QUANTITATIVE-ANALYSIS OF LOW-MOLECULAR-WEIGHT G-ACTIN-BINDING PROTEINS, COFILIN, ADF AND PROFILIN, EXPRESSED IN DEVELOPING AND DEGENERATINGCHICKEN SKELETAL-MUSCLES

A large amount of G-actin is pooled in the cytoplasm of young embryoni c skeletal muscle and, although its concentration is reduced as muscle develops, the total amount of actin in muscle cells increases remarka bly. Three G-actin-binding proteins, cofilin, ADF and profilin, are kn own to be involved in creating the G-actin pool in the embryonic muscl e. To better understand how they are responsible for the regulation of assembly and disassembly of actin in developing and degenerating musc les, we measured the amounts of the three G-actin-binding proteins by means of quantitative immunoblotting and compared them with that of G- actin. The sum of the amounts of the three actin-binding proteins was insufficient at early developmental stages but sufficient at later sta ges to account for the pool of G-actin in young muscle cells. It decre ased in parallel with the decrease in the G-actin pool as muscle devel oped. Expression of thymosin beta 4 which is known to be extremely imp ortant for G-actin-sequestering in a variety of non-muscle cells, was detected at a considerable level in young embryonic but not in adult s keletal muscles according to Northern and Western blotting. In degener ating denervated and dystrophic muscles, cofilin and profilin, but not ADF, were significantly increased in amount. From these results, we c onclude that the G-actin pool in young embryonic skeletal muscle is ma inly due to cofilin, ADF, profilin and thymosin beta 4, but thymosin b eta 4 as well as ADF becomes less important as muscle develops. Cofili n and profilin may also be involved in the redistribution of actin dur ing myofibrillogenesis and in the process of actin disassembly in dege nerating muscles.