Figure 2.

Notch signalling. Notch receptors are synthesized as single precursor proteins that are cleaved in the Golgi by a Furin-like convertase during their transport to the cell surface where they are expressed as heterodimers. Fringe glycosyltransferases modify EGF-like repeats by adding N-acetylglucosamine within the Golgi. Notch signalling is initiated after ligand-receptor interaction, which induces two sequential proteolytic cleavages. The first cleavage within the extracellular domain is mediated by the metalloprotease TACE (tumor necrosis factor α-converting enzyme). The cleaved extracellular subunit of the receptor is ‘trans-endocytosed' by the neighbouring ligand-expressing cell. This process seems to be controlled by Neuralized and/or Mindbomb E3 ubiqutin ligases. The second cleavage occurs within the transmembrane domain and is mediated by the γ-secretase activity of the multi-protein complex of presenilins (PS), which includes Nicastrin, APH-1 and PEN-2. The liberated intracellular domain of Notch (NICD) translocates into the nucleus and binds to the transcription factor CSL (CBF1 in humans, Supressor of Hairless in Drosophila and LAG in C. elegans). This interaction leads to transcriptional activation by displacement of corepressors (CoR) and simultaneous recruitment of coactivators (CoA), including mastermind-like proteins (MAML1). Receptors modified by Fringe glycosyltransferases cannot mediate signalling via Jagged ligands, whereas Delta-mediated Notch signalling is still possible.