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First published online February 6, 2008
doi: 10.1242/10.1242/jcs.015255

Journal of Cell Science 121, 413-420 (2008)
Published by The Company of Biologists 2008
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Building {gamma}-secretase – the bits and pieces

Dragana Spasic and Wim Annaert*

Laboratory for Membrane Trafficking, Center for Human Genetics (KULeuven) and Department of Molecular and Developmental Genetics (VIB), O&N1, Rm. 9.696, Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium


Figure 1
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  		Fig. 1. The membrane topologies of the four known components of the {gamma}-secretase complex: PS1, PEN2, APH1 and NCT. The arrow indicates the endoproteolytic cleavage site in hydrophobic region 7 of PS1, which generates the PS1 NTF-CTF heterodimer that is the active form. Red asterisks denote the position of the catalytic aspartate residues in TMD6 and TMD7 of PS1. TMDs that are involved in intermolecular interactions between {gamma}-secretase components are indicated with the mixed colours of the respective components.

 

Figure 2
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  		Fig. 2. 3D structure of the {gamma}-secretase complex. (A) Surface rendering of the 3D reconstruction. The first row displays side views generated by rotation around a vertical axis, the second row shows tilted views generated by rotation around a horizontal axis. The rotation angles are shown within each view. Two openings at the top and bottom are labelled H1 and H2, respectively, where visible. The top density is labelled NCT because lectin labelling showed that the NCT ectodomain is located at this surface (Lazarov et al., 2006Go). (B) The potential transmembrane segment has a belt-like structure and is indicated by two parallel dashed lines, 60 Å apart. For size comparison, a typical transmembrane {alpha}-helix, taken from the rhodopsin structure (protein data bank ID code 1GZM), is shown to the left of the structure. (C) A cut-open view of the {gamma}-secretase complex, revealing a large central chamber and one opening at the top (H1) and one at the bottom (H2). Two weak-density lateral regions are labelled with asterisks. Reproduced from Lazarov et al. PNAS, 103(18), 6889-6894, 2006 with permission [Copyright (2006) National Academy of Sciences, USA].

 

Figure 3
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  		Fig. 3. Intra- and intermolecular interactions in the {gamma}-secretase complex. Given the hydrophobic nature of {gamma}-secretase, most prominent and primary interactions are likely to be governed through or include their TMDs and hydrophobic domains. Here we present a bird's eye view of the TMDs of PS1 (yellow), NCT (green), APH1 (blue) and PEN2 (red), including the reported intra- (grey arrows) and intermolecular (black arrows) interactions. Suggested interactions are shown by dotted arrows, such as intramolecular interactions in APH1 (via GxxG motifs in TMD4) and the as-yet-uncharacterised interaction domains for APH1 in PS1 and for NCT in APH1. Ectodomain interactions of NCT with APP-CTF are indicated with a green arrow (see text for details). The red sparkle denotes the catalytic aspartate dyad.

 

Figure 4
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  		Fig. 4. Temporal sequence of interactions leading to {gamma}-secretase complex formation. The earliest step in complex assembly is probably the association of NCT (green) with APH1 (blue) in the ER. From then on two `pathways' are proposed, depending on the experimental approaches used. (A) Full-length PS1 (yellow) might bind to NCT-APH1, this is followed by recruitment of PEN2 (red), which promotes endoproteolysis and stabilisation of the full complex. (B) Alternatively, full-length PS1 might first bind to PEN2, leading to endoproteolysis and binding to the pre-existing NCT-APH1 subcomplex.

 

Figure 5
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  		Fig. 5. Percolation-model that integrates {gamma}-secretase assembly into ER-Golgi transport regulation. Individual and newly synthetised {gamma}-secretase components are co-translationally inserted in the ER. Given that NCT interacts very early with APH1 and that Rer1p preferentially binds to immature NCT, we suggest that immature NCT `percolates' between the ER and Golgi compartments (a). It can be captured by Rer1p in the IC (a') or cis-Golgi (a'') and retrieved via COPI-coated organelles to the ER. By competing with the NCT TMD, APH1 can displace Rer1p to form an NCT-APH1 subcomplex. This can occur in the ER (b) as well as IC compartments (b'). Once NCT-APH1 is formed, PS1 and PEN2 (either sequentially or together, see Fig. 4) join it to form a fully assembled complex. Again, this event does not need to be restricted to the ER (c) but can also take place in the IC (c'). Full-length PS1 can exit the ER (Kim et al., 2005Go), which suggests that endoproteolysis (and PEN2 interaction) occurs later and maybe during additional retrieval events from Golgi/IC to ER. The concentration of endogenous PS1 in COPI-coated membranes (Rechards et al., 2003Go) indicates that PS1 percolates through early biosynthetic compartments but potential retrieval mechanisms have not yet been identified. Hence, the ER-Golgi quality control system ensures that monomeric {gamma}-secretase components are selectively retrieved from the cis-Golgi to the ER through interaction with cargo-retrieval receptors. Conversely, full complex assembly leads to a masking of the interactions with these retrieval receptors, allowing escape from ER-Golgi recycling (d) and transport of assembled complexes to their final destination in distal compartments, including the cell surface and endosomes. APP behaves differently because it has a short residence time in ER-Golgi compartments and gets maturely glycosylated quickly. This supports the finding that APP trafficking from the ER is uncoupled from PS1 (Kim et al., 2005Go) (e).

 

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© The Company of Biologists Ltd 2008