Isolation and Characterization of a 30,000-dalton Calcium-sensitive Actin Cross-linking Protein from Dictyostelium discoideurn”

A calcium-sensitive actin-binding protein having a subunit molecular mass of 3 0 , 0 0 0 daltons (30K protein) has been isolated from Dictyostelium discoideum. Structural, immunological, and functional analyses demonstrated that the 30K protein was distinct from other actin-binding proteins of D. discoideum. A native molecular mass of 31,700 daltons was determined by equilibrium sedimentation, indicating that the protein is monomeric. The Stokes radius was 30 A. The frictional coefficient calculated from these measurements was 1.44, indicating an asymmetric shape. The 30K protein induced an increase in the viscosity of a solution of F-actin. Bundles of actin filaments were observed in negatively stained mixtures of actin and the 30K protein. Both the formation of filament bundles and the increases in viscosity of actin induced by the 30K protein were observed in the presence of 1 X lo-’ M but not 2 X lo-‘ M calcium. Variation of the pH from 6.6 to 7.8 had no effect on the activity of the 30K protein. Calcium induced neither a large change in quaternary structure of the 30K protein nor a restriction of the lengths of actin filaments by the 30K protein. The apparent affinity of the 30K protein for actin was decreased in the presence of calcium. Reversible cross-linking of actin filaments by the 30K protein may contribute to regulation of the consistency and contractility of cytoplasm in D. discoideum.