The CF1(alphabeta)-core complex previously isolated from the spinach chloroplast CF0F1-ATP synthase contains equal amounts of CF1alpha- and beta-subunits, functions as a soluble Mg2+-ATPase and forms a hybrid F0F1-ATPase when incorporated into beta-less Rhodospirillum rubrum membrane-bound F0F1 (Avital, S. and Gromet-Elhanan, Z. (1991) J. Biol. Chem. 266, 7067-7072). Here, we demonstrate that this soluble spinach CF1(alphabeta)-Mg2+-ATPase, unlike its latent parent CF1-ATPase, does not respond to activation by octyl glucoside, is only slightly stimulated by sulfite and not inhibited by free Mg2+, azide or tentoxin. The CF1(alphabeta)-ATPase does however bind tentoxin rather tightly and is stimulated by it at concentrations that inhibit the parent CF1-ATPase. Unlike this soluble CF1(alphabeta)-ATPase, the hybrid Mg2+-ATPase formed by incorporation of the same CF1(alphabeta) preparation into beta-less R. rubrum F0F1, is markedly stimulated by sulfite and completely inhibited by azide and tentoxin. These results indicate that (a), for stimulation by tentoxin the presence of alpha- and beta-subunits from a sensitive CF1 is enough, whereas for inhibition by it other F1-subunits are required and these can come from a tentoxin resistant F1-species, such as R. rubrum and (b), although the single-copy subunits are not required for F1-ATPase activity, their presence results in increased rates and large changes in the catalytic properties.
