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

We report a case in which sub-stoichiometric binding of an actin -binding protein has profound structural and functional conse-quences, providing an insight into the fundamental properties of actin regulation. Rng2 is an IQGAP contained in contractile rings in the fission yeast Schizosaccharomyces 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, in-cluding shortening of the filament helical pitch. Sub-stoichiometric binding of Rng2CHD also reduced the affinity between actin fila-ments 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 amoeba Dictyostelium, but in this case, by detaching actin filaments from myosin II-coated surfaces. Thus, sparsely bound Rng2CHD induces apparently cooperative struc-tural changes in actin filaments and inhibits force generation by actomyosin II.

Automated summary

Sub-stoichiometric binding of an actin-binding protein can induce structural changes in actin filaments and inhibit their movement, affecting actomyosin interactions and force generation.