Differential modulation of actin-severing activity of gelsolin by multiple isoforms of cultured rat cell tropomyosin

Multiple isoforms of tropomyosin (TM) of rat cultured cells show differential effects on actin-severing activity of gelsolin. Flow birefringence measurements have revealed that tropomyosin isoforms with high Mrvalues (high MrTMs) partially protect actin filaments from fragmentation by gelsolin, while tropomyosins with low Mrvalues (low MrTMs) have no significant protection even when the actin filaments have been fully saturated with low MrTMs. We have also examined effect of nonmuscle caldesmon on the severing activity of gelsolin because 83-kDa nonmuscle caldesmon stimulates actin binding of rat cell TMs (Yamashiro-Matsumura, S., and Matsumura, F. (1988) J. Cell Biol.106, 1973–1983). While nonmuscle caldesmon alone or low MrTMs alone show no significant protection against fragmentation by gelsolin, the low MrTMs coupled with 83-kDa protein are able to protect actin filaments. Further, high MrTMs together with 83-kDa protein appear to block the severing activity completely. Electron microscopic analyses of length distribution of actin filaments have confirmed the results. The average length of control actin filaments is measured as 1.46 ± 1.0 µM, and gelsolin shortens the average length to 0.084 ± 0.039 µM. Similar short average lengths are obtained when gelsolin severs actin complexed with low MrTMs (0.080 ± 0.045 µM) or with nonmuscle caldesmon (0.11 ± 0.072 µM) while longer average length (0.22 ± 0.18 µM) is measured in the presence of high MrTMs. The simultaneous addition of nonmuscle caldesmon makes the average length considerably longer, i.e.0.61 ± 0.37 µMin the presence of low MrTMs and 1.57 ± 0.97 µMin the presence of high MrTMs. Furthermore, the actin binding of gelsolin is strongly inhibited by co-addition of high MrTMs and nonmuscle caldesmon. These results suggest that TM and gelsolin share the same binding site on actin molecules and that the differences in the actin affinities between TMs are related to their abilities of protection against gelsolin.