This project compares regulation of muscle contraction in different animals. In vertebrate muscle the regulatory system consists of tropomyosin and troponin which prevent sites on actin from reacting with myosin. We have found that in molluscan muscles sites on myosin are prevented from interacting with actin in the absence of calcium. Molluscan muscles do not contain troponin and myosin-linked regulation does not require tropomyosin; it is dependent, however, on a light chain component of myosin. We will study the distribution of the actin-, and myosin-linked regulatory systems in the animal kingdom, compare the fingerprints of the regulatory light chains from myosins of different animals, and see if the various regulatory light chains are interchangeable. We plan to study the mechanism of myosin-linked regulation and its possible role in the control of the "catch" state. The structure of invertebrate striated muscles having myosin-linked regulation (scallop) and actin-linked regulation (Limulus, Balanus) will be compared using X-ray diffraction and electron microscopy. The structure of paramyosin and its contribution to the structure of the thick filaments in the "catch" and "phasic" state will be explored.