The requirements for adenosine triphosphate binding to globular actin.

Most of the present interest in the muscle protein actin stems directly from the fact, demonstrated by Straub and Feuer (2), that G-a&in binds adenosine triphosphate, and that concomitantly with the polymerization of G-actin, the bound adenosine triphosphate is converted to bound adenosine diphosphate (2). The association between protein and nucleotide differs depending on the state of polymerization of the actin, as demonstrated by the findings that in the G-form the nucleotide is comparatively easy to remove and is susceptible to enzymatic attack (3-5), whereas in F-a&in the bound adenosine diphosphate is refractory to removal and is not attacked by enzymes which normally phosphorylate or deaminate free adenosine diphosphate (4, 5). Recent and elegant experiments by Asakura (6), however, show that the binding of adenosine triphosphate to G-actin is quite strong, with a binding constant of 2.3 x 10” M-‘, and proceeds with a large negative AF. In the work to be described here, the exchange reaction, described by Martonosi, Gouvea, and Gergely (7), between exogenous and actin-bound ATP has been examined with the view of obtaining specific information on the mode of nucleotide binding. It has been possible to demonstrate (a) a requirement for divalent cations for G-actin-ATP interaction, (b) a reversible dissociation depending on regulation of the cation, and (c) a requirement for free sulfhydryl groups for ATP binding.