Actin binding domain of Rng2 sparsely bound on F-actin strongly inhibits actin movement on myosin II

Substoichiometric binding of certain actin-binding proteins induces conformational changes in a disproportionally large number of actin protomers in actin filaments. Here, we report a case in which such conformational changes in actin filaments have profound functional consequences. Rng2 is an IQGAP protein implicated in the assembly and contraction of contractile rings in Schizosaccharomyces pombe . We found that the calponin-homology actin-binding domain of Rng2 (Rng2CHD) strongly inhibits the motility of actin filaments on myosin II in vitro. On skeletal muscle myosin II-coated surfaces, Rng2CHD halved the sliding speed of actin filaments at a binding ratio of 1.3% (=1/77), and virtually stopped movement at a binding ratio of 11% (=1/9). Rng2CHD also inhibited actin movements on Dictyostelium myosin II, but in this case by inducing the detachment of actin filaments from myosin II-coated surfaces. Rng2CHD induced cooperative structural changes of actin filaments accompanied by shortening of the filament helical pitch, and reduced the affinity between actin filaments and subfragment 1 (S1) of muscle myosin II in the presence of ADP. Intriguingly, actin-activated ATPase of S1 was hardly inhibited by Rng2CHD. We suggest that sparsely bound Rng2CHD induces global structural changes of actin filaments and interferes with the force generation by actin-myosin II.