Regulation of proton flux through the H+-ATPase of chloroplasts

Catalysis and energy coupling of H(+)-ATPase (ATP synthase): molecular biological approaches

The molecular biological approach has provided important information for understanding the F0F1 H(+)-ATPase. This article focuses on our recent results on the catalytic site in the beta subunit, and the roles of alpha/beta subunit interaction and amino/carboxyl terminal interaction of the gamma subunit in energy coupling. Extensive mutagenesis of the beta subunit revealed that beta Lys-155, beta Thr-156, beta Glu-181 and beta Arg-182 are essential catalytic residues. beta Glu-185 is not absolutely essential, but a carboxyl residue may be necessary at this position. A pseudo-revertant analysis positioned beta Gly-172, beta Ser-174, beta Glu-192 and beta Val-198 in the proximity of beta Gly-149. The finding of the roles of beta Gly-149, beta Lys-155, and beta Thr-156 emphasized the importance of the glycine-rich sequence (Gly-X-X-X-X-Gly-Lys-Thr/Ser, E. coli beta residues between beta Gly-149 and beta Thr-156) conserved in many nucleotide binding proteins. The A subunits of vacuolar type ATPases may have a similar catalytic mechanism because they have conserved glycine-rich and Gly-Glu-Arg (corresponding to beta Gly-180-beta Arg-182) sequences. The results of these mutational studies are consistent with the labeling of beta Lys-155 and beta Lys-201 with AP3-PL, and of beta Glu-192 with DCCD [15]. The DCCD-binding residue of a thermophilic Bacillus corresponds to beta Glu-181, an essential catalytic residue discussed above. The defective coupling of the beta Ser-174-->Phe mutant was suppressed by the second mutation alpha Arg-296-->Cys, indicating the importance of alpha/beta interaction in energy coupling. The gamma subunit, especially its amino/carboxyl interaction, seems to be essential for energy coupling between catalysis and transport judging from studies on gamma Met-23-->Lys or Arg mutation and second-site mutations which suppressed the gamma Lys-23 mutation. Thus the conserved gamma Met-23 is not absolutely essential but is located in the important region for amino/carboxyl interaction for energy coupling.

Escherichia coli F0F1-ATPase. Residues involved in catalysis and coupling

The molecular biological approach has provided important information toward understanding the complexities of the F0F1 ATPase. This article focuses on our recent results on the ATPase catalytic site contained in the beta subunit and the role of the gamma subunit in regulation of proton transport. We used a combination of affinity labeling and mutagenesis to locate several residues of the alpha and beta subunits in the catalytic site. Adenosine triphosphopyridoxal (AP3-PL) labeled beta Lys-155, beta Lys-201 and alpha Lys-201, suggesting that they are near the gamma-phosphate moiety of ATP. Turning to a mutagenesis approach we demonstrated that the two conserved residues, beta Lys-155 and beta Thr-156 in the glycine-rich sequence, are essential for catalysis. Finally, using pseudorevertant analysis, we positioned residue beta Gly-149 (also in the glycine-rich sequence) in proximity to beta Ser-174, beta Glu-192 (binding site for DCCD), and beta Val-198 (only three residues away from the AP3-PL binding site, beta Lys-201). Genetic studies suggested that the gamma subunit plays a role in regulation of catalysis and its coupling with proton conduction. We found that four mutations in the carboxyl-terminal region (gamma Gln-269-->Leu, gamma Gly-275-->Lys, gamma Thr-277-->end, or frameshift) had similar membrane ATPase activities but different ATP-dependent proton pumping and growth by oxidative phosphorylation. These results suggested a perturbation in the coupling between catalysis and proton translocation. We were able to clearly define the "uncoupling" by introducing mutations in the amino-terminal region of the gamma subunit. We were led to gamma Met-23-->Lys and Arg which resulted in an enzyme still regulated by delta microH+, but with profoundly inefficient coupling between ATPase catalytic sites and proton translocation in both ATP-dependent proton pumping and delta microH(+)-driven ATP synthesis. Second-site mutations in the carboxyl-terminal region of the gamma subunit reversed this effect.

The gamma-subunit of ATP synthase from spinach chloroplasts. Primary structure deduced from the cloned cDNA sequence

cDNA clones encoding the gamma-subunit of chloroplast ATP synthase were isolated from a spinach library using synthetic oligonucleotide probes. The predicted amino acid sequence indicated that the mature chloroplast gamma-subunit consists of 323 amino acid residues and is highly homologous (55% identical residues) with the sequence of the cyanobacterial subunit. The positions of the four cysteine residues were identified. The carboxyl-terminal region of the chloroplast gamma-subunit is highly homologous with those of the gamma-subunits from six other sources (bacteria and mitochondria) sequenced thus far.

Anomalous uncoupling of photophosphorylation by palmitic acid and by gramicidin D

Palmitic acid and gramicidin D at low concentrations uncouple photophosphorylation in a mechanism that is inconsistent with classical uncoupling in the following properties: (1) delta pH, H+ uptake, or the transmembrane electric potential is not inhibited. (2) O2 evolution is stimulated under nonphosphorylating conditions but slightly inhibited in the presence of adenosine 5'-diphosphate + inorganic phosphate (Pi). (3) Light-triggered adenosine 5'-triphosphate (ATP)-Pi exchange is hardly affected, and ATPase activity is only slightly stimulated. (4) ATP-induced delta pH formation is selectively inhibited. This characteristic uncoupling is observed only when the native coupling sites of the electron transport system are used for energization such as for methylviologen-coupled phosphorylation. With pyocyanine, which creates an artificial coupling site, 1000-fold higher gramicidin D and higher palmitic acid concentrations are required for inhibition, and the inhibition is accompanied by a decrease in delta pH. Moreover, comparison between photosystem 1 and photosystem 2 electron transport and the effects of membrane unstacking suggest that low gramicidin D preferentially inhibits photosystem 2, while palmitic acid inhibits more effectively photosystem 1 coupling sites. The inhibitory capacity of fatty acids significantly drops when the chain length is reduced below 16 hydrocarbons or upon introduction of a single double bond in the hydrocarbon chain. It is suggested that palmitic acid and gramicidin D interfere with a direct H+ transfer between specific electron transport and the ATP synthase complexes, which provides an alternative coupling mechanism in parallel with bulk to bulk delta microH+. The sites of inhibition seem to be located in chloroplast ATP synthase, photosystem 2, and the cytochrome b6f complexes.

Deletion of seven amino acid residues from the gamma subunit of Escherichia coli H+-ATPase causes total loss of F1 assembly on membranes

A mutant gene for the gamma subunit of H+-translocating ATPase was cloned from Escherichia coli mutant NR70 isolated by B. P. Rosen [J. Bacteriol. 116, 1124-1129 (1973)]. Determination of its nucleotide sequence revealed a deletion of 21 base pairs between nucleotide residues 64 and 84, resulting in a deletion of seven amino acid residues (LysAlaMetGluMetValAla) from the amino-terminal portion. This deletion resulted in the loss of a hydrophobic domain of the subunit estimated by an analysis of its hydropathic character. Since F1 subunits are reported not to be assembled on the normal F0 portion of NR70, it is concluded that the hydrophobic domain deleted in the mutant subunit is important for assembly of the F1 portion. Introduction of a plasmid pNR70 carrying the mutant allele of NR70 into a wild-type strain gave no recombinants resistant to neomycin. This result suggested that the neomycin-resistant phenotype is not directly related to the defect in the gamma subunit of NR70.