Muscle-type tropomyosin of sea urchin egg increases the actin-binding of nonmuscle-type tropomyosin

Tropomyosin isoforms in eggs of several species of sea urchins are classified into two types, muscle-type and nonmuscle-type, based on their antigenicities. Their actin-binding abilities were investigated using muscle-type isoform (32K) and nonmuscle-type isoform (30K), which were purified by the method previously reported and separated by isoelectric focusing from eggs of sea urchin, Strongylocentrotus intermedius. Co-sedimentation assays revealed that 32K could stoichiometrically bind to actin filaments independently of the 30K, but 30K alone bound very poorly. The actin-binding of 30K was, however, considerably increased in the presence of 32K, and the molar ratio of the bound 30K and 32K was approximately 1:1. The increase in the actin-binding of 30K is probably caused by the interaction of 30K with 32K in a head-to-tail manner, as indicated by the higher specific viscosity of the mixture than that of 32K alone.