ATP Hydrolysis Catalyzed by a β Subunit Preparation Purified from the Chloroplast Energy Transducing Complex CF1.CF0

Catalysis by isolated beta-subunits of the ATP Synthase/ATPase from Thermophilic bacillus PS3. Hydrolysis of pyrophosphate

Although the capacity of isolated beta-subunits of the ATP synthase/ATPase to perform catalysis has been extensively studied, the results have not conclusively shown that the subunits are catalytically active. Since soluble F(1) of mitochondrial H(+)-ATPase can bind inorganic pyrophosphate (PP(i)) and synthesize PP(i) from medium phosphate, we examined if purified His-tagged beta-subunits from Thermophilic bacillus PS3 can hydrolyze PP(i). The difference spectra in the near UV CD of beta-subunits with and without PP(i) show that PP(i) binds to the subunits. Other studies show that beta-subunits hydrolyze [(32)P] PP(i) through a Mg(2+)-dependent process with an optimal pH of 8.3. Free Mg(2+) is required for maximal hydrolytic rates. The Km for PP(i) is 75 microM and the Vmax is 800 pmol/min/mg. ATP is a weak inhibitor of the reaction, it diminishes the Vmax and increases the Km for PP(i). Thus, isolated beta-subunits are catalytically competent with PP(i) as substrate; apparently, the assembly of beta-subunits into the ATPase complex changes substrate specificity, and leads to an increase in catalytic rates.

Identification of subunits required for the catalytic activity of the F1-ATPase

F1 (alpha beta) complexes containing equimolar ratios of the alpha and beta subunits have been shown to function as active ATPases, whereas individually isolated alpha and beta subunits show no real ATPase activity. These results indicate that the single-copy subunits are not required for F1-ATPase activity. The minimal F1 (alpha beta)-core complexes exhibit, however, lower rates and some different properties from those of their parent whole F1 or alpha 3 beta 3 gamma complexes. It is therefore concluded that for obtaining a full spectrum of the characteristic functional properties of an F1-ATPase the presence of the F1-gamma subunit is also required. The implications of these findings on the subunit location of both catalytic and noncatalytic nucleotide binding sites is discussed.

Isolation and characterisation of a functional alpha beta heterodimer from the ATP synthase of Rhodospirillum rubrum

An alpha beta heterodimer of the F1-ATPase of Rhodospirillum rubrum was isolated by extraction of chromatophores with LiCl. Each alpha beta heterodimer contains one tightly bound ADP, which is released upon removal of medium Mg2+. The dimer can be reversibly dissociated by removal of Mg(2+)-ions. The alpha beta heterodimer restores both ATP-synthetic and -hydrolytic activities to LiCl-treated chromatophores, saturation being achieved at approximately 2 mmol alpha beta.mol BChl-1. The heterodimer itself hydrolyses Mg-ATP with an activity distinct from RF1, being unaffected by azide or sulphite ions. The Vmax and Km (ATP) for this Mg(2+)-dependent activity were 110 +/- 10 nmol.min-1.mg protein-1 and 100 +/- 30 microM, respectively. The Km did not differ significantly from that of RF1.

Catalytic properties of β subunit isolated from chloroplast coupling factor 1

The chloroplast coupling factor (CF1) was dissociated into subunits by the freezing-thawing procedure in the presence of 0.5 M NaBr and the β subunit was purified by ion-exchange chromatography on a DEAE-cellulose column. The β subunit did not catalyze ATP hydrolysis either in the presence or in the absence of reagents known to activate Mg(2+)-dependent ATPase activity of CF1. However, it manifested appreciable adenylate kinase-like and ATP-ADP γ-phosphate exchange activities. The adenylate kinase-like activity only slightly depended on Mg(2+) ions. Ethanol, and especially diadenosine pentaphosphate, inhibited the reaction effectively. In contrast, the ATP-ADP exchange activity was Mg(2+)-dependent. Ethanol and diadenosine pentaphosphate were poor inhibitors. Sulfite, the CF1-ATPase activator, and quercetin, its inhibitor, had a minor effect on catalytic activity of the β subunit.

Transcription and translation of the chloroplast atpB-gene and assembly of ATP synthase subunit beta

In vitro transcription and subsequent translation of the cloned Chlamydomonas chloroplast atpB gene was used to study assembly of ATP synthase. Translation in the presence of thylakoids resulted in association of the beta subunit with the membrane. The in vitro synthesized polypeptide bound to the membrane copurified with CF1 on sucrose gradients. This provides more evidence for the self-assembly of CF1.