Assembly, Trafficking and Function of γ-Secretase

Abstract

γ-Secretase catalyzes the final cleavage of the β-amyloid precursor protein to generate amyloid-β peptide, the principal component of amyloid plaques in the brains of patients suffering from Alzheimer’s disease. Here, we review the identification of γ-secretase as a protease complex and its assembly and trafficking to its site(s) of cellular function. In reconstitution experiments, γ-secretase was found to be composed of four integral membrane proteins, presenilin (PS), nicastrin (NCT), PEN-2 and APH-1 that are essential and sufficient for γ-secretase activity. PS, which serves as a catalytic subunit of γ-secretase, was identified as a prototypic member of novel aspartyl proteases of the GxGD type. In human cells, γ-secretase could be further defined as a heterogeneous activity consisting of distinct complexes that are composed of PS1 or PS2 and APH-1a or APH-1b homologues together with NCT and PEN-2. Using green fluorescent protein as a reporter we localized PS and γ-secretase activity at the plasma membrane and endosomes. Investigation of γ-secretase complex assembly in knockdown and knockout cells of the individual subunits allowed us to develop a model of complex assembly in which NCT and APH-1 first stabilize PS before PEN-2 assembles as the last component. Furthermore, we could map domains in PS and PEN-2 that govern assembly and trafficking of the complex. Finally, Rer1 was identified as a PEN-2-binding protein that serves a role as an auxiliary factor for γ-secretase complex assembly.

Trafficking and Proteolytic Processing of APP

“…This apparent contradiction was coined the spatial paradox by Annaert and De Strooper (1999). Although still not resolved completely, the work of many labs now indicates that mature, proteolytically active PS/g-secretase is principally localized not in ER, Golgi, or postGolgi transport vesicles but rather at the PM and in the endosomal/lysosomal system, including phagosomes and autophagosomes (reviewed in Pasternak et al 2004;Kaether et al 2006a;Nixon 2007;Dries and Yu 2008). The g-secretase subunits found in the ER/early secretory pathway most likely represent unassembled or partially assembled subcomplexes, but not the active enzyme.…”

Cell and molecular biology of Notch

“…It has been argued that the presenilin complex could act at the cell surface (Kaether et al 2006) and this has always been somewhat at odds with observation that Psn resides somewhere in the endocytic pathway (Ray et al 1999, Pasternak et al 2003. The recent observations emphasize that accumulation of Notch in the endocytic pathway, probably in a post Rab5 and/orRab7 compartment, can lead to its activation via S3 cleavage.…”

The Mitochondrial Secret(ase) of Alzheimer's Disease

“…The ability of the four components to form up to six distinct complexes suggests each may possess different substrate affinities and serve importance in specific environments. Some authors have also suggested that other proteins associated with the γ-secretase complex, like TMP21, may assist in determining the cleavage specificity of the enzyme with its substrates [87][88][89][90], further suggesting a physiological regulation of substrate recognition and target processing. Mutations in PS1 are responsible for over 50% of fAD cases and are highly penetrant before the age of 65 [88][89][90].…”

“…Some authors have also suggested that other proteins associated with the γ-secretase complex, like TMP21, may assist in determining the cleavage specificity of the enzyme with its substrates [87][88][89][90], further suggesting a physiological regulation of substrate recognition and target processing. Mutations in PS1 are responsible for over 50% of fAD cases and are highly penetrant before the age of 65 [88][89][90]. These PS mutations tend to shift the cleavage specificity of C99 to increase generation of Aβ 42 [91], possibly by altering γ-secretase conformation to decrease or increase binding of the enzyme with the Aβ residues responsible for forming Aβ 40 or Aβ 42 , respectively [92].…”