Question: What is the composition of the gamma-secretase complex?

Context:
Gamma-secretase is a widely expressed multisubunit enzyme complex which is involved in the pathogenesis of Alzheimer disease and hematopoietic malignancies through its aberrant processing of the amyloid precursor protein (APP) and Notch1, respectively.
Presenilin (PS1 or PS2) is the catalytic component of the gamma-secretase complex, which mediates the final proteolytic processing step leading to the Alzheimer's disease (AD)-characterizing amyloid beta-peptide. 
gamma-Secretase is a membrane-embedded multi-protein complex that catalyzes the final cut of the Alzheimer's disease-related amyloid precursor protein (APP) to amyloid-beta peptides of variable length (37-43 amino acids) via an unusual intramembrane cleavage.
The gamma-secretase complex processes substrate proteins within membranes and consists of four proteins: presenilin (PS), nicastrin, Aph-1 and Pen-2. PS harbours the enzymatic activity of the complex, and there are two mammalian PS homologues: PS1 and PS2. 
gamma-Secretase is known to contain four major protein constituents: presenilin (PS), nicastrin, Aph-1, and Pen-2, all of which are integral membrane proteins. 
Here we describe the association of all four components of the gamma-secretase complex, namely presenilin 1 (PS1)-derived fragments, mature nicastrin, APH-1, and PEN-2, with cholesterol-rich detergent insoluble membrane (DIM) domains of non-neuronal cells and neurons that fulfill the criteria of lipid rafts. 
In contrast, the authentic TMD of NCT is critically required for the interaction with gamma-secretase complex components and for formation of an active gamma-secretase complex.
Active gamma-secretase is a tetrameric protein complex consisting of presenilin-1 (or -2), nicastrin, PEN-2, and Aph-1a (or -1b).
The gamma-secretase complex processes substrate proteins within membranes and consists of four proteins: presenilin (PS), nicastrin, Aph-1 and Pen-2.
Maturation of exogenous NCT, gamma-secretase complex formation and proteolytic function was then investigated.
gamma-Secretase complexes containing N- and C-terminal fragments of different presenilin origin retain normal gamma-secretase activity.
Co-overexpression of presenilin-1 or APH-1 abrogated gamma-secretase inhibition probably through prevention of the incorporation of CRB2 into the gamma-secretase complex
We conclude that a PS1/Pen2/Aph1a trimeric complex is an active enzyme, displaying biochemical properties similar to those of gamma-secretase and roughly 50% of its activity when normalized to PS1 N-terminal fragment levels
γ-Secretase is involved in the regulated intramembrane proteolysis of amyloid-β protein precursor (AβPP) and of many other important physiological substrates. γ-secretase is a multiproteic complex made of four main core components, namely presenilin 1 or 2, APH-1, PEN-2, and Nicastrin
Amyloid precursor protein associates with a nicastrin-dependent docking site on the presenilin 1-gamma-secretase complex in cells demonstrated by fluorescence lifetime imaging.
Activity-dependent isolation of the presenilin- gamma -secretase complex reveals nicastrin and a gamma substrate.
Characterization of the reconstituted gamma-secretase complex from Sf9 cells co-expressing presenilin 1, nicastrin [correction of nacastrin], aph-1a, and pen-2.
p53-Dependent Aph-1 and Pen-2 anti-apoptotic phenotype requires the integrity of the gamma-secretase complex but is independent of its activity.
Here we show that all known gamma-secretase complexes are active in APP processing and that all combinations of APH-1 variants with either FAD mutant PS1 or PS2 support pathogenic Abeta(42) production. 
To characterize the functional similarity between complexes of various PS composition, we analysed PS1, PS2, and chimeric PS composed of the NTF from PS1 and CTF from PS2, or vice versa, in assembly and function of the gamma-secretase complex.
While beta-secretase is a classical aspartyl protease, gamma-secretase activity is associated with a high molecular weight complex. One of the complex components, which is critically required for gamma-secretase activity is nicastrin (NCT). Here we investigate the assembly of NCT into the gamma-secretase complex.
Maturation of exogenous NCT, gamma-secretase complex formation and proteolytic function was then investigated. This revealed that the cytoplasmic tail of NCT is dispensable for gamma-secretase complex assembly and function. In contrast, the authentic TMD of NCT is critically required for the interaction with gamma-secretase complex components and for formation of an active gamma-secretase complex.
This revealed that the cytoplasmic tail of NCT is dispensable for gamma-secretase complex assembly and function. In contrast, the authentic TMD of NCT is critically required for the interaction with gamma-secretase complex components and for formation of an active gamma-secretase complex.
While beta-secretase is a classical aspartyl protease, gamma-secretase activity is associated with a high molecular weight complex. One of the complex components, which is critically required for gamma-secretase activity is nicastrin (NCT).
This revealed that the cytoplasmic tail of NCT is dispensable for gamma-secretase complex assembly and function. In contrast, the authentic TMD of NCT is critically required for the interaction with gamma-secretase complex components and for formation of an active gamma-secretase complex. Neither soluble NCT lacking any membrane anchor nor NCT containing a heterologous TMD were inserted into the gamma-secretase complex.
In contrast, the authentic TMD of NCT is critically required for the interaction with gamma-secretase complex components and for formation of an active gamma-secretase complex. Neither soluble NCT lacking any membrane anchor nor NCT containing a heterologous TMD were inserted into the gamma-secretase complex.
Maturation of exogenous NCT, gamma-secretase complex formation and proteolytic function was then investigated. This revealed that the cytoplasmic tail of NCT is dispensable for gamma-secretase complex assembly and function.
Here we investigate the assembly of NCT into the gamma-secretase complex.
Co-overexpression of presenilin-1 or APH-1 abrogated gamma-secretase inhibition probably through prevention of the incorporation of CRB2 into the gamma-secretase complex.
This could be demonstrated also in cell-free assays, where in addition presenilin-1, the catalytic subunit of the gamma-secretase complex, was shifted out of lipid rafts.
In PS1(-/-)/PS2(-/-) and NCT(-/-) fibroblasts, gamma-secretase components that still remain fail to become detergent-resistant, suggesting that raft association requires gamma-secretase complex assembly.

Answer: Gamma-secretase is a multisubunit enzyme complex which is consists of four proteins: presenilin 1 (PS1) or presenilin 2 (PS2), nicastrin, Aph-1 and Pen-2.