Our view about the Ca regulation of smooth muscle contraction can be summarized as follows: the regulatory system is actin-linked and consists of tropomyosin and leiotonin; the latter is a complex of two proteins, leiotonin A, the regulatory moiety, and leiotonin C, the Ca-binding moiety. Leiotonin resembles troponin in some respects, but is clearly different in that it has no affinity for tropomyosin and is effective at a leiotonin/actin molar ratio of less than 1:50. The methods of preparing leiotonin were critically reviewed and a suggestion was provided for the development of a new procedure. References were also made to our idea that the myosin light chain phosphorylation may not play an essential role in smooth muscle contraction. With reference to a new type of actin-linked factor from Physarum polycephalum through which Ca2+ represses the actin-myosin-ATP interaction, the diversity of the modes of action of Ca2+ was discussed.
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