PubMed1983
The uncB402 mutation in Escherichia coli results in formation of an H+-ATPase complex that is defective in energy-transducing capacity. The mutation, originally described by Butlin et al. (Butlin, J.D., Cox, G.B., and Gibson, F. (1973) Biochim. Biophys. Acta 292, 366-375), alters the F0 sector of the H+-ATPase complex. Here, we show that uncB402 is an amber-suppressible, chain-terminating mutation that results in loss of the chi subunit from F0. This was demonstrated in crude membrane fractions after overproduction of the ATPase complex by heat induction of a lambda transducing phage carrying the unc operon of uncB402. The lambda-uncB402 DNA was used as a template in an in vitro transcription-translation system. A synthesis product that may correspond to the truncated form of the chi subunit was observed. Despite the absence of chi, the F1-ATPase was still bound to the membrane, although more weakly than in wild type. The omega subunit of F0 ("dicyclohexylcarbodiimide-binding protein") shows normal reactivity with dicyclohexylcarbodiimide, indicating that at least this portion of F0 integrates properly in the membrane in the absence of the chi subunit. The F0 of uncB402 was not functional in H+ translocation activity. This was shown by direct H+ flux measurements with crude membrane vesicles that were treated with guanidine to disrupt the binding of F1 to F0. Secondly, a method was developed for isolation of F0 from F1-depleted membranes. The F0 from uncB402 was shown to have less than 5% the proton-translocase activity of wild type F0 when reconstituted into liposomes. Although the uncB402 mutant shows these defects, the question of whether the chi subunit plays a direct role in F1-binding or H+ translocation remains open, since the loss of chi may lead to subtle changes in the assembly of the other F0 subunits. Analysis of other mutants should permit a more definitive assignment of function.
Adenosine Triphosphatases↗