Preparation and polymerization properties of monomeric ADP-actin.

An improved method for the preparation of Mg-ADP-actin and Ca-ADP-actin which minimizes denaturation of the protein has been developed. Using ADP-actin prepared by this method, we have measured the polymerization characteristics of Mg-ADP-actin and Ca-ADP-actin. In contrast to the significant difference in Mg-ATP-actin and Ca-ATP-actin polymerization characteristics that we reported previously (J. Muscle Res. Cell Motility 7 (1986) 215-224), we show here that values for the critical concentration, the relative rate constant of elongation (mk+) and the relative rate constant of depolymerization (mk-) for Mg-ADP-actin are similar to those for Ca-ADP-actin. The value of mk+ for Mg-ATP-actin is about 8-fold higher than that for Mg-ADP-actin and the value of mk- for Mg-ADP-actin is 3-4-fold higher than that for Mg-ATP-actin. These factors may help explain the observation that the spontaneous nucleation rates of both types of ADP-actin are low in contrast to the rapid nucleation of Mg-ATP-actin.