CA2-AFFINITY DIVALENT CATION-BINDING SITE OF ACTIN ENHANCES ACTOPHORIN-INDUCED DEPOLYMERIZATION OF MUSCLE F-ACTIN BUT INHIBITS ACTOPHORIN-INDUCED DEPOLYMERIZATION OF ACANTHAMOEBA F-ACTIN( BOUND TO THE HIGH)

The cation tightly bound to actin, Mg2+ or Ca2+, affects the ability o f actophorin to accelerate depolymerization of filaments and bind to m onomers of actin prepared from rabbit skeletal muscle and Acanthamoeba castellanii. Actophorin interacted similarly with muscle and Acantham oeba Mg2+-F-actin but depolymerized muscle Mg2+-F-actin more efficient ly. Muscle Ca2+-F-actin depolymerized about 5 times more rapidly than Mg2+-F-actin in the presence of actophorin but Acanthamoeba Ca2+-F-act in was highly resistant to actophorin. Muscle actin subunits dissociat ed more rapidly than Acanthamoeba actin subunits from copolymers of mu scle 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 f or 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 incompatibl e with the formation of 1:1 actin:actophorin complexes and, thus, Kd v alues could not be calculated. While it may not be surprising that act ophorin would interact differently with Mg2+- and Ca2+-actin, it is un expected that the nature of the tightly bound cation would have such d ramatically opposite effects on the ability of actophorin to depolymer ize muscle and Acanthamoeba F-actin. Differential severing by actophor in, with Acanthamoeba Ca2+-actin being almost totally resistant, is su fficient to explain the results but other possibilities cannot be rule d out.