Ca2+-calmodulin regulates fesselin-induced actin polymerization

Fesselin is a proline-rich actin-binding protein that was isolated from avian smooth muscle. Fesselin bundles actin and accelerates actin polymerization by facilitating nucleation. We now show that this polymerization of actin can be regulated by Ca2+-calmodulin. Fesselin was shown to bind to immobilized calmodulin in the presence of Ca2+. The fesselin-calmodulin interaction was confirmed by a Ca2+-dependent increase in 2-(4-maleimidoanilino)naphthalene-6-sulfonic acid (MIANS) fluorescence upon addition of fesselin to MIANS-labeled wheat germ calmodulin. The affinity was estimated to be similar to10(9) M-1. The affinity of Ca2+-calmodulin to the fesselin F-actin complex was approximately 10(8) M-1. Calmodulin binding to fesselin appeared to be functionally significant. In the presence of fesselin and calmodulin, the polymerization of actin was Ca2+-dependent. Ca2+-free calmodulin either had no effect or enhanced the ability of fesselin to accelerate actin polymerization. Ca2+-calmodulin not only reversed the stimulatory effect of fesselin but reduced the rate of polymerization below that observed in the absence of fesselin. While Ca2+-calmodulin had a large effect on the interaction of fesselin with G-actin, the effect on F-actin was small. Neither the binding of fesselin to F-actin nor the subsequent bundling of F-actin was greatly affected by Ca2+-calmodulin. Fesselin may function as an actin-polymerizing factor that is regulated by Ca2+ levels.