Invariance and heterogeneity in the major structural and regulatory proteins of chick muscle cells revealed by two-dimensional gel electrophoresis (100-A filaments/muscle contractile proteins/two-dimensional isoelectric focusing/sodium dodecyl sulfate gel electrophoresis)

A two-dimensional gel electrophoresis system is used to investigate some of the properties of desmin, the major subunit of the 100-A filaments from chick muscle cells, and to compare these properties to those of the other major contractile and regulatory proteins of muscle. Desmin from embryonic and adult smooth, skeletal, and cardiac muscle cells is resolved into two isoelectric variants, a and A, which possess slightly different electrophoretic mobilities in sodium dodecyl sulfate/polyac- rylamide gel electrophoresis. Both the a and the A variants from all six preparations appear to be identical in isoelectric point and apparent molecular weight. The a and A desmin are present in approximately equal amounts in all three types of muscle, suggesting that both isoelectric variants of desmin serve as the structural subunits of the 100-A filaments in chick muscle cells. Tropomyosin also can be resolved into two subunits, a and A, in all three types of muscle. However, in each type of muscle both subunits differ from their counterparts in the other types of muscle, either by molecular weight or by isoelectric point. These results indicate that, with regard to apparent isoelectric point and molecular weight, desmin, a major muscle structural protein, is invariant, while tropomyosin, a major muscle regu- latory protein, exhibits heterogeneity in the three types of muscle.