Physarum action. Observations on its presence, stability, and assembly in plasmodial extracts and development of an improved purification procedure.

Actin is readily extracted from plasmodia of Physarum polycephalum by low ionic strength solutions which do not solubilize the plasmodial myosin. The actin in such extracts exists predominantly as a monomer which slowly denatures, apparently via removal of bound nucleotide, and is subsequently proteolyzed. However, the native monomeric actin can be induced to assemble into polymeric arrays under appropriate solvent conditions. Actin assembly is dependent on the addition of ATP and is a function of KCl and CaCl2 concentrations. These observations have allowed the development of an improved actin purification scheme which is simple, rapid, and efficient, yielding approximately 60 mg of protein from 100 g of plasmodium. The actin thus obtained is pure, stable, and comparable to that obtained by previously described procedlres. Furthermore, the observations suggest that actin polymers may be metastably assembled in vivo and raise the possibility that actin assembly, and plasmodial movements, could be regulated via alterations in intracellular concentrations of nucleotide and/or divalent cation.