex rather than their degradation. This is supported by high values for the initial fluorescence Fo in fluorescence-induction experiments and, in part, is indicated by the shift of the maximal fluorescence emission from 693 nm in +N to 684 nm in-N cells. The lack of the CP1 band in the gels of sodium dodecyl sulfate-solubilized thylakoid membranes from-N cells after electrophoresis demonstrates that photosystem I is also severely affected.

N-Ethylmaleimide inhibition of the catalytic activities of the Dunaliella salina coupling factor 1 (CF1) and the restoration of the inhibition of the CF1 ATPase activity by N-ethylmaleimide

The sensitivity of the catalytic activities of the D. salina chloroplast coupling factor 1 (CF1) to chemical modification by N-ethylmaleimide has been investigated. When D. salina thylakoid membranes are treated with N-ethylmaleimide, both photophosphorylation and the inducible CF1 ATPase activity are partially (approx. 60%) inhibited. The inhibition of both activities does not require the presence of a proton-motive force, and the inhibition of photophosphorylation is directly related to the N-ethylmaleimide-covalent modification of CF1 as shown by the time-course for the inhibition and the maximal extent of inhibition. Treatment of the purified, latent, D. salina CF1 with low concentrations of N-ethylmaleimide also results in the partial (approx. 60%) inhibition of the inducible ATPase activity (I50 approximately 50 microM). The inhibition does not require the presence of the chemical modifier during the activation of the enzyme. N-ethylmaleimide-induced inhibition of the ATPase activity of either membrane-bound or solubilized CF1 is partially reversed by either prolonged incubation at low concentrations of N-ethylmaleimide or short incubation times at high concentrations of N-ethylmaleimide. The results are interpreted as indicating multiple binding sites on the D. salina CF1 that have different rates of reactivity with N-ethylmaleimide. Those sites (or site) that react rapidly with N-ethylmaleimide cause(s) an inhibition of both ATP synthase and ATPase activities, whereas those sites (or site) that react more slowly partially restore(s) the original ATPase activity. The effects of N-ethylmaleimide on the catalytic activity of D. salina CF1 are probably mediated by N-ethylmaleimide-induced conformational changes of the enzyme.