The smallest subunit of F1-ATPase, the ε subunit acts as an endogenous inhibitor of the ATPase activity in both bacterial and chloroplast F1-ATPase, where it
CCT129202 is believed to play a regulatory role in FoF1[6] and [7]. A recent single molecule study revealed its importance not only in the regulation but also in the efficient coupling of rotation of γ subunit and ATP synthesis [8]. The ε subunit consists of two distinct domains, an N-terminal β sandwich domain and a C-terminal α helical domain. Structural and biochemical studies have shown that the ε subunit adopts at least two different conformational states in F1 and FoF1[9], [10], [11], [12], [13], [14], [15], [16] and [17]. The conformation that causes inhibition of ATP hydrolysis activity is an extended one, in which the C-terminal α helices of the ε subunit extended and run parallel to the coiled-coil of the γ subunit. The other non-inhibitory conformation is characterized by a hairpin-fold of C-terminal α helices. These two states of the ε subunit, termed as an extended ε and folded ε hereafter, are controlled by the concentration of ATP, as well as the membrane potential [16] and [17]. It has been suggested that when FoF1 catalyzes ATP synthesis the ε subunit takes extended-conformation and that ATP synthesis is not inhibited by the extended ε [18]. The folded ε itself can bind ATP, but not ADP or other nucleotides, and is stabilized by
annuals ATP binding in the FoF1’s from some bacteria including thermophilic Bacillus PS3 [19].
