FURTHER PURIFICATION AND CHARACTERIZATION OF SLIME MOLD MYOSIN AND SLIME MOLD ACTIN

Starting with the portion of partially purified slime mold actomyosin which was voided from Sephadex G-200 at low ionic strength slime mold myosin was further purified by DEAE-cellulose chromatography and salt fractionation with ammonium sulfate. Slime mold myosin had about three times the specific Ca2+-adenosine triphosphatase activity of rabbit myosin assayed under identical conditions. The enzyme was free of nucleic acids and nearly all material present sedimented as a single species with S020,w = 6.40 S in 0.50 M KCl at pH 7.4. By gel chromatography on Sepharose 4B the enzyme was found to have a high particle asymmetry with a diffusion coefficient similar to rabbit myosin An approximate molecular weight of 4.6 × 105 was obtained. Slime mold myosin formed an actomyosin complex with rabbit actin and was similar to rabbit myosin in some of its enzymatic properties. The main differences from the rabbit enzyme were its solubility and low degree of aggregation at low ionic strength and the lack of a strong magnesium inhibition of the adenosine triphosphatase activity. Slime mold actin was prepared from the protein peak which was retarded on the G-200 column. The material was first polymerized by the addition of KCl and MgCl2 and separated by high-speed centrifugation. At low ionic strength slime mold actin existed as a low molecular weight protein with S020,w = 3.2 S. In the presence of salt the actin formed rapidly sedimenting asymmetric particles: several schlieren boundaries were present. Slime mold actin had adenosine triphosphate binding properties similar to muscle actin, formed an actomyosin complex with rabbit myosin, and activated the Mg2+- adenosine triphosphatase activity of the mammalian enzyme at low ionic strength. © 1969, American Chemical Society. All rights reserved.