Ribosomes in the skeletal muscle filament lattice

The compartmentalization of myosin isoforms within a muscle cell (Gauthier: J. Cell Biol. 110:693-701, 1990) suggests that myosin might be assembled directly into thick filaments at sites where it is synthesized. We therefore examined myofibrils by immunoelectron microscopy to determine whether ribosomes are associated with thick filaments under conditions in which new myosin can be identified. We used the embryonic chick anterior latissimus dorsi (ALD), a slow muscle that is induced, by curare, to synthesize a fast myosin isoform that is not normally present. Myosin was localized in situ, using a gold-labeled monoclonal antibody that recognizes the new isoform. The gold marker, as expected, was localized preferentially to the A band. There was an overall increase of fivefold in the number of gold particles per micron2 of A band in the curare-treated compared to the normal ALD, indicating that the labeled isoform was largely newly formed. There was a corresponding preferential distribution of ribosomes at the A band, especially in the H-band region, and the number of ribosomes per micron2 of A band was nearly twice as high in the curare-treated as in the normal muscle. Ribosomes were located between thick filaments, often aligned in rows. We conclude that ribosomes are located within the filament lattice, and therefore that they are available for local myosin synthesis.