Actin concentration and monomer-polymer ratio in developing chicken skeletal muscle.

The actin concentration and monomer-polymer ratio in developing chicken skeletal muscle were determined by means of a DNase I inhibition assay. The concentration of G-actin in embryonic muscle was much higher than the critical concentration for polymerization of purified actin. As muscle development progressed, the amount of total actin remarkably increased, whereas the concentration of G-actin markedly decreased, and finally in adults reached the critical concentration for polymerization of purified actin. When the monomeric actin in the soluble fraction of embryonic muscle was purified, the critical concentration for polymerization of the embryonic actin decreased to the same value as that of adult skeletal muscle actin. On the other hand, there was no difference between the crude and purified actin in the type of actin. They consisted of alpha-, beta-, and gamma-actins; their amounts were in the order, beta greater than gamma greater than alpha. Furthermore, polymerization of the monomeric actin in the soluble fraction of embryonic muscle was induced by the addition of myosin or HMM. The large amount of monomeric actin in the embryonic skeletal muscle may be due to the presence of some factor(s) which inhibits actin polymerization and also to an insufficiency of myosin.