We report a case in which sub-stoichiometric binding of an actin-binding protein has profound structural and functional consequences, providing an insight into the fundamental properties of actin regulation. Rng2 is an IQGAP contained in contractile rings in the fission yeastSchizosaccharomyces pombe. Here, we used high-speed atomic force microscopy and electron microscopy and found that sub-stoichiometric binding of the calponin-homology actin-binding domain of Rng2 (Rng2CHD) induces global structural changes in skeletal muscle actin filaments, including shortening of the filament helical pitch. Sub-stoichiometric binding of Rng2CHD also reduced the affinity between actin filaments and muscle myosin II carrying ADP and strongly inhibited the motility of actin filaments on myosin II in vitro. On skeletal muscle myosin II–coated surfaces, Rng2CHD stopped the actin movements at a binding ratio of 11%. Rng2CHD also inhibited actin movements on myosin II of the amoebaDictyostelium, but in this case, by detaching actin filaments from myosin II–coated surfaces. Thus, sparsely bound Rng2CHD induces apparently cooperative structural changes in actin filaments and inhibits force generation by actomyosin II.

The contraction of contractile rings (CRs) depends on sliding between actin filaments and myosin II filaments. The rate of contraction in the fission yeast Schizosaccharomyces pombe is less than 1/120 of the velocity of acto-myosin II movement in vitro, but the mechanism of inhibition has not been described. Here, we found that the calponin-homology actin binding domain of fission yeast IQGAP Rng2 (Rng2CHD) strongly inhibits the motility of actin filaments on skeletal muscle myosin II fragments in vitro, even at a low ratio of bound Rng2CHD to actin protomers, reducing the sliding velocity to half when the binding ratio was 1/75. Rng2CHD also induced structural changes of actin filaments and reduced the affinity between actin filaments and subfragment 1 (S1) of muscle myosin II carrying ADP. Intriguingly, actin-activated ATPase of S1 was only mildly inhibited, even by high concentrations of Rng2CHD. Moreover, the motility of actin filaments by myosin V was not inhibited by Rng2CHD. We propose a new regulatory mechanism for acto-myosin II movement that involves Rng2CHD-induced structural changes of actin filaments.