Acumentin, an actin-modulating protein of rabbit pulmonary macrophages.

An actin-modulating protein has been purified from rabbit alveolar macrophages utilizing DEAE-Sepharose and gel filtration chromatography. The purified protein which we have named acumentin is similar in structure and function to a protein found in human granulocytes [Southwick, F.S., & Stossel, T.P. (1981) J. Biol. Chem. 256, 3030-3036] and has a Stokes radius of 34 A and s20,w of 4.02 S, consistent with a globular protein with a native molecular weight of 63 500. Acumentin has a molecular weight of 65 000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. This protein is present in high concentrations in macrophages, representing about 6% of the total protein in cytoplasmic extracts. Acumentin caps the pointed end of actin filaments labeled with heavy meromyosin [Southwick, F.S., & Hartwig, J.H. (1982) Nature (London) 297, 303-307], 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 concentrations above 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 calcium-insensitive capping protein in both the human granulocyte and rabbit alveolar macrophage suggests acumentin may be of general importance in constitutively maintaining a shortened actin filament length distribution in the cytoplasm of the nonmuscle cell.