Phosphorylation of subunit proteins of intermediate filaments from chicken muscle and nonmuscle cells

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Abstract
The phosphorylation of the subunit proteins of intermediate (10-nm) filaments has been investigated in chicken muscle and nonmuscle cells by using a two-dimensional gel electrophoresis system. Desmin, the 50,000-dalton subunit protein of the intermediate filaments of muscle, had previously been shown to exist as two major isoelectric variants—α and β—in smooth, skeletal, and cardiac chicken muscle. Incubation of skeletal and smooth muscle tissue with 32 PO 4 3- reveals that the acidic variant, α-desmin, and three other desmin variants are phosphorylated in vivo and in vitro . Under the same conditions, minor components of α- and β-tropomyosin from skeletal muscle, but not smooth muscle, are also phosphorylated. Both the phosphorylated desmin variants and the nonphosphorylated β-desmin variant remain insoluble under conditions that solubilize actin and myosin filaments, but leave Z-discs and intermediate filaments insoluble. Primary cultures of embryonic chicken muscle labeled with 32 PO 4 3- possess, in addition to the desmin variants described above, a major nonphosphorylated and multiple phosphorylated variants of the 52,000-dalton, fibroblast-type intermediate filament protein (IFP). Filamentous cytoskeletons, prepared from primary myogenic cultures by Triton X-100 extraction, contain actin and all of the phosphorylated and nonphosphorylated variants of both desmin and the IFP. Similarly, these proteins are the major components of the caps of aggregated 10-nm filaments isolated from the same cell cultures previously exposed to Colcemid. These results demonstrate that a nonphosphorylated and several phosphorylated variants of desmin and IFP are present in assembled structures in muscle and nonmuscle cells.