Identification of a factor in conventional muscle actin preparations which inhibits actin filament self-association

Gel filtration of depolymerized conventionally purified muscle actin separates from the actin monomers a fraction of minor contaminants with a Stokes' radius of 4.7 nm which has the ability to block actin filament network formation. On the basis of heat and trypsin sensitivity, this inhibitory activity appears to be a protein. The inhibitory activity binds to actin filaments and reduces their low shear viscosity by up to 99% in a concentration dependent fashion while reducing polymerization to only a minor extent.