[Regulatory proteins of the cytoskeleton system and their changes associated with carcinogenesis]

Microfilaments are known to participate in several cellular functions. Proteins which cross-link, sever or cap F-actin filaments, and those inhibit polymerization of G-actin may regulate intracellular distribution, length and amount of F-actin to cope with wide varieties of cellular functions of microfilaments. Actinogelin, a calcium-regulated crosslinking protein of F-actin, was discovered in and purified from Ehrlich tumor cells. Native molecule of the protein was found to be consisted of two 110,000-115,000 dalton subunits. Gelation of F-actin can be induced by the addition of the purified protein, and this can be inhibited by Ca2+ (half maximal inhibition, 2 microM). Actinogelin is distributed in several types of cells including fibroblasts, epithelial cells, macrophages and lymphocytes. This protein is found to localize at crossing or converging points of stress fibers (bundle of microfilaments) by immunofluorescence staining using anti-actinogelin antibody. Adhesion plaques of fibroblasts are also stained. Since oncogene product of Rous sarcoma virus, pp 60src, is also present in adhesion plaques, possible phosphorylation of actinogelin in a RSV-transformed cells was studied by the immunoblotting technique. It was found that phosphorylation of actinogelin occurred only at permissive temperature. This modification of the protein might be a cause of disappearance of stress fibers from cancer cells.