Repolymerizability of native thin filaments of rabbit skeletal muscle

The repolymerizability of native thin filaments of rabbit skeletal muscle was examined under various conditions. The native thin filaments usually did not repolymerize after depolymerization, but it was found that the process was only inhibited at the nucleation step of the G-F transformation of actin. The inhibited state was released by the addition of sonicated F-actin fragments as seeds or by direct sonication. An attempt to isolate the actin moiety from native thin filaments showed that the isolated actin was very similar to Straub-type G-actin2 in its polymerizability. However, a protein factor which inhibits the polymerization of G-actin was obtained by ammonium sulfate fractionation of the depolymerized native thin filaments. Thus, the apparent lack of repolymerizability of native thin filaments was found to be due to the action of this factor on the polymerization of G-actin.