24 January 2002. Amyloid (Aβ), deposited as
the major component in the plaques found in Alzheimer's brains, is a product
of sequential cleavage of the amyloid precursor protein (AβPP)
by β- and γ-secretases.
The true identity of the γ-secretase, thought by
many to be presenilin-1, is still debated. Furthermore, both AβPP
and the signaling molecule Notch are cleaved in their transmembrane domains
(TMD) by a process that requires presenilin-1. If Notch is also a γ-secretase
substrate, then the enzyme would become a less attractive therapeutic target.
The AβPP and Notch cleavage sites are not identical,
however. AβPP is cleaved in the middle of its TMD,
whereas notch is processed close to the C-terminal end. So are they really cleaved
by the same enzyme? An international collaboration led by Konrad Beyreuther,
University of Heidelberg, Germany, has attempted to answer this question by
examining how the length of the AβPP transmembrane
domain affects the γ-secretase cleavage site. Their
work appeared in the 22 January PNAS online.
The researchers expressed mutated AβPP, with lengthened
or shortened TMDs, in Cos7 cells and then determined, by mass spectrometry,
the site of cleavage. They found that if two amino acids were removed from the
N-terminal, or two amino acids added to the C terminal of the TMD, the cleavage
site shifted toward the C-terminus. In contrast, adding two residues to the
N-terminal end of the TMD shifted cleavage toward the N terminus. The results
show that the site of γ-secretase cleavage is substrate-dependent,
supporting the hypothesis that Notch and AβPP are
cleaved by the same enzyme.-Tom Fagan.
Reference:Lichtenthaler SF, Beher D, Grimm HS, Wang R, Shearman MS, Masters CL, Beyreuther K. The intramembrane cleavage site
of the amyloid precursor protein depends on the length of its transmembrane
domain. PNAS. Advanced online publication 22 January 2002. Abstract