Studies on the regulatory mechanism of myofibril assembly

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1. Regulation of Actin Assembly during Myofibrillogeneses A large amount of unpolymrized actin was detected in the skelctal muscle of young chicken embryos. We succeeded in purifying three actin-binding proteins of 16 kDa, 19 kDa and 20 kDa, respectively, which regulate actin assembly in the embryonic skeletal muscle. -and -actins of non-muscle type and -striated muscle actin are co-expressed in the embryonic muscle. The 16 kDa protein was identified with profilin which is widely distributed in non-muscle cells. It exhibited higher affinity to -, -actins than to -actin to inhibit speci-fically their assembly in the embryonic muscle. The 19 kDa protein identified with ADF which was discovered in chicken brain, while the 20 kDa protein resembled cofilin. Both proteins inhibited actin polymerization regardless of the type of actin, but the inhibitory action was removed by myosin. We conclude that actin assembly in developing muscle is enhanced by decrease in the amount of the inhibitory proteins, increase in myosin amount and the change in the type of actin.2. Role of Microtubules in Muscle Formation We demonstrated that microtubles make up the cytoskeletal element which is most responsible for elongation of myotubes during development by immunocytochemistry.3. Characterization of Muscle Structural Proteins in Developing Muscle C-protein and troponin T in embryonic muscle were characterized. The primary structure of embryonic mysoin light chain (L23) which is transiently expressed in embryonic skeletal muscle was determined by gene technology and its assembly in myofilaments was visualized with a specific monoclonal antibody.