• Gamma-secretase activity 23/01/07

  Reconstituted gamma-secretase activity depends on the presence of four complex components including presenilin (PS), nicastrin (Nct), APH-1  and PEN-2 , is associated with endoproteolysis of PS, and produces Abeta and AICD in vitro. Nat Cell Biol 2003 Apr 7 Reconstitution of gamma-secretase activity. Edbauer D, Winkler E, Regula JT, Pesold B, Steiner H, Haas.

  Recently, the minimal molecular subunit composition of the γ-secretase complex has been solved, namelypresenilin (either PS-1 or PS-2), nicastrin (Nct), presenilin enhancer 2 (PEN-2), and the anterior pharynx defective 1 protein Aph-1, in mammals Aph-1aS, -1aL, or -1b. In total six complexes with different subunit compositions can be formed because of the two presenilin proteins and the three Aph-1 proteins.

  Both CT99 ( b stub) and CT83 (alpha stub) are substrates for g-secretase, which performs an unusual proteolysis in the middle of the transmembrane domain to produce the 4-kDa A and CT57–59 [amyloid intracellular domain (AICD)] from CT99, and a 3-kDa peptide called p3 and CT57–59 from CT83.

  Proteolysis by g-secretase is heterogeneous: Most of the full-length Abspecies produced is a 40-residue peptide (A40), whereas a small proportion is a 42-residue COOH-terminal variant (A42).

• After either alpha- or beta-secretase releases the bulk of APP, the remaining carboxyl-terminal fragments, CT83 and CT99, undergo proteolysis within their plasma membrane domain—regulated intramembrane proteolysis (RIP)—and the intracellular portion moves to the nucleus where it may affect the transcription of target genes .

• Proteins that undergo RIP include APP; Notch, a receptor involved in fate decisions during embryonic development; sterol regulatory element binding proteins (SREBPs), transmembrane proteins of the endoplasmic reticulum that regulate lipid metabolism (Brown et al., 2000); and ErbB-4, a member of the epidermal growth factor tyrosine kinase receptor family (Ni et al., 2001).

• How hydrolysis takes place in what is otherwise a waterexcluded environment is unclear.

  • There is evidence that the intracellular portions of Notch and SREBP modulate gene transcription (Brown et al., 2000). Notch interacts with the coactivator p300 (Oswald et al., 2001), and SREBPs contain well characterized DNA binding and transactivating domains that interact with a number of other transcription factors and coactivators (Edwards and Ericsson, 1999).

  • Furthermore, the intracellular portion of APP forms a complex with the nuclear adaptor protein Fe65 and with Tip60, which has histone acetyltransferase activity like p300, and stimulates transcription when fused to the DNA binding domains of the heterologous transcription factors Gal4 or LexA (Cao and Sudhof, 2001; Kimberly et al., 2001).

  • However, the term RIP may be misleading, because proteolytic cleavage within a membrane has never been demonstrated directly. For example, it is possible that _- or _-secretase cleavage of APP results in movement of the C-terminal protein and exposure of the _-secretase sites to the aqueous environment (Nunan and Small, 2000)

  • The enzyme that catalyzes the secondary cleavage of APP, Notch, ErbB-4, and SREBPs is g-secretase. This g-secretase has pharmacological characteristics of an aspartyl protease and remarkably loose sequence specificity for its substrate because many mutations in APP near the g-secretase site still allow Ab production in transfected cells (Maruyama et al., 1996; Tischer and Cordell, 1996; Lichtenthaler et al., 1997, 1999; Wolfe et al., 1999a–c) and is a multiprotein complex.

  • A crucial question is how protein cleavage itself is regulated. It seems that the triggering event is the removal of most of the extracytoplasmic part of the protein. This seems to be a prerequisite for the intramembrane cleavage by g-secretase (Heldin and Ericsson, 2001).

 Many thanks to Suh and Checler, 2002 for their analysis.