Synthesis of heat-shock proteins by cells undergoing myogenesis.

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Abstract
Subjecting 24-h-old cultures of quail myoblasts to incubation at an elevated temperature causes the pattern of protein synthesis to shift from the production of a broad spectrum of different proteins to the enhanced synthesis of a small number of heat-shock proteins. The synthesis of 4 major heat-induced polypeptides with Mr [molecular ratios] of 88,000, 82,000, 64,000 and 25,000 achieve levels comparable to that of the major structural protein, actin. 2-Dimensional electrophoretic separation and fluorographic analysis of these polypeptides establish that those with Mr of 94,000, 88,000, 82,000 and 64,000 and pl [isoelectric point] of 5.1, 5.2, 5.2 and 5.4, respectively, are synthesized by heat-shocked as well as by control (albeit not as intense) cultures. The synthesis of polypeptides with Mr of 94,000, 64,000 and 25,000 and pl'' of 5.2, 5.8 and 5.4, respectively, is detectable only in myoblasts shifted to a higher temperature. Recovery of heat-shocked myoblasts, to a normal preinduction pattern of polypeptide synthesis, takes .apprx. 8 h. Similar studies completed in older, more differentiated myogenic cells demonstrate that as cells progress through myogenesis their ability to respond to a similar temperature shift is diminished. The synthesis of some myoblastlike heat-shock proteins by fusing of cells or by myotubes requires that they be maintained at an elevated temperature at least twice as long as myoblasts. This observation and the demonstration that heat-shocked myotubes do not synthesize detectable levels of the 25,000-dalton polypeptide found in heat-shocked myoblasts, suggest that the synthetic response of myogenic cells to heat shock is dependent on the differentiative state of these cells.