REGULATION OF ACTIN-BINDING AND ACTIN BUNDLING ACTIVITIES OF FASCIN BY CALDESMON COUPLED WITH TROPOMYOSIN

Human fascin is an actin-bundling protein and is thought to play a rol e in the formation of microfilament bundles of microspikes and stress fibers in cultured cells. To explore the regulation of fascin-actin in teraction, we have examined the effects of culture cell caldesmon and tropomyosin (TM) on actin binding activity of human fascin, Caldesmon alone or TM alone has little or no effect on the actin binding of fasc in, However, caldesmon together with TM completely inhibits actin bind ing of human fascin, When calmodulin is added, the inhibition of fasci n-actin interaction by caldesmon and TM becomes Ca2+ dependent because Ca2+/calmodulin blocks actin binding of caldesmon, Furthermore, as ph osphorylation of caldesmon by cdc2 kinase inhibits actin binding of ca ldesmon, phosphorylation can also control actin binding of fascin in t he presence of TM. As expected by the inhibition of fascin-actin bindi ng, caldesmon coupled with TM also inhibits actin bundling activity of fascin, Whereas smooth muscle caldesmon alone or TM alone shows no ef fect, caldesmon together with TM completely inhibits actin bundling ac tivity of fascin. This inhibition is again Ca2+ dependent when calmodu lin is added to the system. These results suggest important roles for caldesmon and TM in the regulation of the function of human fascin.