Actinogelin, which induces gelation of F-actin at Ca2+ concentrations below micromolar concentrations but not at higher concentrations, was isolated in the pure state from Ehrlich tumor cells. The protein consists of subunits of 112,000-115,000 daltons and under physiological conditions is present mostly as a dimer. Up to 1 mol of actinogelin (dimer) binds to 10-12 mol of actin monomer. The binding was slightly decreased by the presence of 50 microM Ca2+ and almost completely inhibited by 300 mM KCl. Antibodies against actinogelin giving a single precipitation line with Ehrlich cell extract and with pure actinogelin were raised in rabbits. Antibody preparations were purified before use in an affinity column containing purified actinogelin. In mouse embryo fibroblasts and 3T3 cells, staining of actin bundles by the antiactinogelin antibody usually was discontinuous or gave a striated appearance. Most of the crossing points of the actin bundles were intensively stained. In epithelial cells from mouse small intestine, actinogelin was distributed throughout the cell, with the exception of the microvilli, which were devoid of staining. In mouse peritoneal cells, the antibody staining patterns were similar to those of tetramethylrhodamine isothiocyanate-labeled heavy meromyosin, but the former usually were sharper than the latter. Intracellular localization of actinogelin was drastically altered by cytochalasin D treatment at 10 microgram/ml. We conclude that actinogelin is present in a wide variety of cell types and discuss the possible participation of actinogelin in the Ca2+-dependent regulation of microfilament distribution.

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