Abstract
The premyofibril model proposes a three-stage process for the de novo assembly of myofibrils in cardiac and skeletal muscles: premyofibrils to nascent myofibrils to mature myofibrils. FRAP experiments and jasplakinolide, a drug that stabilizes F-actin, permitted us to determine how decreasing the dynamics of actin filaments affected the dynamics of tropomyosin, troponin-T, troponin-C, and two Z-Band proteins (alpha-actinin, FATZ) in premyofibrils versus mature myofibrils. Jasplakinolide reduced markedly the dynamics of actin in premyofibrils and in mature myofibrils in skeletal muscles. Two isoforms of tropomyosin-1 (TPM1α, TPM1κ) are more dynamic in premyofibrils than in mature myofibrils in control skeletal muscles. Jasplakinolide reduced the exchange rates of tropomyosins in premyofibrils but not in mature myofibrils. The reduced tropomyosin recoveries did not match the YFP-actin recoveries in premyofibrils in jasplakinolide. There were no significant differences in the effects of jasplakinolide on the dynamics of troponins in the thin filaments or of two Z-band proteins in premyofibrils or skeletal mature myofibrils. Cardiac control mature myofibrils lack nebulin, and small decreases in actin (∼5%) and two tropomyosin isoforms (∼10-15%) dynamics are detected in premyofibril to mature myofibril transformations compared with skeletal muscle. In contrast to skeletal muscle, jasplakinolide lowered the dynamics of actin and tropomyosin isoforms in the cardiac mature myofibrils. These results suggest that the dynamics of tropomyosins in control muscle cells are related to actin exchange. These results also suggest a stabilizing role for nebulin, an actin and tropomyosin-binding protein, present in mature myofibrils but not in premyofibrils of skeletal muscles.