Ca(2+)-dependent inhibition of actin-activated myosin ATPase activity by S100C (S100A11), a novel member of the S100 protein family.

S100C (S100A11, calgizzarin) inhibits the actin-activated myosin Mg(2+)-ATPase activity of smooth muscle in a dose-dependent manner: its half-maximal effect occurs at a S100C/actin molar ratio of 0.05 and its maximal effect occurs at a ratio of 0.20. Furthermore, S100C was found to bind to actin with a stoichiometry of 1:6-7 in the presence of Ca(2+), with an affinity of 1 x 10(-6) M determined by cosedimentation assays. Other Ca(2+)-binding proteins such as S100A1, S100A2, S100B, and calmodulin did not inhibit actin-activated myosin Mg(2+)-ATPase activity. Calmodulin, S100A1, and S100B reversed the inhibitory effect of calponin in a Ca(2+)-dependent manner, S100A2 had no effect, and S100C had additional inhibitory effects. The results suggest that S100C might be involved in the regulation of actin-activated myosin Mg(2+)-ATPase activity through its Ca(2+)-dependent interaction with actin filaments. Copyright 2000 Academic Press.