TRANSREGULATION OF MYOGENIN PROMOTER ENHANCER ACTIVITY BY C-SKI DURING SKELETAL-MUSCLE DIFFERENTIATION - THE C-TERMINUS OF THE C-SKI PROTEIN IS ESSENTIAL FOR TRANSCRIPTIONAL REGULATORY ACTIVITY IN MYOTUBES/

c-ski gene product is a nuclear protein with myogenesis-promoting and transforming activities. We have analysed the effects of c-ski transfe ction on the promoter/enhancer activity of the upstream region of the myogenin gene during in vitro myogenesis using CAT reporter assay. Whe n co-transfected with c-ski into myogenic C2C12 cells, promoter/enhanc er activity was efficiently suppressed in proliferating cells, but the myogenesis-induced increase in activity was potentiated approximately ten limes more (150-fold in the ski-transfected cells) than the ordin ary increase (12-fold in the mock) 48 h after induction of differentia tion. In non-myogenic 10T1/2 cells, c-ski transfection caused persiste nt suppression of promoter/enhancer activity in both proliferating and growth-arrested (i.e. myogenesis-inducing) conditions. Thus the ski-d ependent potentiation of myogenin gene transcriptional activity appear s to be specific for myogenesis. The C-terminal region (amino acids 59 5-663) of the c-Ski protein was essential for the potentiating activit y in myotubes, Other members of the ski-gene family, snoN and snoA, we re ineffective in transactivation, possibly because of the defect in t he corresponding C-terminal region. c-Ski protein underwent a mobility shift on SDS/PAGE after in vitro myogenesis which may explain the con version of the activity from suppressive in myoblasts to potentiating in myotubes. Deletion analysis of the upstream region of the myogenin gene revealed that a responsive element to c-ski in myotubes is locate d at a distinct site upstream of the basal promoter/enhancer region.