# Make these paragraphs plagiarism-free. Following ligand binding, signaling is initiated when endocytosis of ligand-rec
Posted: Fri Jul 15, 2022 5:13 pm
# Make these paragraphs plagiarism-free.
Following ligand binding, signaling is initiated whenendocytosis of ligand-receptor complexes induces the unfolding of ajuxtamembrane negative control region (NRR) unique to Notchproteins. Unfolding of the NRR allows access by the protease ADAM10(also known as KUZ), which removes the Notch extracellular domainby cleaving at site 2 (S2); g-secretase then cleaves Notch withinits transmembrane domain at site 3 (S3) to release various forms ofthe NICD. Those that have valine or methionine residues at theamino terminus escape the N-end-rule degradation pathway (Tagami etal. 2008) and are stable enough to impact transcription (seebelow). Interestingly, productive interactions between Notch andits ligands occur when these are present on neighboring cells(i.e., in trans); when receptor and ligand are present on the samecell (i.e., interactions are in cis), activation is inhibited. cisinteractions thus determine whether a cell will signal (the ligandis more abundant than Notch) or receive (Notch is more abundantthan the ligand) (Sprinzak et al. 2010).
Alternatively, in some cases, ligands and receptors can besegregated into different subdomains to allow simultaneoustransmission and reception of signals (Luty et al. 2007). Only onenuclear protein is known to mediate the bulk of Notch signals: CSL(Kopan and Ilagan 2009). CSL is a DNA-binding adaptor thatinteracts with many proteins to build either repressor complexes,which include histone deacetylases (HDACs) that preserve a closedchromatin conformation, or activating complexes, which containNICD, along with other proteins including histoneacetyltransferases (HATs) that open up chromatin. In canonical,CSL-mediated Notch signaling, NICD translocates to the nucleus,binds to CSL, and helps recruit the adaptor protein Masterminated-like (MAML) (Kopan and Ilagan 2009). MAML recruits theHAT p300 and components of the transcription machinery. Thus, everycleaved Notch molecule generates one signaling unit, and tuning theeffectiveness of receptor–ligand interaction directly determinesthe amount of NICD in the nucleus. During the transcriptionalactivation process, NICD is phosphorylated on a degron within itsPEST domain by kinases such as cyclin-dependent kinase 8 (CDK8) andtargeted for proteasome-mediated degradation by E3 ubiquitinligases such as Sel10 (also known as Fbw7). This limits thehalf-life of a canonical Notch signal and resets the cell for thenext pulse of signaling. In addition to the canonical signals,mounting evidence indicates that CSL-independent activities ofNotch also regulate vertebrates (Rangarajan et al. 2001; Demehri etal. 2008) and invertebrate (Ramain et al. 2001) development, butthe biochemical details of this aspect of the pathway are yet to beuncovered. In the absence of ligand, Notch may also be involved inother cellular processes, such as regulating the stability ofb-catenin (Sanders et al. 2009), a component of the Wnt signalingpathway (Nusse 2012). Under most physiological conditions, unboundNotch receptors simply recycle or are targeted for lysosomaldegradation. With one exception (Mukherjee et al. 2011), onlypathological or experimental conditions are known to lead toreceptor activation without ligand. These include mutations in theNRR domain (Weng et al. 2004), overexpression of Notch with ADAMproteases, and exposure to calcium chelators (Bozkulak andWeinmaster 2009; van Tetering et al. 2009), all of which exposeNotch to shedding by ADAM17 (also known as TACE). Notch can alsobecome activated when ESCRT components are mutated (Moberg et al.2005; Thompson et al. 2005; Vaccari and Bilder 2005), which delaysentry into the lysosome and permits ligand-independent activation.The frequent activation of Notch by mutations in T-cell acutelymphoblastic leukemia and its frequent inactivation in head andneck squamous cell carcinomas (Agrawal et al. 2011; Stransky et al.2011) illustrate the importance of the pathway for control of cellfate and proliferation and the severe consequences of itsdysregulation.
