In the present work, we show that actin is present in considerable quantities in the oocyte nucleus of the newt Pleurodeles waltlii. The nuclear sap, extracted in saline buffer containing Ca++, is fluid. DNAase I inhibition assays have shown that 90% of actin is under a globular state in such conditions. Chelation of Ca++ by EGTA leads to the formation of a nuclear gel composed of individual microfilaments. This nuclear gel contains approximately 50% of total nuclear actin in a filamentous form. Phalloidin, a drug known to stabilize F-actin, induces the formation of a network of actin cables in the nuclei. This network contains nearly 100% of total nuclear actin in the filamentous form. The observation of the cables in the electron microscope shows that they are made of tightly associated microfilaments to which RNP-like particles are bound. The actin antibodies stain the cables and the particles by the indirect immunoperoxidase technique; myosin antibodies mainly stain the particles. The formation of the phalloïdin-induced network seems to require the presence of Ca++, Mg++, and ATP. We propose a scheme for the regulation of the supramolecular forms of actin in oocyte nuclei in which a delicate equilibrium seems to exist between globular actin, microfilaments, and actin cables. This equilibrium would be controlled by the concentration of Ca++, ATP, and various actin-associated proteins.

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