ATP causes a large change in the conformation of the isolated alpha subunit of Escherichia coli F1 ATPase.

The physiochemical properties of the isolated alpha subunit of the Escherichia coli coupling factor ATPase, and changes resulting from the interaction of alpha with ATP, were studied. Amino acid analysis of alpha revealed 42% polar residues, 4 cysteine residues, and a single tryptophan residue. The partial specific volume, v, of alpha was 0.74 cm3 g-1. Molecular weight value of alpha, determined by sedimentation equilibrium, of 55,000 to 59,000 were observed in guanidine hydrochloride, or in nondenaturing buffer in either the presence of absence of ATP, which alpha binds with high affinity. Sedimentation velocity experiments gave a value of s20,w0-3.52 S for alpha. In the presence of ATP, this value increased to 4.00 S, indicating a large conformational change of alpha when ATP is bound. A slow dissociation rate of alpha x ATP was suggested by the finding that a substantial portion of [2-3H]ATP mixed with alpha remained bound to the protein during native polyacrylamide gel electrophoresis, causing alpha to migrate with a higher relative mobility. A dissociation rate constant, koff, of 0.21 min-1 at 22 degrees C was measured by following the rate at which unlabeled ATP displaced [2-3H]ATP from the protein. The properties of the interaction of alpha with ATP suggest that this subunit may be the site of the "tightly bound" nucleotides of the coupling factor ATPase.