Biochemical and structural studies of actomyosin-like proteins from non-muscle cells. II. Purification, properties, and membrane association of actin from amoebae of Dictyostelium discoideum.

Actin, an apparently universal component of eukaryotic cells, has been purified from Dictyostelium amoebae to electrophoretic homogeneity. Purification was facilitated by newly discovered solubility properties of actomyosin in 30% sucrose. A quantitative assay for the Dictyostelium actin is described which is based on its ability to activate muscle heavy meromyosin ATPase activity. The specific activity of purified amoeba actin for activation of heavy meromyosin is nearly the same as that of purified muscle actin. The molecular weight of the Dictyostelium actin is identical with that of muscle actin, as judged by co-electrophoresis onsodium dodecyl sulfate acrylamide gels. The amoeba actin, like muscle actin, forms Mg2+-paracrystals with a helical repeat of about 360 A. Some of the Dictyostelium actin is associated with membrane in a MgATP-stable linkage. Myosin is found in membrane preparations in a MgATP-labile linkage, presumably due to association with the membranelinked actin.