Neuralized-like proteins differentially activate Notch ligands

Notch signalling is a major signalling pathway coordinating cellular processes between neighbouring animal cells. In Drosophila, two E3 ubiquitin ligases, Neuralized (Neur) and Mindbomb1 (Mib1), regulate Notch ligand activation and are essential for development. However, the mammalian orthologs of Neur, Neuralized-like (NEURL) 1 and 1B, appear to be dispensable for development, as double knock-out mice show no overt developmental defects. Thus, it is unclear if and how NEURL proteins regulate the mammalian Notch ligands. To address this question, we examined NEURL proteins' ability to activate Notch ligands in a humanized Drosophila model and mammalian cell culture. We found that, unlike MIB1, NEURL proteins activate Notch only with a subset of mammalian ligands, which contain a Neuralized binding motif. This motif has the consensus sequence NxxN and is present only in Notch ligands DLL1 and JAG1, but not in DLL4 and JAG2. Thus, our results reveal a differential regulatory mechanism of Notch activation in mammals, which can potentially explain the limited role of NEURL proteins in mammalian development and homeostasis.