Following ligand binding, signaling is initiated whenendocytosis of ligand-receptor complexes induces the unfolding of ajuxtamembrane negative control region (NRR) unique to Notchproteins. Unfolding of the NRR allows access by the protease ADAM10(also known as KUZ), which removes the Notch extracellular domainby cleaving at site 2 (S2); g-secretase then cleaves Notch withinits transmembrane domain at site 3 (S3) to release various forms ofthe NICD. Those that have valine or methionine residues at theamino terminus escape the N-end-rule degradation pathway (Tagami etal. 2008) and are stable enough to impact transcription (seebelow). Interestingly, productive interactions between Notch andits ligands occur when these are present on neighboring cells(i.e., in trans); when receptor and ligand are present on the samecell (i.e., interactions are in cis), activation is inhibited. cisinteractions thus determine whether a cell will signal (the ligandis more abundant than Notch) or receive (Notch is more abundantthan the ligand) (Sprinzak et al. 2010).
Alternatively, in some cases, ligands and receptors can besegregated into different subdomains to allow simultaneoustransmission and reception of signals (Luty et al. 2007). Only onenuclear protein is known to mediate the bulk of Notch signals: CSL(Kopan and Ilagan 2009). CSL is a DNA-binding adaptor thatinteracts with many proteins to build either repressor complexes,which include histone deacetylases (HDACs) that preserve a closedchromatin conformation, or activating complexes, which containNICD, along with other proteins including histoneacetyltransferases (HATs) that open up chromatin. In canonical,CSL-mediated Notch signaling, NICD translocates to the nucleus,binds to CSL, and helps recruit the adaptor protein Masterminated-like (MAML) (Kopan and Ilagan 2009). MAML recruits theHAT p300 and components of the transcription machinery. Thus, everycleaved Notch molecule generates one signaling unit, and tuning theeffectiveness of receptor–ligand interaction directly determinesthe amount of NICD in the nucleus. During the transcriptionalactivation process, NICD is phosphorylated on a degron within itsPEST domain by kinases such as cyclin-dependent kinase 8 (CDK8) andtargeted for proteasome-mediated degradation by E3 ubiquitinligases such as Sel10 (also known as Fbw7). This limits thehalf-life of a canonical Notch signal and resets the cell for thenext pulse of signaling. In addition to the canonical signals,mounting evidence indicates that CSL-independent activities ofNotch also regulate vertebrates (Rangarajan et al. 2001; Demehri etal. 2008) and invertebrate (Ramain et al. 2001) development, butthe biochemical details of this aspect of the pathway are yet to beuncovered. In the absence of ligand, Notch may also be involved inother cellular processes, such as regulating the stability ofb-catenin (Sanders et al. 2009), a component of the Wnt signalingpathway (Nusse 2012). Under most physiological conditions, unboundNotch receptors simply recycle or are targeted for lysosomaldegradation. With one exception (Mukherjee et al. 2011), onlypathological or experimental conditions are known to lead toreceptor activation without ligand. These include mutations in theNRR domain (Weng et al. 2004), overexpression of Notch with ADAMproteases, and exposure to calcium chelators (Bozkulak andWeinmaster 2009; van Tetering et al. 2009), all of which exposeNotch to shedding by ADAM17 (also known as TACE). Notch can alsobecome activated when ESCRT components are mutated (Moberg et al.2005; Thompson et al. 2005; Vaccari and Bilder 2005), which delaysentry into the lysosome and permits ligand-independent activation.The frequent activation of Notch by mutations in T-cell acutelymphoblastic leukemia and its frequent inactivation in head andneck squamous cell carcinomas (Agrawal et al. 2011; Stransky et al.2011) illustrate the importance of the pathway for control of cellfate and proliferation and the severe consequences of itsdysregulation.