Purification and Partial Characterization of a New Protein in Porcine Brain which Bundles Actin Filaments1

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Abstract
A new protein capable of bundling actin filaments was purified from porcine brain by ammonium sulfate fractionation and Sephacryl S-300, hydroxyapatite and Whatman DE 52 column chromatographies. Co-sedimentability of this protein with actin filaments on low speed centrifugation was used as an index in the purification process. This protein had a molecular weight of 53,000 as estimated by SDS-polyacrylamide gel electrophoresis. The Stokes' radius of the protein was determined to be 3.3 nm by the gel filtration method, which indicates a monomeric form of the protein in solution. On isoelectric focusing, it showed a single protein band having a pI of 5.62. This protein caused bundling of actin filaments as seen on electron microscopy, thereby lowering the specific viscosity of the actin solution in a concentration-dependent fashion. The bundling activity of the 53K protein was modulated by changes in ionic strength and pH of the medium as well as by ATP and Mg ions. Low shear falling ball viscometry showed the formation of a gelling structure on mixing of actin filaments with this protein.