page 1065, Ono and Ono show that competition between two actin-binding proteins, ADF/cofilin and tropomyosin, determines the stability of actin thin filaments in C. elegans muscle.
Previously, the authors found that worms with mutations in a muscle-specific ADF/cofilin isoform could not assemble normal myofibrils. ADF/cofilin appears to increase actin turnover, but myofibrils are highly stable structures, suggesting that some additional factor must inhibit ADF/cofilin in order to stabilize the myofibrils. The new study demonstrates that purified tropomyosin and ADF/cofilin compete for binding to purified F-actin, and that ADF/cofilin cannot bind to isolated myofibrils unless the attached tropomyosin is removed from the myofibrils first. RNAi suppression of tropomyosin disrupts myofibril organization in wild-type worms, but not in ADF/cofilin mutant worms. The results suggest that in vivo tropomyosin preserves myofilaments by blocking the destabilizing effects of ADF/cofilin. ▪