MUSCLE-TYPE TROPOMYOSIN OF SEA-URCHIN EGG INCREASES THE ACTIN-BINDINGOF NONMUSCLE-TYPE TROPOMYOSIN

Tropomyosin isoforms in eggs of several species of sea urchins are cla ssified 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 wer e purified by the method previously reported and separated by isoelect ric focusing from eggs of sea urchin, Strongylocentrotus intermedius. Go-sedimentation assays revealed that 32K could stoichiometrically bin d to actin filaments independently of the 30K, but 30K alone bound ver y poorly, The actin-binding of 30K was, however, considerably increase d 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 pr obably caused by the interaction of 30K with 32K in a head-to-tail man ner, as indicated by the higher specific viscosity of the mixture than that of 32K alone.