Human Platelet Actin

A rapid purification procedure for actin from human blood platelets is described. The isolated protein was found to be homogeneous by ultracentrifugation, polyacrylamide gel electrophoresis, and 3-methylhistidine analysis. The following physical parameters of the protein were established: sedimentation coefficient (s020,w) 3.09 S; partial specific volume (v), 0.718 ml g-1; molecular weight (Mw) from acrylamide gel electrophoresis, 46,100 ± 2,499; molecular weight (Mw0) from high speed equilibrium ultracentrifugation, 44,700 ± 2,590; and molecular weight (Mw0) of the reduced carboxymethylated protein, from high speed equilibrium ultracentrifugation, 45,000 ± 2,080. The amino acid composition of platelet actin is reported. ATP-salt-induced polymerization of platelet G-actin is concurrent with an increase in relative viscosity and release of inorganic phosphate. Addition of platelet actin to human platelet and rabbit skeletal muscle myosins enhanced the myosin ATPase activities and increased the relative viscosity of a platelet actin-platelet myosin protein mixture. In addition, electron micrographs of negatively stained (uranyl acetate) polymerized platelet actin show varying length, single filament-like F-actin polymers with a diameter of 61.5 ± 5.2 A. Platelet F-actin also formed typical arrowhead-shaped complexes with rabbit heavy meromyosin. Finally, comparison of the CNBr-produced fragments of human platelet and rabbit skeletal muscle actins by polyacrylamide gel electrophoresis indicates distinct differences between these two proteins.