PREPARATION AND CHARACTERIZATION OF AN ALPHA-BETA HETERODIMER FROM THE ATP SYNTHASE OF RHODOSPIRILLUM-RUBRUM

1. An alphabeta heterodimer of the F1-ATPase of Rhodospirillum rubrum (RF1) was isolated from extracts of chromatophores. This fragment of R F1 is highly active in restoring ATP hydrolysis and ATP synthesis to L iCl-treated chromatophores, maximal activity being reached at approxim ately 2 mol alphabeta per original RF1 molecule. Both alpha and beta s ubunits of the heterodimer bind to chromatophore membranes on reconsti tution. It was concluded that the alphabeta heterodimer was able to re constitute RF1 in these membranes. 2. Reconstitutive activity in LiCl extracts purifies with the alphabeta heterodimer and away from the mon omeric beta subunit. Prolonged exposure to LiCl leads to irreversible dissociation of the heterodimer and loss of reconstitutive activity. I t was concluded that the alphabeta heterodimer was required for recons titution in LiCl-treated membranes, and that the monomeric beta subuni t was inactive. 3. The alphabeta heterodimer contains a non-catalytic nucleotide binding site. It can also catalyse CaATP and MgATP hydrolys is at similar rates, with K(m) values similar to RF1. However, hydroly sis is insensitive to activators/inhibitors of RF1 such as sulphite, o ctyl glucoside and azide. It was concluded that the isolated heterodim er represents a functional unit of RF1 but that, as it has a single ca talytic site, catalytic cooperativity is absent.