Notch signaling regulates many aspects of metazoan development and tissue renewal. Accordingly, misregulation or loss of Notch signaling underlies multiple human disorders, from developmental syndromes to adult onset diseases and cancer. Notch receptor activation is irreversible as it involves proteolysis-mediated release of the Notch intracellular domain, translocation to the nucleus, and association with a DNA-bound protein. Even though each Notch molecule signals only once without amplification by secondary messenger cascades, Notch signaling is remarkably robust in most tissues. In this review, we highlight the recent studies that reveal new molecular details involved in regulating ligand-mediated activation, receptor proteolysis and target selection.

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Notch receptors and the amyloid precursor protein are type I membrane proteins that are proteolytically cleaved within their transmembrane domains by a presenilin (PS)-dependent ␥-secretase activity. In both proteins, two peptide bonds are hydrolyzed: one near the inner leaflet and the other in the middle of the transmembrane domain. Under saturating conditions the substrates compete with each other for proteolysis, but not for binding to PS. At least some Alzheimer's disease-causing PS mutations reside in proteins possessing low catalytic activity. We demonstrate (i) that differentially tagged PS molecules coimmunoprecipitate, and (ii) that PS N-terminal fragment dimers exist by using a photoaffinity probe based on a transition state analog ␥-secretase inhibitor. We propose that ␥-secretase contains a PS dimer in its catalytic core, that binding of substrate is at a site separate from the active site, and that substrate is cleaved at the interface of two PS molecules.

Ligand-induced proteolysis of Notch produces an intracellular effector domain that transduces essential signals by regulating target gene transcription. This function relies on formation of transcriptional activation complexes that include intracellular Notch, a Mastermind co-activator, and the CSL transcription factor bound to cognate DNA. These complexes form higher order assemblies on paired, head-to-head CSL recognition sites. Here, we report the X-ray structure of a dimeric human Notch1 transcription complex loaded on the paired site from the human HES1 promoter. The small interface between the Notch ankyrin domains can accommodate DNA bending and untwisting to allow a range of spacer lengths between the two sites. Remarkably, cooperative dimerization occurs on the Hes5 promoter at a sequence that diverges from the CSLbinding consensus at one of the sites. These studies reveal how promoter organizational features control cooperativity and thus, the responsiveness of different promoters to Notch signaling. Notch proteins are highly conserved transmembrane receptors that regulate numerous developmental events through cell-cell contact in multicellular organisms. The importance of Notch as a developmental regulator in mammals is underscored by the developmental defects and embryonic lethality associated with loss-of-function of core components of the Notch pathway in both mice and humans 1 . The requirement for tight control of Notch activity is also highlighted by the association of dysregulated Notch activity with cancer. Most notably, mutations in human Notch1 that lead to increased Notch signaling activity are found in more than half of T cell acute lymphoblastic leukemia/lymphomas 2 .Notch receptors are normally activated by regulated intramembrane proteolysis (RIP)3. After engagement with ligand, Notch becomes sensitive to metalloprotease cleavage at a juxtamembrane site 2 (refs. 4 , 5), resulting in ectodomain shedding. This step creates a transient membrane-tethered species that is subsequently cleaved by the γ-secretase multiprotein enzyme complex6 -10 to release the intracellular portion of Notch (ICN, NICD Though the X-ray structures explain how Notch nuclear effector complexes readily assemble onto DNA containing a single CSL binding site, how the arrangement of multiple CSL binding sites influences the transcriptional activity of various Notch-responsive promoters has remained elusive. Some progress into understanding how cooperativity might take place has emerged from consideration of crystal contacts between two copies of the ANK domain related by a 2-fold symmetry axis 18 in the structure of the human monomeric complex. NIH Public AccessThe interface between the symmetry-related complexes orients their respective single-site DNA duplexes head-to-head in a near-linear orientation about 65 Å apart. Biochemical studies showed that mutation of interacting residues at the crystal-packing interface of the Notch intracellular domains (NICD) disrupts cooperative complex assembly on the human H...