Annealing of gelsolin-severed actin fragments by tropomyosin in the presence of Ca²⁺. Potentiation of the annealing process by caldesmon

Previous results from our laboratory have shown that 1) cultured rat cells contain two classes of tropomyosin (TM), one (high M(r) TMs) with higher M(r) values and greater affinity for actin than the other (low M(r) TMs); 2) presaturation of F-actin with high M(r) TMs, but not with low M(r) TMs, inhibits both actin-severing and actin binding activities of gelsolin; and 3) nonmuscle caldesmon not only enhances the inhibitory effects of high M(r) TMs but also makes low M(r) TMs capable of inhibiting the severing activity of gelsolin (Ishikawa, R., Yamashiro, S., and Matsumura, F. (1989) J. Biol. Chem. 264, 7490-7497). These results suggest that gelsolin has much lower affinity for F-actin-TM-caldesmon complexes than for pure F-actin. We have therefore examined whether addition of TM and/or caldesmon to gelsolin-severed actin filaments can make gelsolin dissociate from barbed ends of actin filaments, resulting in annealing of short actin filaments into long ones. Flow birefringence and electron microscopic studies have suggested that high M(r) TMs slowly and partially anneal gelsolin-severed actin fragments in 3 h, whereas low M(r) TMs have no effects. Nonmuscle caldesmon greatly potentiates the effects of high M(r) TMs and accelerates the process to 20 min, whereas nonmuscle caldesmon alone shows no effects. Furthermore, nonmuscle caldesmon makes low M(r) TMs capable of reversing gelsolin-severing action. Actin binding assay has shown that gelsolin (or a gelsolin-actin complex) is dissociated from these annealed actin filaments. Smooth muscle TM and smooth muscle caldesmon also appear to anneal gelsolin-severed actin fragments as do high M(r) TMs and nonmuscle caldesmon. Calmodulin decreases the potentiation effects of caldesmon as calmodulin inhibits actin binding of caldesmon. These results suggest that tropomyosin and caldesmon may regulate both capping and severing activities of gelsolin.