1. The Canonical Notch Signaling Pathway: Unfolding the Activation Mechanism 
Raphael Kopan, Ma. Xenia G. Ilagan 
Cell 
Volume 137, Issue 2, Pages (April 2009)
DOI: /j.cell
Copyright © 2009 Elsevier Inc. Terms and Conditions

2. Figure 1
The Core Notch Signaling Pathway Is Mediated by Regulated Proteolysis
The newly translated Notch receptor protein is glycosylated by the enzymes O-fut and Rumi, which are essential for the production of a fully functional receptor. The mature receptor is produced after proteolytic cleavage by PC5/furin at site 1 (S1). It is then targeted to the cell surface as a heterodimer that is held together by noncovalent interactions. In cells expressing the glycosyltransferase Fringe, the O-fucose is extended by Fringe enzymatic activity, thereby altering the ability of specific ligands to activate Notch. The Notch receptor is activated by binding to a ligand presented by a neighboring cell. Endocytosis and membrane trafficking regulate ligand and receptor availability at the cell surface. Ligand endocytosis is also thought to generate mechanical force to promote a conformational change in the bound Notch receptor. This conformational change exposes site 2 (S2) in Notch for cleavage by ADAM metalloproteases (perhaps after heterodimer dissociation at site 1). Juxtamembrane Notch cleavage at site 2 generates the membrane-anchored Notch extracellular truncation (NEXT) fragment, a substrate for the γ-secretase complex. γ-Secretase then cleaves the Notch transmembrane domain in NEXT progressively from site 3 (S3) to site 4 (S4) to release the Notch intracellular domain (NICD) and Nβ peptide. γ-Secretase cleavage can occur at the cell surface or in endosomal compartments, but cleavage at the membrane favors the production of a more stable form of NICD. NICD then enters the nucleus where it associates with the DNA-binding protein CSL (CBF1/RBPjκ/Su(H)/Lag-1). In the absence of NICD, CSL may associate with ubiquitous corepressor (Co-R) proteins and histone deacetylases (HDACs) to repress transcription of some target genes. Upon NICD binding, allosteric changes may occur in CSL that facilitate displacement of transcriptional repressors. The transcriptional coactivator Mastermind (MAM) then recognizes the NICD/CSL interface, and this triprotein complex recruits additional coactivators (Co-A) to activate transcription.
Cell  , DOI: ( /j.cell )
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3. Figure 2
Domain Organization of Notch Pathway Receptors, Ligands, and Coligands
(A) Notch receptors are large type I proteins that contain multiple extracellular EGF-like repeats. Drosophila Notch (dNotch) and the four mammalian Notch paralogs (mNotch1–4) differ in the number of repeats (29–36) but all are much longer than the C. elegans Notch proteins (cLIN-12 and cGLP-1). EGF repeats 11–12 (red) and 24–29 (green) mediate ligand interactions. EGF repeats may contain consensus motifs for fucosylation by O-Fut1 and glucosylation by Rumi; the putative distribution of fucosylation sites (common, green; unique, light blue) and glucosylation sites (common, dark blue; unique, magenta) are shown for mNotch1 and mNotch2. Note that the ligand-binding regions differ in their modification patterns. EGF repeats are followed by the negative regulatory region (NRR), which is composed of three cysteine-rich Lin12-Notch repeats (LNR-A, -B, and -C) and a heterodimerization domain (HD). Notch also contains a transmembrane domain (TMD), a RAM (RBPjκ association module) domain, nuclear localization sequences (NLSs), a seven ankyrin repeats (ANK) domain, and a transactivation domain (TAD) that habors conserved proline/glutamic acid/serine/threonine-rich motifs (PEST). The transactivation domain in Drosophila Notch also has a glutamine-rich repeat (OPA). In contrast to Drosophila Notch, mammalian Notch proteins are cleaved by furin-like convertases at site 1 (S1), which converts the Notch polypeptide into an NECD-NTMIC (Notch extracellular domain-Notch transmembrane and intracellular domain) heterodimer that is held together by noncovalent interactions between the N- and C-terminal halves of the heterodimerization domain. (Inset) Details of the mouse Notch1 transmembrane domain (TMD) (purple box) and flanking residues showing the Notch cleavage sites and corresponding cleavage products. After ligand binding, Notch is cleaved at site 2 by metalloproteases. γ-Secretase can cleave multiple scissile bonds at site 3 (arrows), but only NICD molecules initiating at valine 1744 (V1744) (NICD-V) evade N-end rule degradation. Cleavage then proceeds toward site 4 until the short Nß peptides (most are 21 amino acids) can escape the membrane lipid bilayer. Amino acid substitution of V1744 to glycine (V1744G, red) and lysine 1749 to arginine (K1749R, blue) shift the cleavage site.
