Transitions in contractile protein isozymes during muscle cell differentiation

Abstract

The nature of the myosin synthesized in fused cultures of primary rat muscle cells and the myogenic cell line L6 was investigated. The myosin light chain subunits can be separated on two dimensional gels and the study of these proteins has led to the identification of a new form of light chain, called embryonic LC1 or LC1 emb (Whalen et al. (1978) J. Mol. Biol., 126 , 415–431). The relative rates of synthesis of the adult LC1 F and LC1 emb were measured during the several days following cell fusion. Although LC1 emb is the predominant Lc1 species synthesized two days after cell fusion, an evolution occurs such that by six days after fusion the synthesis of LC1 F is predominant. This result suggests that during normal differentiation myotubes exist which contain a myosin isozyme whose light chain composition is LC1 emb + LC2 F . Myotubes formed by L6 cells are unique in that they do not synthesize any significant amounts of LC1 F . These myotubes may thus represent the embryonic phase of development with respect to the light chains. The myosin heavy chain aggregates during isoelectric focusing and thus no qualitative information can be obtained concerning its nature using two dimensional gels. However, after partial chymotryptic digestion of SDS denatured myosin, a number of large polypeptides are obtained which can be conveniently separated on two dimensional gels. In this way it was shown that the L6 myosin heavy chain is a different isozyme when compared to adult fast and slow myosin heavy chains. Thus L6 cells harbor a heavy chain isozyme which may be of an embryonic type.