Actin (green) and ADF/cofilin (red) are disorganized in the absence of tropomyosin (bottom).

The actin cytoskeleton is a highly adaptable framework. In some structures actin polymerizes and depolymerizes rapidly to facilitate quick movement, whereas in others actin monomers turn over quite slowly to maintain structural integrity. On 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. ▪