The highly conserved coronins were first identified as actin-binding proteins. But although they are crucial to the dynamics of actin filaments and cell movement, their actin-binding sites have been difficult to pin down. Now, on p. 1779, James Bear and colleagues report a point mutation in a highly conserved region of coronin 1B that prevents its binding to F-actin. They show that the protein has to bind to F-actin at the leading edge in order to affect cell motility, and they go on to describe why. The turnover of actin filaments is regulated by another actin-binding protein, cofilin, which severs and depolymerises filaments. Previous work had suggested that coronin helps cofilin to bind to and sever actin, but Bear et al. find that the binding of cofilin to F-actin is not enhanced by coronin 1A or 1B. In fact, they report that coronin 1B protects actin filaments from being depolymerised by cofilin, and conclude that the two proteins bind antagonistically to actin filaments.