THE INTRACELLULAR DOMAIN OF MOUSE NOTCH - A CONSTITUTIVELY ACTIVATED REPRESSOR OF MYOGENESIS DIRECTED AT THE BASIC HELIX-LOOP-HELIX REGION OF MYOD

We show that Myf-5 and mNotch mRNA are both present in the presomitic mesoderm before muscle cell commitment and before muscle structural ge ne activation. The failure of presomitic mesoderm to respond to Myf-5 and express myogenic properties implies that there may be a mechanism in presomitic mesoderm to suppress muscle differentiation. Here we sho w that ectopic expression of the intracellular domain of mNotch (mNotc hIC) functions as a constitutively activated repressor of myogenesis b oth in cultured cells and in frog embryos. Mutagenesis experiments ind icate that the target for inactivation by mNotch is the MyoD basic hel ix-loop-helix domain. mNotchIC contains a nuclear localization signal and localizes to the nucleus. Removal of the nuclear localization sign al (NLS) reduces nuclear localization and diminishes the inhibition of myogenesis caused by Myf-5 or MyoD. Additional experiments show that the CDC10/SW16/ankyrin repeats are also necessary for myogenic inhibit ion.