I-MF, A NOVEL MYOGENIC REPRESSOR, INTERACTS WITH MEMBERS OF THE MYOD FAMILY

During embryogenesis, cells from the ventral and dorsal parts of the s omites give rise to sclerotome and dermomyotome, respectively. Dermomy otome contains skeletal muscle precursors that are determined by the M yoD family of myogenic factors. We have isolated a novel myogenic repr essor, I-mf (Inhibitor of MyoD family), which is highly expressed in t he sclerotome. In contrast, MyoD family members are concentrated in th e dermomyotome. We demonstrate that I-mf inhibits the transactivation activity of the MyoD family and represses myogenesis. I-mf associates with MyoD family members and retains them in the cytoplasm by masking their nuclear localization signals. I-mf can also interfere with the D NA binding activity of MyoD family members. We postulate that I-mf pla ys an important role in the patterning of the somite early in developm ent.