Chicken gizzard tropomyosin: head-to-tail assembly and interaction with F-actin and troponin

Chicken gizzard and rabbit striated muscle tropomyosins have been compared with respect to their abilities to polymerize head-to-tail and to interact with rabbit skeletal F-actin and troponin. By both viscosity and sedimentation equilibrium measurements, the smooth muscle protein was shown to aggregate more extensively than the cardiac (skeletal) tropomyosin. The stoichiometry and cooperativity of binding of gizzard tropomyosin to F-actin were shown to be similar to the striated muscle protein, as were also the effects of MgCl2 and KCl concentration. Although the gizzard protein was found to interact with rabbit striated troponin as indicated by sedimentation velocity measurements, it was eluted at a lower KCl concentration from a column of immobilized troponin than the cardiac (skeletal) protein. The absence of a significant increase in viscosity upon addition of troponin to gizzard tropomyosin is consistent with a minimal influence on its head-to-tail aggregation. Thus while gizzard tropomyosin resembles rabbit cardiac (skeletal) tropomyosin in its propensity for head-to-tail polymerization and in its F-actin binding properties, it is similar to the nonmuscle tropomyosin from platelets in its interactions with skeletal muscle troponin. These observations suggest that the gizzard protein shares structural features with both striated muscle and platelet tropomyosins.