(B) Known ligands and putative ligands of Notch receptors can be divided into several groups on the basis of their domain composition. Classical DSL ligands (DSL/DOS/EGF ligands) contain the DSL (Delta/Serrate/LAG-2), DOS (Delta and OSM-11-like proteins), and EGF (epidermal growth factor) motifs and are not found in C. elegans. C. elegans and mammalian DSL-only ligands lacking the DOS motif (DSL/EGF ligands) are a subtype of DSL ligands that may act alone (for example, mDLL4) or in combination with DOS coligands (for example, cDSL-1 and possibly Dll3). This subfamily includes soluble/diffusible ligands. Noncanonical ligands lacking DSL and DOS domains have been reported to activate Notch in some contexts.
Cell  , DOI: ( /j.cell )
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4. Figure 3
Atomic Resolution of Domains in Human Notch1, Notch2, and Jagged1
(A) (Top) The ligand-binding domain (EGF 11–13) from the crystal structure of human Notch1 (PDB:2VJ3). The ligand-binding domain of Notch1 is centered on EGF repeat 12, which contains residues that coordinate Ca2+ binding (blue), as well as O-glucosylation (serine 458, serine 496) and O-fucosylation (threonine 466) sites. Mutation of glutamic acid 455 to valine (E455V) abolishes ligand binding in Drosophila. This region in human Notch1 (hNotch1) was suggested to interact with human Jagged1 (hJagged1) DSL ligand on the basis of computational docking models (Cordle et al., 2008a). However, glycosylation at serine 458 may block access to this site. Threonine 466 is essential for productive Notch activation in the mouse but not in the fly. (Bottom) The negative regulatory region from the crystal structure of human Notch2 (PDB:2OO4). The negative regulatory region folds to protect the S2 cleavage site (green), which is located in a pocket protected by LNR-A, the HD-C helix, and the LNR-A/B linker. The furin cleavage site (S1) lies within an unstructured loop that was removed to facilitate crystallization. LNR repeats bind to calcium ions; chelation of Ca2+ leads to negative regulatory region dissociation and Notch activation.
(B) The crystal structure of the hJagged1 Notch-binding domain (PDB:2VJ2) containing the DSL, DOS, and EGF repeat 3 (EGF-3) regions. (Left) The hJagged1 ribbon structure.The DSL fold is distinct from the EGF fold; DSL amino acids required for interaction with Notch are labeled in red (Cordle et al., 2008a). The phenylalanine 207 to alanine (F207A) substitution generates a null protein. In contrast, arginine 203 to alanine (R203A) and phenylalanine 199 to alanine (F199A) substitutions ablate binding of ligand presented from another cell (trans) but not cis binding of ligand present in the same cell. Aspartic acid 205 to alanine (D205A) and arginine 201 to alanine (R201A) substitutions are hypomorphic. The DOS domain contains two conserved atypical EGF repeats (defined by the presence of the conserved amino acids in blue) (Komatsu et al., 2008). Tyrosine 255 (Y255) is characteristic of Jagged DSL ligands and is replaced by a small hydrophobic amino acid in Delta-like ligands (this residue may be involved in defining sensitivity to Fringe glycosyltransferase activity). (Right) Surface rendering of the hJagged1 ribbon structure. The structure is rotated so that the Notch-binding interface (dotted circles) is facing the reader. Surface exposed residues are indicated by white circles, whereas buried residues are indicated by green circles. Mapping of known Jagged mutations in humans and mouse to the structure indicates that the DOS domain could form part of the Notch-binding interface. Human Alagille syndrome (ALGS)-associated missense mutations (orange) are likely to affect disulfide bonds (and thus the structural integrity of these domains). A positively charged cluster of highly conserved surface-exposed residues within the DSL domain (red) identifies a putative Notch-binding surface. Interestingly, missense mutations (blue) associated with Tetralogy of Fallot in humans and autosomal dominant inner ear malformations in mice (headturner, slalom, and Nodder) cluster near a common DOS region. Mutations in headturner and Tetralogy of Fallot affect amino acids buried under the surface defined by slalom and Nodder mutations and may impact the structure of the potential Notch binding site within DOS. Note that arginine 252 (mutated to lysine in ALGS1) and tyrosine 255 (Y255), which is unique to Jagged, are also aligned with the putative Notch binding surfaces on the DSL and DOS domains. The exact topology of the Notch/ligand interface remains to be explored by cocrystallization. Structures were generated with MacPymol (
Cell  , DOI: ( /j.cell )
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