4.G.1. The γ-Secretase (γ-Secretase) Family
γ-secretase is an unusual membrane-embedded protease complex, which cleaves the transmembrane domains (TMSs) of type I membrane proteins, including amyloid-beta precursor protein and Notch receptor as well as about 80 other proteins. A hydrophilic pore is thought to be formed by TMS6 and TMS7 of presenilin 1 (PS1), the catalytic subunit of γ-secretase (but see below). TMS8, TMS9 and the C-terminus of PS1, which encompass the conserved PAL motif and the hydrophobic C-terminal tip, are critical for the catalytic activity and formation of the γ-secretase complex. The amino acid residues around the PAL motif and the extracellular/luminal portion of TMS9 are highly water accessible and located in proximity to the catalytic pore (Sato et al., 2008). Furthermore, the region starting from the luminal end of TMS9 toward the C terminus forms an amphipathic α-helix-like structure that extends along the interface between the membrane and the extracellular milieu. Competition analysis using γ-secretase inhibitors revealed that TMS9 is involved in the initial binding of substrates. TMS9 in part forms the catalytic pore, allowing substrate entry, crucial for intramembrane proteolysis by γ-secretase, Aph-1C, which shows sequence similarity with a putative ABC membrane proteins of Thermobifida fusca (Q47P80). This is the NCBI Anterior Pharynx defective (Aph-1) family.
γ-Secretase consists of Presenilin (PS) and three indispensable subunits: Nicastrin, Aph-1 and Pen-2. PS forms a hydrophilic catalytic pore structure within the lipid bilayer. Takeo et al. (2012) showed that the hydrophilic pore with an open conformation is formed by PS within an immature γ-secretase complex. The binding of the subunits induces close proximity between transmembrane domains facing the catalytic pore. Both γ- and β-secretases have been reported to have affinity for inclusion in membrane nanodomains (Sanders and Hutchison 2018). The effect of bilayer lipid composition on the GS structural ensemble and its function have also been studied (Aguayo-Ortiz et al. 2018). The role of the protease subunit, nicastrin as a gatekeeper, the effects of Alzheimer-causing mutations in presenilin on processive proteolysis of APP, and evidence that three pockets in the active site (S1', S2', and S3') determine carboxypeptidase cleavage of substrates in intervals of three residues have been reviewed (Wolfe 2019). CD147 is a regulatory subunit of the gamma-secretase complex in Alzheimer's disease amyloid beta-peptide production (Zhou et al. 2005). ER retention of the gamma-secretase complex component Pen2 involves Rer1 (Kaether et al. 2007).
Aberrant cleavage of Notch by γ-secretase leads to several types of cancer. The cryo-EM structure of human γ-secretase in complex with a Notch fragment at a resolution of 2.7 Å has been determined (Yang et al. 2019). The TMS of Notch is surrounded by three TMSs of PS1, and the carboxyl-terminal β-strand of the Notch fragment forms a β-sheet with two substrate-induced β-strands of PS1 on the intracellular side. Formation of the hybrid β-sheet is essential for substrate cleavage, which occurs at the carboxy-terminal end of Notch TMSx. PS1 undergoes pronounced conformational rearrangement upon substrate binding (Yang et al. 2019). Moreover, cleavage of amyloid precursor protein (APP) by γ-secretase is linked to Alzheimer's disease (AD). Zhou et al. 2019 reported an atomic structure of human γ-secretase in complex with a transmembrane (TM) APP fragment at 2.6 Å resolution. The TMS of APP closely interacts with five surrounding TMSs of PS1 (the catalytic subunit of γ-secretase). A hybrid β sheet, which is formed by a β strand from APP and two β strands from PS1, guides γ-secretase to the scissile peptide bond of APP between its TM and β strand. Residues at the interface between PS1 and APP are heavily targeted by recurring mutations from AD patients. This structure, together with that of γ-secretase bound to Notch (see above in this paragraph), revealed contrasting features of substrate binding (Zhou et al. 2019).
The γ-secretase complex, contains presenilin (bearing the active site aspartates), nicastrin, Aph-1, and Pen-2 with at least 18 TMSs (Lazarov et al. 2006). EM and single-particle image analyses have been applied to the purified enzyme, which produces physiological ratios of Abeta40 and Abeta42. The 3D EM structure revealed a large, cylindrical interior chamber, approximately 20-40 Å in length, consistent with a proteinaceous proteolytic site that is occluded from the hydrophobic environment of the lipid bilayer. Lectin tagging of the nicastrin ectodomain enabled proper orientation of the globular, approximately 120-A-long complex within the membrane and revealed approximately 20-Å pores at the top and bottom that provide potential exit ports for cleavage products to the extra- and intracellular compartments. The reconstructed 3D map provided a physical basis for hydrolysis of transmembrane substrates within a lipid bilayer and release of the products into distinct subcellular compartments (Lazarov et al. 2006).
γ-Secretase generates the toxic species of the amyloid-beta peptide (Abeta) that is responsible for the pathology of Alzheimer disease (AD). The catalytic subunit, presenilin 1 (PS1), contains the hydrophilic catalytic pore. The length of the C-terminus of Abeta is proteolytically determined by its processive trimming by gamma-secretase. Cai et al. 2019 showed that TMS 3 of human PS1 is involved in the formation of the intramembranous hydrophilic pore. The water accessibility of TMS3 is altered by point mutations and compounds which modify gamma-secretase activity. Changes in the water accessibility of TMS3 correlated with Abeta42 production. Therefore, the conformational dynamics of TMS3 may be a prerequisite for regulation of the Abeta trimming activity of gamma-secretase (Cai et al. 2019). APH-1A, a component of gamma-secretase, forms an internal water and ion-containing cavity (Aguayo-Ortiz and Dominguez 2019).
The γ-secretase complex functions to cleave type I transmembrane proteins within their TM domains. These include the amyloid precursor protein, which is central to Alzheimer's disease pathogenesis, as well as N-cadherin and Notch, which regulate transcription. This complex is composed of four requisite integral membrane proteins: presenilin 1 (PS1) or presenilin 2 (PS2), nicastrin, Pen-2, and Aph-1. (Crystal et al. 2004) demonstrate that PS1 selectively enhances the stability of Pen-2 protein but not that of nicastrin or Aph-1. In the absence of PS1, Pen-2 is rapidly degraded by the proteasome. As PS1 levels increased, so too did the half-life of Pen-2 and therefore its steady-state levels. Presenilin (PS) regulates Pen-2 levels posttranslationally by preventing its degradation by the proteasome.