Selective isoactin release from cultured embryonic skeletal muscle cells.

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Abstract
The culture medium of embryonic quail myoblasts, labeled for 24 h with [35S]L-methionine, was analyzed by 2-dimensional gel autoradiography. The major polypeptide observed had a 43,000 MW and an isoelectric point of 5.4. This polypeptide could be specifically adsorbed to DNase-I Sepharose. A tryptic peptide map of the [35S]methionine-labeled peptides of intracellular actin and the extracellular major polypeptide were virtually identical. These findings identify the released polypeptide as actin. A comparison of 2-dimensional gel patterns of intracellular and extracellular labeled polypeptides showed a large number of differences indicating the actin release did not result from general cellular breakdown. The released actin was not filamentous as judged by its behavior during Bio-Gel A-5m chromatography. The released actin did not originate solely from contaminating fibroblasts in the culture because actin was observed in the medium in clonal myoblast cultures and in purified myotube preparations. The nonmuscle isoactins, as opposed to muscle .alpha.-isoactin, were released preferentially. Within the developing muscle cell where both muscle and nonmuscle specific isoactins are simultaneously present, the different isoactins may be physically or functionally compartmentalized with the nonmuscle isoactins existing primarily at or near the cell surface.