The Notch 3 intracellular domain represses Notch 1-mediated activation through Hairy/Enhancer of split (HES) promoters

The Notch signaling pathway is important for cellular differentiation. The current view is that the Notch receptor is cleaved intracellularly upon lig and activation, The intracellular Notch domain then translocates to the nuc leus, binds to Suppressor of Hairless (RBP-Jk in mammals), and acts as a tr ansactivator of Enhancer of Split (HES in mammals) gene expression. In this report we show that the Notch 3 intracellular domain (IC), in contrast to ail other analysed Notch ICs, is a poor activator, and in fact acts as a re pressor by blocking the ability of the Notch 1 IC to activate expression th rough the HES-1 and HES-5 promoters. We present a model in which Notch 3 IC interferes with Notch 1 IC-mediated activation at two levels. First, Notch 3 IC competes with Notch 1 IC for access to RBP-Jk and does not activate t ranscription when positioned close to a promoter. Second, Notch 3 IC appear s to compete with Notch 1 IC for a common coactivator present in limiting a mounts. In conclusion, this is the first example of a Notch IC that functio ns as a repressor in Enhancer of Split/HES upregulation, and shows that mam malian Notch receptors have acquired distinct functions during evolution.