Shedding of cytoplasmic actins by developing muscle cells.

Young myotubes develop spots or macules of actin and alpha actinin under and adjacent to slender actin strands. Macules are found only before sarcomere formation. They contain cytoplasmic actin, small amounts of alpha actin, and alpha actinin but are devoid of tubulin, myosin and vimentin. In the electron microscope, they are seen to contain small filaments but no other organelles. Macules are found on the culture dish and in the culture medium, as well as on the lower and lateral surfaces of the cells. Both emetine and cycloheximide, at doses that inhibit protein synthesis, accelerate formation and shedding of macules. This presents the first observation of a cellular system in which the changeover from a generalized cytoskeleton to a tissue-specific cytoskeleton involves shedding of the cytoplasmic isoforms of cytoskeletal proteins.