Expression of myosin light chains during fetal development of human skeletal muscle.

The expression of myosin light chains (MLCs) during the development of human skeletal muscle was investigated by using two different two-dimensional electrophoretic techniques. In both electrophoretic systems the predominant light chain 1 (LC1) expressed during the whole fetal period was found to co-migrate with the adult fast LC1 (LC1F). The main LC2 expressed during the whole fetal period was found to be different from the main fast LC2 (LC2F) and slow LC2 (LC2S) usually present in adult muscle, but co-migrated with a minor component often present in adult muscle. This fetal LC2 was phosphorylatable, and the phosphorylated form co-migrated with the main component of LC2F expressed in the adult. The adult fast LC3 appeared as early as week 20 of gestation, whereas the adult slow light chains (LC1S and LC2S) appeared only during the late fetal period. A minor component of LC1, previously described in humans as an 'embryonic LC' (LCemb.) [Strohman, Micou-Eastwood, Glass & Matsuda (1983) Science 221, 955-957], was only expressed in the early fetal period and was found to co-migrate with atrial LC1 (ALC1). We discuss the expression of these specific developmental forms of MLCs co-existing with immature myosin heavy chains during fetal life.