F-1-ATPase (F-1) is an ATP-driven rotary motor wherein the gamma subunit rotates against the surrounding alpha(3)beta(3) stator ring. The 3 catalytic sites of F-1 reside on the interface of the alpha and beta subunits of the alpha(3)beta(3) ring. While the catalytic residues predominantly reside on the beta subunit, the alpha subunit has 1 catalytically critical arginine, termed the arginine finger, with stereogeometric similarities with the arginine finger of G-protein-activating proteins. However, the principal role of the arginine finger of F1 remains controversial. We studied the role of the arginine finger by analyzing the rotation of a mutant F-1 with a lysine substitution of the arginine finger. The mutant showed a 350-fold longer catalytic pause than the wild-type; this pause was further lengthened by the slowly hydrolyzed ATP analog ATP gamma S. On the other hand, the mutant F1 showed highly unidirectional rotation with a coupling ratio of 3 ATPs/turn, the same as wild-type, suggesting that cooperative torque generation by the 3 beta subunits was not impaired. The hybrid F-1 carrying a single copy of the alpha mutant revealed that the reaction step slowed by the mutation occurs at + 200 degrees from the binding angle of the mutant subunit. Thus, the principal role of the arginine finger is not to mediate cooperativity among the catalytic sites, but to enhance the rate of the ATP cleavage by stabilizing the transition state of ATP hydrolysis. Lysine substitution also caused frequent pauses because of severe ADP inhibition, and a slight decrease in ATP binding rate.