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 RF1 is highly active in restoring ATP hydrolysis and ATP synthesis to LiCl-treated chromatophores, maximal activity being reached at approximately 2 mol alphabeta per original RF1 molecule. Both alpha and beta subunits of the heterodimer bind to chromatophore membranes on reconstitution. It was concluded that the alphabeta heterodimer was able to reconstitute RF1 in these membranes. 2. Reconstitutive activity in LiCl extracts purifies with the alphabeta heterodimer and away from the monomeric beta subunit. Prolonged exposure to LiCl leads to irreversible dissociation of the heterodimer and loss of reconstitutive activity. It was concluded that the alphabeta heterodimer was required for reconstitution in LiCl-treated membranes, and that the monomeric beta subunit was inactive. 3. The alphabeta heterodimer contains a non-catalytic nucleotide binding site. It can also catalyse CaATP and MgATP hydrolysis at similar rates, with K(m) values similar to RF1. However, hydrolysis is insensitive to activators/inhibitors of RF1 such as sulphite, octyl glucoside and azide. It was concluded that the isolated heterodimer represents a functional unit of RF1 but that, as it has a single catalytic site, catalytic cooperativity is absent.