The interaction between ATP-actin and ADP-actin. A tentative model for actin polymerization.

The involvement of interactions between ATP-actin and ADP-actin in actin polymerization has been studied. It has been found that ATP-actin and ADP-actin can copolymerize and that the rate of nucleation is enhanced when both ATP-actin and ADP-actin are present in solution. The fact that the heterologous interaction between ATP-actin (T) and ADP-actin (D) is stronger than either of the homologous reactions, T-T and D-D, agrees with the kinetic data in the accompanying paper (Carlier, M.-F., Pantaloni, D., and Korn, E.D. (1985) J. Biol. Chem. 260, 6565-6571) which show that filament ends having the DT conformation are more stable than those having the TT conformation. These data are incorporated into a model for actin polymerization in ATP in which the kinetic parameters for polymerization depend on the nature of the nucleotide (ADP or ATP) bound to the three terminal subunits of the actin filament.