The Dissociation of the Phalloidin-Actin Complex

Abstract

Phalloidin reduces the critical concentration [G]c = .**GRAPHIC**. for the depolymerization of rabbit muscle actin. Adding 1 eq of toxin reduces the [G]c by a factor of 30; adding 2 eq reduces the [G]c by a factor of 90. The depolymerization of actin was measured by the exchangeability of 45Ca and [14C]ADP in equilibrium dialysis. Half dissociation of both the metal ion and the nucleotide were found at the same concentration. From this value the critical concentration for actin [G]c = 1.05 .times. 10-6 M was calculated. The analogous value in the presence of 1 eq of toxin is .**GRAPHIC**. = 3.7 .times. 10-8 M. The dissociation from actin for a labeled phallotoxin, [3H]demethylphalloin, was likewise studied by equilibrium dialysis. The apparent Kd for this toxin, and for the natural toxin phalloidin, is 3.6 .times. 10-8 M. The value is identical to that of .**GRAPHIC**. The dissociation of the toxin and the breakdown of the filaments together with the concomitant release of Ca and ADP are interdependent events.