Switching of subunit composition of muscle spectrin during myogenesis in vitro

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Abstract
Spectrin comprises a family of polypeptides thought to be involved in mediating linkage of actin filaments to the plasma membrane in a wide variety of cell types (for reviews see refs 1–3). Spectrin is present as a tetramer composed of two non-identical subunits4–6 . Most cells express a common subunit with a molecular weight (Mr) of 240,000 (240 K; termed α-spectrin)7 in association with a polymorphic cell type-specific sub-unit: Mr 260 K in the intestinal terminal web (termed TW260)8, Mr 235 K in nervous tissue6,8,9, liver6, lymphocytes10 and fibroblasts11 (termed γ-spectrin10; also referred to as fodrin9), and Mr 225/220 K in erythrocytes and adult cardiac and skeletal muscle12 (termed β′- and β-spectrin, respectively). We show here that primary chicken myoblasts express predominantly αγ-spectrin, but on terminal differentiation in vitro the cells gradually switch to αβ-spectrin as a result of the onset of β- and β′-spectrin synthesis and by the subsequent differential stabilization of β- and γ-spectrin. This switching correlates with known changes in the biophysical properties and function of the developing muscle sarcolemma and cytoskeleton.

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