Tropomyosin in the sea urchin egg cortex.

Tropomyosin was purified from the Triton-treated cortex fraction of fertilized sea urchin egg. Egg tropomyosin showed characteristics typical of nonmuscle tropomyosins such as low molecular mass, short periodicity of Mg2+-paracrystals, low lysine/arginine ratio, high Mg2+ requirement in binding to F-actin, in addition to the properties of all tropomyosins, namely, stability to high temperature, anomalous migration of SDS/urea gel, dissociation from F-actin under high ionic conditions and very acidic isoelectric point. Co-sedimentation assay of egg tropomyosin with actin in the presence of the previously purified high-molecular-mass actin binding protein (260-kDa protein) showed that these two proteins bind to actin filaments in a non-competitive manner. This suggested that both the proteins play a cooperative role in the formation of actin-filament-based cytoskeletal structure in the cortex.