Acumentin, an actin-modulating protein of rabbit pulmonary macrophages

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Abstract
An actin-modulating protein was purified from rabbit alveolar macophages by DEAE-Sepharose and gel filtration chromatography. The purified protein, acumentin, is similar in structure and function to a protein found in human granulocytes and has a Stokes radius of 34 .ANG. and s20,w [sedimentation coefficient] of 4.02 S, consistent with a globular protien with a native MW of 63,500. Acumentin has a MW of 65,000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. This protein is present in high concentrations in macrophages, representing .apprx. 6% of the total protein in cytoplasmic extracts. Acumentin caps the pointed end of actin filaments labeled with heavy meromyosin, thereby decreasing the final viscosity of monomeric actin polymerized in its presence without detectably increasing the critical monomer concentration. The activity of this protein is inhibited by KCl concentration > 0.1 M and is completely inactive at a KCl concentration of 0.3 M. Acumentin''s function is equivalent in the presence or absence of CaCl2. The presence of such a Ca-insensitive capping protein in both the human granulocyte and rabbit alveolar macrophage suggests that acumentin may be of general importance in constitutively maintaining a shortened actin length distribution in the cytoplasm of the nonmuscle cell.