Chemical modification of thiol groups of mitochondrial F1-ATPase from the yeast Schizosaccharomyces pombe. Involvement of alpha- and gamma-subunits in the enzyme activity.

Abstract

Mitochondrial F1-ATPase from the yeast Schizosaccharomyces pombe has been prepared under a stable form and in relatively high amounts by an improved purification procedure. Specific chemical modification of the enzyme by the thiol reagent N-ethylmaleimide (NEM) at pH 6.8 leads to complete inactivation characterized by complex kinetics and pH dependence, indicating that several thiols are related to the enzyme activity. A complete protection against NEM effect is afforded by low concentrations of nucleotides in the presence of Mg2+, with ADP and ATP being more efficient than GTP. A total binding of 5 mol of [14C]NEM/mol of F1-ATPase is obtained when the enzyme is 85% inactivated: 3 mol of the label are located on the alpha-subunits and 2 on the gamma-subunit. Two out of the 3 mol on the alpha-subunits bind very rapidly before any inactivation occurs, indicating that the two thiols modified are unrelated to the inactivation process. Complete protection by ATP against inactivation by NEM prevents the modification of three essential thiols out of the group of five thiols labeled in the absence of ATP: one is located on a alpha-subunit and two on the gamma-subunit. These two essential thiols of the gamma-subunit can be differentiated by modification with 6,6'-dithiodinicotinic acid (CPDS), another specific thiol reagent. A maximal binding of 4 mol of [14C]CPDS/mol of enzyme is obtained, concomitant to a 25% inhibition. Sequential modification of the enzyme by CPDS and [14C]NEM leads to the same final deep inactivation as that obtained with [14C]NEM alone. One out of the two thiols of the gamma-subunit is no longer accessible to [14C]NEM after CPDS treatment. When incubated at pH 6.8 with [3H]ATP in the presence of Mg2+, F1-ATPase is able to bind 3, largely exchangeable, mol of nucleotide/mol of enzyme. Modification of the three essential thiols by NEM dramatically decreases the binding of 3H-nucleotide down to about 1 mol/mol of enzyme. Partial modification modifies the cooperative properties, the enzyme being no longer sensitive to anion activation.