The mechanism of inhibition of yeast F(0)F(1)-ATPase by its naturally occurring protein inhibitor (IF1) was investigated in submitochondrial particles by studying the IF1-mediated ATPase inhibition in the presence and absence of a protonmotive force. In the presence of protonmotive force, IF1 added during net NTP hydrolysis almost completely inhibited NTPase activity. At moderate IF1 concentration, subsequent uncoupler addition unexpectedly caused a burst of NTP hydrolysis. We propose that the protonmotive force induces the conversion of IF1-inhibited F(0)F(1)-ATPase into a new form having a lower affinity for IF1. This form remains inactive for ATP hydrolysis after IF1 release. Uncoupling simultaneously releases ATP hydrolysis and converts the latent form of IF1-free F(0)F(1)-ATPase back to the active form. The relationship between the different steps of the catalytic cycle, the mechanism of inhibition by IF1 and the interconversion process is discussed.
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