Characterization of the Relationship between ADP- and ε-induced Inhibition in Cyanobacterial F1-ATPase

The ATPase activity of chloroplast and bacterial F-1-ATPase is strongly inhibited by both the endogenous inhibitor epsilon and tightly bound ADP. Although the physiological significance of these inhibitory mechanisms is not very well known for the membrane-bound F0F1, these are very likely to be important in avoiding the futile ATP hydrolysis reaction and ensuring efficient ATP synthesis in vivo. In a previous study using the alpha(3)beta(3)gamma complex of F-1 obtained from the thermophilic cyanobacteria, Thermosynechococcus elongatus BP-1, we succeeded in determining the discrete stop position, similar to 80 degrees forward from the pause position for ATP binding, caused by epsilon-induced inhibition (epsilon-inhibition) during gamma rotation (Konno, H., Murakami-Fuse, T., RIM, F., Koyama, F., Ueoka-Nakanishi, H., Pack, C. G., Kinjo, M., and Hisabori, T. (2006) EMBO J. 25, 4596 4604). Because gamma in ADP-inhibited F-1 also pauses at the same position, ADP-induced inhibition (ADP-inhibition) was assumed to be linked to epsilon-inhibition. However, ADP-inhibition and epsilon-inhibition should be independent phenomena from each other because the ATPase core complex, alpha(3)beta(3)gamma, also lapses into the ADP-inhibition state. By way of thorough biophysical and biochemical analyses, we determined that the E subunit inhibition mechanism does not directly correlate with ADP-inhibition. We suggest here that the cyanobacterial ATP synthase epsilon subunit carries out an important regulatory role in acting as an independent braking system for the physiologically unfavorable ATP hydrolysis reaction.