Ligand-induced cleavage and regulation of nuclear entry of Notch in Drosophila melanogaster embryos

Notch, a transmembrane protein found in a wide range of organisms, is a com ponent of a pathway that mediates cell-fate decisions that involve intercel lular communication. In this paper, we show that in Drosophila melanogaster , Notch (N) is processed in a ligand-dependent fashion to generate phosphor ylated, soluble intracellular derivatives. Suppressor of Hairless [Su(H)] i s predominantly associated with soluble intracellular N. It has been demons trated by others that N has access to the nucleus, and we show that when te thered directly to DNA, the cytoplasmic domain of N can activate transcript ion. Conversely, a viral activator fused to Su(H) can substitute for at lea st some N functions during embryogenesis. We suggest that one function of s oluble forms of N is to bind to Su(H), and in the nucleus, to act directly as a transcriptional transactivator of the latter protein. Although N has f unctional nuclear localization signals, the N/Su(H) complex accumulates in the cytoplasm and on membranes suggesting that its nuclear entry is regulat ed. Localization studies in cultured cells and embryos suggest that Su(H) p lays a role in this regulation, with the relative levels of Delta, N and Su (H) determining whether a N/Su(H) complex enters the nucleus.