AN ACTIN-MODULATING PROTEIN FROM PHYSARUM-POLYCEPHALUM .2. CA++-DEPENDENCE AND OTHER PROPERTIES

  • 1 January 1981
    • journal article
    • research article
    • Vol. 23  (2) , 234-240
Abstract
Actin modulating (AM-) protein from Physarum binds as monomer and as a stable heterodimer with 1 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 and 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 Ca2+-dependent as it requires micromolar amounts of Ca2+ for full activation. The inhibitory activity of both the natural and the reconstituted heterodimer has little Ca2+-sensitivity: A prolonged exposure to Ca2+-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 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 Ca2+-sensitivity of the AM-protein indicates that these processes are controlled by Ca.