Evidence for multiple effects in the methanol activation of chloroplast coupling factor 1.

Activation of the latent ATPase of soluble CF1 by methanol is shown to involve several distinct effects. CaATPase activity of whole, but not epsilon-deficient or heat-activated CF1, is stimulated by methanol. This suggests that one effect of methanol is to overcome inhibition by the epsilon subunit. In contrast, the MgATPase activities of both whole and epsilon-deficient CF1 are further stimulated by methanol. This second activating effect can be traced in part to a greatly increased affinity of CF1, due to methanol, for those anions which reverse the inhibitory effect of Mg2+. Since the inhibition by free Ca2+ is much less severe than that caused by Mg2+, anions have relatively little effect on CaATPase. Thus methanol has little or no effect when Ca2+ is the divalent cation, but stimulates the reaction when Mg2+ is used. Methanol also stimulates the MgATPase activity of epsilon-deficient CF1 in the complete absence of activating anions. This additional effect is shown to arise from an increase in the Vmax rather than from changes in either the Km for MgATP or the Ki for free Mg2+. Since this change in Vmax occurs with the MgATPase but not the CaATPase, it can be inferred that different steps are rate-limiting in the two activities.