Construction and function of chimeric beta subunits containing regions from the beta subunits of the F1F0 ATPases of Escherichia coli and Bacillus megaterium

Structure/function of the beta-barrel domain of F1-ATPase in the yeast Saccharomyces cerevisiae

The first 90 amino acids of the alpha- and beta-subunits of mitochondrial F1-ATPase are folded into beta-barrel domains and were postulated to be important for stabilizing the enzyme (Abrahams, J. P., Leslie, A. G., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628). The role of the domains was studied by making chimeric enzymes, replacing the domains from the yeast Saccharomyces cerevisiae enzyme with the corresponding domains from the enzyme of the thermophilic bacterium Bacillus PS3. The enzymes containing the chimeric alpha-, beta-, or alpha- and beta-subunits were not functional. However, gain-of-function mutations were obtained from the strain containing the enzyme with the chimeric PS3/yeast beta-subunit. The gain-of-function mutations were all in codons encoding the beta-barrel domain of the beta-subunit, and the residues appear to map out a region of subunit-subunit interactions. Gain-of-function mutations were also obtained that provided functional expression of the chimeric PS3/yeast alpha- and beta-subunits together. Biochemical analysis of this active chimeric enzyme indicated that it was not significantly more thermostable or labile than the wild type. The results of this study indicate that the beta-barrel domains form critical contacts (distinct from those between the alpha- and beta-subunits) that are important for the assembly of the ATP synthase.

Exchangeability of the b subunit of the Cl(-)-translocating ATPase of Acetabularia acetabulum with the beta subunit of E. coli F1-ATPase: construction of the chimeric beta subunits and complementation studies

The gene encoding the b subunit of the Cl(-)-translocating ATPase (aclB) was isolated from total RNA and poly(A)+ RNA of Acetabularia acetabulum and sequenced (total nucleotides of 3038 bp and an open reading frame with 478 amino acids). The deduced amino acid sequence showed high similarity to the beta subunit of the F type ATPases, but was different in the N-terminal 120 amino acids. The role of the N-terminal region was investigated using an F -ATPase beta-less mutant of E. coli, JP17. The JP17 strain expressing the aclB could not grow under conditions permitting oxidative phosphorylation, although ACLB was detected in the membrane fraction. The beta subunit was divided into three portions: amino acid position from 1 to 95 (portion A), 96 to 161 (portion B) and 162 to the C-terminus (portion C). The corresponding regions of ACLB were designated as portions A' (from 1 to 106), B' (from 107 to 172) and C' (from 173 to 478). Chimeric proteins with combinations of A-B'-C', A-B-C' and A'-B-C restored the function as the beta subunit in E. coli F0F1-complex, but those with combinations of A'-B'-C and A-B'-C had no function as the beta subunit. These findings suggested that portion B plays an important role in the assembly and function of the beta subunit in the F0F1-complex, while portion B' of ACLB exhibited inhibitory effects on assembly and function. In addition, portion A was also important for interaction of the beta subunit with the alpha subunit in E. coli F0F1-complex. These findings also suggested that the b subunit of the Cl(-)-translocating ATPase of A. acetabulum has a different function in the Cl(-)-translocating ATPase complex, although the primary structure resembled to the beta subunit of the F1-ATPase.

Localization of a conformational energy-coupling determinant near the C terminus of the beta subunit of the F1F0-ATPase

Escherichia coli mutants in the beta subunit of the F1F0-ATPase can be complemented with the beta subunit from the obligate aerobe Bacillus megaterium. It has been shown that cells carrying such hybrid ATPases have an unusual energy-coupling phenotype. Although they are able to grow on minimal succinate medium, and therefore carry a functional ATP synthase, they are defective in the ability to grow anaerobically, indicating some defect in ATP-driven proton pumping (Scarpetta, M., Hawthorne, C. A., and Brusilow, W. S. A. (1991) J. Biol. Chem. 266, 18567-18572). In this study, chimeric beta subunits were constructed consisting of the E. coli or the B. megaterium beta subunit carrying the C-terminal 18% of the other's beta subunit. The phenotypes of an E. coli beta mutant complemented with these chimeric subunits showed that the energy-coupling defect was located in this C-terminal region. The E. coli beta subunit carrying the B. megaterium C-terminal region displayed the energy-coupling defect, while the B. megaterium beta subunit carrying the E. coli C-terminal region did not. In ATP-dependent fluorescence quenching assays, membranes isolated from cells displaying the energy-coupling defect also pumped protons less well than membranes isolated from cells that were able to grow anaerobically. These results demonstrate that the C terminus of the beta subunit is involved in the conformational coupling pathway, which, through the polypeptide backbone of the beta subunit, physically links ATP synthesis or hydrolysis to the energy of proton translocation.