Actin filaments are intrinsically dynamic but their stabilisation is essential for actomyosin-based contractility and for maintaining stable cell structures. Tropomyosin stabilises actin filaments by preventing binding of the actin-depolymerising factor cofilin. But, report Shoichiro Ono and colleagues, the nematode protein UNC-87 also antagonises the effects of cofilin (see ). UNC-87 contains seven calponin-like (CLIK), actin-binding repeats. The authors show that UNC-87 competes with UNC-60B (a muscle-specific form of nematode cofilin) for binding to actin filaments in vitro and inhibits cofilin-dependent actin severing and depolymerisation. In vivo, actin filament stability in the C. elegans body wall muscle is reduced in unc-87 mutants; this disorganised actin phenotype is suppressed in unc-87 unc-60B double mutants. Other results indicate that UNC-87 has a more potent antagonistic effect on cofilin activity than tropomyosin and that the functions of the two actin-stabilising proteins are not identical in vivo. The authors propose, therefore, that tropomyosin and UNC-87 have distinct cellular roles in regulation of actin dynamics.
