Ca2+ bound to the high affinity divalent cation-binding site of actin enhances actophorin-induced depolymerization of muscle F-actin but inhibits actophorin-induced depolymerization of Acanthamoeba F-actin.

The cation tightly bound to actin, Mg2+ or Ca2+, affects the ability of actophorin to accelerate depolymerization of filaments and bind to monomers of actin prepared from rabbit skeletal muscle and Acanthamoeba castellanii. Actophorin interacted similarly with muscle and Acanthamoeba Mg2(+)-F-actin but depolymerized muscle Mg2(+)-F-actin more efficiently. Muscle Ca2(+)-F-actin depolymerized about 5 times more rapidly than Mg2(+)-F-actin in the presence of actophorin but Acanthamoeba Ca2(+)-F-actin was highly resistant to actophorin. Muscle actin subunits dissociated more rapidly than Acanthamoeba actin subunits from copolymers of muscle and Acanthamoeba Ca2(+)-actin upon addition of actophorin although Acanthamoeba actin dissociated much more rapidly from copolymers than from its homopolymer. The Kd of the 1:1 complex between actophorin and monomeric actin was somewhat lower for muscle Mg2(+)-ATP-G-actin than for both Acanthamoeba Mg2(+)-ATP-G-actin and muscle Ca2(+)-ATP-G-actin. The data for the interactions of actophorin with Acanthamoeba Ca2(+)-ATP-G-actin or muscle and amoeba Mg2(+)- and Ca2(+)-ADP-G-actin were incompatible with the formation of 1:1 actin: actophorin complexes and, thus, Kd values could not be calculated. While it may not be surprising that actophorin would interact differently with Mg2(+)- and Ca2(+)-actin, it is unexpected that the nature of the tightly bound cation would have such dramatically opposite effects on the ability of actophorin to depolymerize muscle and Acanthamoeba F-actin. Differential severing by actophorin, with Acanthamoeba Ca2(+)-actin being almost totally resistant, is sufficient to explain the results but other possibilities cannot be ruled out.