In the Notch signalling pathway, upon ligand binding, Notch is cleaved by γ-secretase at the membrane. The Notch intracellular domain (NICD) then translocates into the nucleus and acts as a co-activator for the transcription factor, CSL. However, how CSL reaches the nucleus has been unclear. Now, Sarah Bray and colleagues employ an optogenetic approach in Drosophila to interrogate the Notch signalling mechanism from a new perspective. The authors engineer ‘OptIC-Notch’, a membrane-tethered NICD, which is released upon light activation. OptIC-Notch can sequester CSL in the cytoplasm, suggesting that the NICD interacts with CSL and could chaperone it into the nucleus. Indeed, CSL nuclear localisation increases after blue light exposure or γ-secretase-dependent cleavage, indicating that the CSL-interacting domain is hidden in full-length Notch. The researchers designed OptIC-Notch {ω}, in which the hydrophobic ΦWΦP motif of NICD is masked unless exposed to light. In darkness, OptIC-Notch {ω} does not sequester CSL in the cytoplasm. Light induction reveals the ΦWΦP motif and, when accompanied by cleavage to releases NICD, results in target gene activation. The authors propose a model by which Notch cleavage reveals the CSL-interacting ΦWΦP domain in the NICD, which is capable of shuttling cytoplasmic CSL into the nucleus.
