APP intracellular domain

A key step in the pathogenesis of Alzheimer s disease is APP proteolysis that results in the formation of the amyloid-fi peptide (AfS), the principle protein component of the characteristic cerebral plaques of the disease (18). The N-terminus of Afi is produced from the amyloid fS-protein precursor (APP) by the action of fS-secretase, which leads to membrane shedding of the large luminal/extracellular APP domain (Fig. 3a). The 99-residue remnant (C99) then is cleaved in the middle of its transmembrane region by y-secretase, which releases Afi and is cleaved again near the inner leaflet at the 6 site to release the APP intracellular domain (AlCD). As described below, chemical probes played important roles in the characterization, identification, purification, and mechanistic understanding of the I-CLiP that now is known as the y-secretase complex. 

Fig. 4.9 Processing scheme for the j8-amyloid precursor protein. j8-APP (amyloid precursor protein) is processed in two steps. First, j8-secretase generates two fragments, the j8-amyloid precursor peptide and the C-terminal fragment (CTF-jS) C99. In a second step, the y-secretase cleaves the C99 fragment into an Afl fragment and AlCD (APP Intracellular Domain). Sometimes, y-secretase gen-...

Further processing of plO by y-secretase yields truncated Ap fragments of about 3 kDa (p3) and the APP intracellular domain (AlCD). p3 is generally not found in amyloid cores of classical plaques, or in amyloid deposits in the cerebral vasculature. The AICD fragment can form multiprotein complexes, which are transported to the nucleus and may have a role in nuclear signaling (Shoji et al., 1992). 

In this study, a number of C-terminal APP polypeptides were synthesised and efficiently employed as antigens in the structural determination of epitopes recognised by monoclonal and polyclonal antibodies specific for the Carboxy-terminal, intracellular domain of APP, using proteolytic epitope-excision and -extraction in combination with high resolution FTICR-mass spectrometry. The results of these experiments specifically define the epitope of the cAPP polyclonal antibody (36BO) as the sequence, APP(727-737), while the epitope recognised by the Jonas-mAb was defined at the... 

Figure 3 Presenilin, the y-secretase complex, and the proteolysis of APP to Ap. (a) Presenilin is processed into two pieces, an N-terminal fragment (NTF, dark portion) and a C-terminal fragment (CTF, light portion) that remain associated. Each fragment donates one aspartate that is essential for y-secretase activity. APP is cleaved first in the extracellular domain by p-secretase, and the remnant is cleaved twice within the membrane by y-secretase to produce the Ap peptide of Alzheimer s disease (secreted) and the intracellular domain (AlCD, freed into the cytosol), (b) Presenilin interacts with three other membrane proteins, nicastrin, Aph-1, and Pen-2, to form active y-secretase.

Figure 30-34 Cleavage of the amyloid precursor protein APP with liberation of amyloid A 3 protein. The proteins are represented as sticks (not to scale) but in reality contain both intracellular and extracellular globular domains.

In addition to the metal interactions with APP and Ab that may directly affect the generation of Ab and its aggregation and toxicity, biometals have been found to interact with many of the proteins and activities that surround APP. These interactions are hkely to subserve physiological purposes, and may reflect a role for APP metabolism in metal homeostasis. Zn has been shown to interact with and inhibit the gamma-secretase complex [ 172]. The intracellular carboxyl terminus of APP interacts with XIla (MINT), which in turn interacts with the Cu Chaperone of SODl (CCSl) directing Cu away from SODl [173]. CCSl interacts with a Cu(I)-binding site on the intracellular carboxyl terminus of BACEl [174], although it is not yet clear whether this influences BACE activity. This may be part of the mechanism by which Cu added to cell culture increases Ab release into the culture medium (unpublished data). A metalloproteinase inhibitory domain has been identified in the portion of APP immediately upstream from the Ab domain [175]. 

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