Nicastrin binds to membrane-tethered Notch

APH-2/nicastrin functions in LIN-12/Notch signaling in the Caenorhabditis elegans somatic gonad

Nicastrin is a recently identified member of high-molecular weight complexes containing presenilin. The Caenorhabditis elegans homolog of nicastrin, aph-2, was shown to be required for GLP-1/Notch signaling in the early embryo. In addition to the maternal-effect embryonic lethal phenotype, aph-2 mutant animals also display an egg-laying defect. We show that this latter defect is related to the SEL-12/presenilin egg-laying defect. We also show that aph-2 and sel-12 genetically interact and cooperate to regulate LIN-12/Notch signaling in the development of the somatic gonad. In addition, aph-2 and lin-12/Notch genetically interact. We illustrate a new role for aph-2 in facilitating lin-12 signaling in the somatic gonad, thus providing evidence that APH-2 is involved in both GLP-1/Notch- and LIN-12/Notch-mediated signaling events. Finally, we demonstrate that nicastrin can partially substitute for aph-2, suggesting a conservation of function between these proteins.

Nicastrin is required for Presenilin-mediated transmembrane cleavage in Drosophila

The transmembrane glycoprotein Nicastrin was identified in a complex with the multipass membrane protein Presenilin. Presenilin mediates transmembrane cleavage of single-pass transmembrane proteins with short extracellular domains, including the ligand-activated form of the receptor Notch and beta-amyloid precursor protein (beta-APP). Transmembrane cleavage of Notch is essential for signal transduction, and transmembrane cleavage of beta-APP generates pathogenic amyloid peptides implicated in Alzheimer's disease. Here, we investigate the requirement for Nicastrin in Presenilin-mediated transmembrane cleavage. We show that, in Drosophila, loss of Nicastrin activity blocks the accumulation of Presenilin associated with the apical plasma membrane, abolishes Presenilin-dependent cleavage of the transmembrane domains of Notch and beta-APP, and abrogates Notch signal transduction.

Notch and presenilin: a proteolytic mechanism emerges

Presenilins are needed for proteolytic processing of transmembrane proteins of the Notch/Lin-12 family and for cleavage of the amyloid precursor protein. Accumulating evidence now strongly implicates Presenilin as the catalytic core of a multiprotein complex that executes an unusual intramembranous cleavage of its substrates. In the case of amyloid precursor protein, this cleavage contributes to the generation of small, toxic amyloid peptides that trigger the pathological development of Alzheimer's disease. In the Notch/Lin-12 pathway, Presenilin-mediated cleavage of the receptor is a crucial feature of ligand-induced receptor activation and signal transduction. In this pathway, the Presenilins perform a regulated cleavage event that follows additional processing steps during receptor maturation and ligand-induced ectodomain removal.