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Myogenin expression is necessary for commitment to differentiation and is closely related to src tyrosine kinase activity in quail myoblasts transformed with Rous sarcoma virus.

Quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) proliferate at 35.5 degrees C, a permissive temperature for RSV, but differentiate at 41 degrees C, a nonpermissive temperature, with the formation of multinucleated myotubes and the synthesis of muscle-specific proteins. Tyrosine kinase activity of the src gene product derived from RSV is closely related to regulation of this temperature-dependent differentiation, and the cells obtain commitment to differentiation by incubation for about 12 h at 41 degrees C with dephosphorylation of tyrosine-phosphorylated protein(s). It was examined how myogenin, a member of myogenic regulatory factors, participates in commitment to differentiation and tyrosine dephosphorylation of QM-RSV cells. Myogenin was expressed within 8 h and reached a plateau within 10 h at 41 degrees C. Each cell clone whose differentiation proceeded faster or slower than the parental QM-RSV cells was reflected by a faster or slower myogenin expression, corresponding to the time that is required for commitment to differentiation. It was suggested that there is a lag time between myogenin expression and the acquisition of commitment in QM-RSV cells. On the other hand, at 35.5 degrees C, a condition which suppresses differentiation, myogenin expression was not detected. However, herbimycin A, an inhibitor of protein tyrosine kinase, induced myogenin expression even at 35.5 degrees C. On the contrary, myogenin expression was inhibited at 41 degrees C by sodium orthovanadate, an inhibitor of tyrosine-phosphorylated protein phosphatase. Furthermore, forced induction of myogenin into the cells cultured at 35.5 degrees C resulted in the formation of multinucleated myotubes and the synthesis of muscle-specific proteins. These results suggest that myogenin expression is one of the indispensable conditions for the acquisition of commitment to differentiation and is regulated by tyrosine phosphorylation and dephosphorylation of some protein(s) in QM-RSV cells.