30 Jan 2003

A report in the January 24 online Journal of Biological Chemistry indicates that the Notch ligands Delta and Jagged are also substrates for γ-secretase, the intramembrane aspartyl protease responsible for processing of Notch and AβPP.

Like Notch, Delta and Jagged are transmembrane proteins. Delta, also like Notch, can be cleaved by a metalloprotease that removes the extracellular domain and leaves a mostly intracellular C-terminal fragment anchored to the cell membrane. Now, Takeshi Ikeuchi and Sangram Sisodia at the University of Chicago, using a version of Delta engineered with a traceable tag, shows that a slightly smaller version of the C-terminal fragment, which the authors dub D-CTF2, shows up in cultured cells. The size of D-CTF2 suggests that the C-terminal fragment has been cleaved at or close to the cell membrane. This alone does not necessarily implicate γ-secretase in the process, however, when Ikeuchi added a potent γ-secretase inhibitor to the cells, D-CTF2 no longer appeared, nor did it appear in cells expressing inactive γ-secretase. Similar experiments revealed the presence of a Jagged C-terminal fragment (J-CTF1) that is cleaved by γ-secretase to a smaller form (J-CTF2).

But what is the role, if any, of D-CTF2? Could it be translocated to the nucleus to activate transcription, as Notch and AβPP intracellular domains may (see ARF related news story)? To test this hypothesis, Ikeuchi and Sisodia coupled the C-terminal end of Delta to a transcriptional activator and expressed it in cells containing a luciferase gene coupled to the activator's promoter; translocation of the hybrid Delta to the nucleus should be detected by the luciferase's bioluminescence. Sure enough, cells with the hybrid had 70 times more bioluminescence than cells with the luciferase gene alone, indicating that D-CTF2 can indeed travel into the nucleus. A similar fate may await J-CTF2, as the intracellular domains of both Delta and Jagged have putative nuclear localization signals, the cellular equivalent of an entry visa.

These findings lead the authors to speculate that Delta and Jagged may deliver a double whammy to the nucleus-activation of transcription via the Notch pathway, and a similar activation via their own intracellular domains, the targets of which remain to be found.—Tom Fagan