Making of a retinal cell: insights into retinal cell-fate determination.
Oncogenic forms of Notch1, Notch2, and Notch4 appear to mimic signaling intermediates of Notch1 and suggest that the role of proteolysis in Notch signaling has been conserved. Here we demonstrate that extracellularly truncated Notch homologs are substrates for a presenilin-dependent gamma-secretase activity. Despite minimal conservation within the transmembrane domain, the requirement for a specific amino acid (P1' valine) and its position at the cleavage site relative to the cytosolic border of the transmembrane domain are preserved. Cleaved, untethered Notch intracellular domains from each receptor translocate to the nucleus and interact with the transcriptional regulatory protein CSL. All four Notch proteins display presenilin-dependent transactivating potential on a minimal promoter reporter. Thus, this study increases the number of biochemically characterized gamma-secretase substrates from two to five. Despite a high degree of structural homology and the presenilin-dependent activity of truncated Notch proteins, the extent that this reflects functional redundancy is unknown.