Tropomyosin in the sea urchin egg cortex

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Abstract
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.