An actin-modulating protein from Physarum polycephalum. II. Ca++-dependence and other properties.

Actin modulating (AM-) protein from Physarum binds as monomer and as a stable heterodimer with one actin molecule to actin and induces the formation of oligomeric actin complexes and short-chained filaments with no definite stoichiometry. While inhibiting the extent of actin polymerization by reducing the size of the aggregates formed, AM-protein increases the velocity of polymerization. The AM-protein monomer depolymerizes F-actin very rapidly by breaking the filaments into small pieces and oligomeric complexes. When a heterodimer of actin an AM-protein is reconstituted the polymerization inhibitor activity is identical to that of the monomer, but depolymerization of actin is almost completely abolished. The action of the monomeric AM-protein on actin is highly Ca++-dependent as it requires micromolar amounts of Ca++ for full activation. The inhibitory activity of both the natural and the reconstituted heterodimer has only little Ca++-sensitivity: A prolonged exposure to Ca++-chelating agents is necessary to obtain a partial inactivation of the heterodimers. The depolymerizing effect on actin of the AM-protein monomer is inhibited by tropomyosin and also by heavy meromyosin. Addition of phalloidin to the actin reduces only the velocity of depolymerization by AM-protein. In the presence of AM-protein the actomyosin ATPase or Acto-HMM ATPase is strongly inhibited. The supposed function of the AM-protein in Physarum is that of a powerful regulator of the polymer state of actin. Such a regulation system is necessary for the dynamic actin transformation processes during ectoplasm-endoplasm transitions and the assembly-disassembly of contractile and cytoskeletal structures. The Ca++-sensitivity of the AM-protein indicates that these processes are controlled by calcium.