F1-ATPase, the catalytic sector of Fo-F1 ATPases-ATPsynthases, displays an apparent negative cooperativity for ATP hydrolysis at high ATP concentrations which involves noncatalytic and catalytic nucleotide binding sites. The molecular mechanism of such cooperativity is currently unknown. To get further insights, we have investigated the structural consequences of the single mutation of two residues: Q173L in the alpha-subunit and Q170Y in the beta-subunit of the F1-ATPase of the yeast Schizosaccharomyces pombe. These residues are localized in or near the Walker-A motifs of each subunit and their mutation produces an opposite effect on the negative cooperativity. The betaQ170 residue (M167 in beef heart) is located close to the binding site for the phosphate-Mg moiety of the nucleotide. Its replacement by tyrosine converts this site into a close state with increased affinity for the bound nucleotide and leads to an increase of negative cooperativity. In contrast, the alphaQ173L mutation (Q172 in beef heart) abolishes negative cooperativity due to the loss of two H-bonds: one stabilizing the nucleotide bound to the noncatalytic site and the other linking alphaQ173 to the adjacent betaT354, localized at the alpha(DP)-beta(TP) interface. The properties of these mutants suggest that negative cooperativity occurs through interactions between neighbor alpha- and beta-subunits. Indeed, in the beef heart enzyme, (i) the alpha(DP)-beta(TP) interface is stabilized by a vicinal alphaR171-betaD352 salt bridge (ii) betaD352 and betaT354 belong to a short peptidic stretch close to betaY345, the aromatic group of which interacts with the adenine moiety of the nucleotide bound to the catalytic site. We therefore propose that the betaY345-betaT354 stretch (beef heart numbering) constitutes a short link that drives structural modifications from a noncatalytic site to the neighbor catalytic site in which, as a result, the affinity for ADP is modulated.