Switching of filamin polypeptides during myogenesis in vitro.

Abstract

During chicken skeletal myogenesis in vitro, the actin-binding protein filamin is present at first in association with actin filament bundles both in myoblasts and myotubes early after fusion. Later in mature myotubes it is found in association with myofibril Z discs. These 2 associations of filamin are separated by a period of several days, during which the protein is absent from the cytoplasm of differentiating myotubes. In order to characterize the 2 classes of filamin polypeptides, 125I-labeled filamin immunoprecipitated from myoblasts and fibroblasts was compared, by 2-dimensional peptide mapping, to filamin immunoprecipitated from mature myotubes and adult skeletal myofibrils. Myoblast filamin is highly homologous to fibroblast and purified chicken gizzard filamins. Mature myotube and adult myofibril filamins are highly homologous but exhibit extensive peptide differences with respect to the other 3 classes of filamin. Comparison of peptide maps from immunoprecipitated 35S-methionine-labeled filamins also shows that fibroblast and myoblast filamins are highly homologous but show substantial peptide differences with respect to mature myotube filamin. Filamins from both mature myotubes and skeletal myofibrils exhibit a slightly higher electrophoretic mobility than gizzard, fibroblast and myoblast filamins. Short pulse-labeling studies show that mature myotube filamin is synthesized as a lower MW variant and is not derived from a higher MW precursor. Myoblast and mature myotube filamins are distinct gene products, and during skeletal myogenesis in vitro, 1 class of filamin polypeptides is replaced by a new class of filamin polypeptides, which is maintained into adulthood.