Second stalk of ATP synthase. Cross-linking of gamma subunit in F1 to truncated Fob subunit prevents ATP hydrolysis.

ATP synthase consists of two portions, F(1) and F(o), connected by two stalks: a central rotor stalk containing gamma and epsilon subunits and a peripheral, second stalk formed by delta and two copies of F(o)b subunits. The second stalk is expected to keep the stator subunits from spinning along with the rotor. We isolated a TF(1)-b'(2) complex (alpha(3)beta(3)gammadeltaepsilonb'(2)) of a thermophilic Bacillus PS3, in which b' was a truncated cytoplasmic fragment of F(o)b subunit, and introduced a cysteine at its N terminus (bc'). Association of b'(2) or bc'(2) with TF(1) did not have significant effect on ATPase activity. A disulfide bond between the introduced cysteine of bc' and cysteine 109 of gamma subunit was readily formed, and this cross-link caused inactivation of ATPase. This implies that F(o)b subunit bound to stator subunits of F(1) with enough strength to resist rotation of gamma subunit and to prevent catalysis. Contrary to this apparent tight binding, some detergents such as lauryldodecylamine oxide tend to cause release of b'(2) from TF(1).