Cotranslational assembly of myosin heavy chain in developing cultured skeletal muscle.

To examine how nascent myosin heavy chains associate with the cytoskeletons of developing muscle cells, we used pulse labeling, cell fractionation, and immunoprecipitation. More than 80% of nascent myosin heavy chains associate with the cytoskeleton. More than one-third of these nascent chains are not released by puromycin and/or RNase. The fraction of nascent heavy chains that resists release increases during development of muscle cells in culture. Treatment with cytochalasin D but not nocodazole decreases myosin heavy chain cotranslational assembly. These results indicate that (i) cotranslational assembly of myosin heavy chains is developmentally regulated, (ii) structures containing actin and not microtubules may mediate initial association of the heavy chains with the cytoskeleton, and (iii) the site of translation dictates where a significant fraction of the heavy chains will be inserted into the cytoskeleton.