Effect of valine on the efficiency and precision at S4 cleavage of the Notch-1 transmembrane domain

Permissive Conformations of a Transmembrane Helix Allow Intramembrane Proteolysis by γ-Secretase

The intramembrane protease γ-secretase activates important signaling molecules, such as Notch receptors. It is still unclear, however, how different elements within the primary structure of substrate transmembrane domains (TMDs) contribute to their cleavability. Using a newly developed yeast-based cleavage assay, we identified three crucial regions within the TMDs of the paralogs Notch1 and Notch3 by mutational and gain-of-function approaches. The AAAA or AGAV motifs within the N-terminal half of the TMDs were found to confer strong conformational flexibility to these TMD helices, as determined by mutagenesis coupled to deuterium/hydrogen exchange. Crucial amino acids within the C-terminal half may support substrate docking into the catalytic cleft of presenilin, the enzymatic subunit of γ-secretase. Further, residues close to the C-termini of the TMDs may stabilize a tripartite β-sheet in the substrate/enzyme complex. NMR structures reveal different extents of helix bending as well as an ability to adopt widely differing conformational substates, depending on the sequence of the N-terminal half. The difference in cleavability between Notch1 and Notch3 TMDs is jointly determined by the conformational repertoires of the TMD helices and the sequences of the C-terminal half, as suggested by mutagenesis and building molecular models. In sum, cleavability of a γ-secretase substrate is enabled by different functions of cooperating TMD regions, which deepens our mechanistic understanding of substrate/non-substrate discrimination in intramembrane proteolysis.

Conformationally Flexible Sites within the Transmembrane Helices of Amyloid Precursor Protein and Notch1 Receptor

Intramembrane proteases typically cleave multiple substrates within their transmembrane domains (TMDs). Because substrate TMDs lack a consensus sequence around their scissile sites, it remains unclear how the enzyme discriminates substrates from nonsubstrates at the level of their TMDs. Here, we compare the previously well investigated TMDs of γ-secretase substrates C99 and Notch1 in terms of helix flexibility. Our results reveal that the low-stability site neigboring a functionally relevant diglycine hinge of C99 has an equivalent in the Notch1 TMD. This suggests that the tetra-alanine motif of Notch1 also functions as a hinge which may facilitate its cleavage.

Proteolytic cleavage of Notch: "HIT and RUN"

The Notch pathway is a highly conserved signaling pathway in multicellular eukaryotes essential in controlling spatial patterning, morphogenesis and homeostasis in embryonic and adult tissues. Notch proteins coordinate cell-cell communication through receptor-ligand interactions between adjacent cells. Notch signaling is frequently deregulated by oncogenic mutation or overexpression in many cancer types. Notch activity is controlled by three sequential cleavage steps leading to ectodomain shedding and transcriptional activation. Here we review the key regulatory steps in the activation of Notch, from receptor maturation to receptor activation (HIT) via a rate-limiting proteolytic cascade (RUN) in the context of species-specific differences.

Pharmacological analysis of Drosophila melanogaster gamma-secretase with respect to differential proteolysis of Notch and APP

The gamma-secretase aspartyl protease is responsible for the cleavage of numerous type I integral membrane proteins, including amyloid precursor protein (APP) and Notch. APP cleavage contributes to the generation of toxic amyloid beta peptides in Alzheimer's disease, whereas cleavage of the Notch receptor is required for normal physiological signaling between differentiating cells. Mutagenesis studies as well as in vivo analyses of Notch and APP activity in the presence of pharmacological inhibitors indicate that these substrates can be differentially modulated by inhibition of mammalian gamma-secretase, although some biochemical studies instead show nearly identical dose-response inhibitor effects on Notch and APP cleavages. Here, we examine the dose-response effects of several inhibitors on Notch and APP in Drosophila melanogaster cells, which possess a homogeneous form of gamma-secretase. Four different inhibitors that target different domains of gamma-secretase exhibit similar dose-response effects for both substrates, including rank order of inhibitor potencies and effective concentration ranges. For two inhibitors, modest differences in inhibitor dose responses toward Notch and APP were detected, suggesting that inhibitors might be identified that possess some discrimination in their ability to target alternative gamma-secretase substrates. These findings also indicate that despite an overall conservation in inhibitor potencies toward different gamma-secretase substrates, quantitative differences might exist that could be relevant for the development of therapeutically valuable substrate-specific inhibitors.

The insulin-like growth factor 1 (IGF-1) receptor is a substrate for gamma-secretase-mediated intramembrane proteolysis

Several type-1 membrane proteins undergo regulated intramembrane proteolysis resulting in the generation of biologically active protein fragments. Presenilin-dependant gamma-secretase activity is central to this event and includes amyloid precursor protein (APP), Notch and ErbB4 as substrates. Here we show that the insulin-like growth factor 1 receptor (IGF-IR) undergoes regulated intramembrane proteolysis. A metalloprotease-dependant ectodomain-shedding event generates a approximately 52 kDa IGF-IR-carboxyl terminal domain (CTD). The IGF-IR-CTD is consequentially a substrate for gamma-secretase cleavage, liberating a approximately 50 kDa intracellular domain (ICD) that can be inhibited by a specific gamma-secretase inhibitor. This study suggests that the IGF-IR is a substrate for gamma-secretase and may mediate a function independent of its role as a receptor tyrosine kinase.