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 Rou s sarcoma virus (QM-RSV cells) proliferate at 35.5 degrees C, a permis sive temperature for RSV, but differentiate at 41 degrees C, a nonperm issive 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 regulatio n of this temperature-dependent differentiation, and the cells obtain commitment to differentiation by incubation for about 12 h at 41 degre es C with dephosphorylation of tyrosine-phosphorylated protein(s). It was examined how myogenin, a member of myogenic regulatory factors, pa rticipates in commitment to differentiation and tyrosine dephosphoryla tion 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 differentia tion proceeded faster or slower than the parental QM-RSV cells was ref lected by a faster or slower myogenin expression, corresponding to the time that is required for commitment to differentiation. It was sugge sted that there is a lag time between myogenin expression and the acqu isition of commitment in QM-RSV cells. On the other hand, at 35.5 degr ees C, a condition which suppresses differentiation, myogenin expressi on was not detected. However, herbimycin A, an inhibitor of protein ty rosine kinase, induced myogenin expression even at 35.5 degrees C. On the contrary myogenin expression was inhibited at 41 degrees C by sodi um orthovanadate, an inhibitor of tyrosine-phosphorylated protein phos phatase, Furthermore, forced,induction of myogenin into the cells cult ured at 35.5 degrees C resulted in the formation of multinucleated myo tubes and the synthesis of muscle-specific proteins. These results sug gest that myogenin expression is one of the indispensable conditions f or the acquisition of commitment to differentiation and is regulated b y tyrosine phosphorylation and dephosphorylation of some protein(s) in QM-RSV cells.