The cysteine introduced into the alpha subunit of the Escherichia coli F1-ATPase by the mutation alpha R376C is near the alpha-beta subunit interface and close to a noncatalytic nucleotide binding site

Abstract

Mutation of the alpha subunit of the Escherichia coli F1-ATPase to convert Arg-376 to a Cys (alpha R376C) lowers multisite ATPase activity 400-1,000-fold while affecting unisite catalysis only around 6-fold, suggesting that the mutation is in a region important for transmission of conformational changes between catalytic sites (Soga, S., Noumi, T., Takeyama, M., Maeda, M., and Futai, M. (1989) Arch. Biochem. Biophys. 268, 643-648; this study). To learn more of the structural features of the segment of the alpha subunit around Arg-376, mutant enzyme with a Cys at this position was modified with several maleimides. N-[14C]Ethylmaleimide reacted rapidly with this Cys in one of the three alpha subunits/F1 (2,500 M-1 s-1); more slowly with a second alpha subunit (390 M-1 s-1); and the same Cys in the third copy of the alpha subunit was completely unreactive to the reagent, indicating asymmetry of alpha subunits in the ECF1 complex. The photoactivatable cross-linker N-(4-azido-2,3,5,6-tetrafluorobenzyl)-3-maleimidopropionamide++ +, when reacted via its maleimide to alpha Cys-376 of the mutant, covalently linked alpha to beta subunits upon photolysis, indicating that Cys-376 of alpha is close to an interface between the alpha and beta subunits. The EDTA-induced exchangeable noncatalytic site could be filled by TNP-ATP in both wild type and alpha R376C mutant ECF1. Occupancy of this site in the alpha R376C mutant altered the rate of reaction of the second-fastest reacting Cys-376 from 390 M-1 s-1 to below 130 M-1 s-1, suggesting that the two sites are on the same alpha subunit. TNP-ATP in the EDTA-induced exchangeable noncatalytic site was quenched by reacting Cys-376 with 4-maleimido-(2,2,6,6-tetramethylpiperidine-N(oxyl), indicating that the region around Cys-376, which is involved in transmission of conformational changes between alpha and beta, and noncatalytic sites are maximally 10-12 A from each other.