Roles of proteolysis and lipid rafts in the processing of the amyloid precursor protein and prion protein.

In the amyloidogenic pathway, the APP (amyloid precursor protein) is proteolytically processed by the beta- and gamma-secretases to release the Abeta (amyloid-beta) peptide that is neurotoxic and aggregates in the brains of patients suffering from Alzheimer's disease. In the non-amyloidogenic pathway, APP is cleaved by alpha-secretase within the Abeta domain, precluding deposition of intact Abeta peptide. The cellular form of the PrP(C) (prion protein) undergoes reactive oxygen species-mediated beta-cleavage within the copper-binding octapeptide repeats or, alternatively, alpha-cleavage within the central hydrophobic neurotoxic domain. In addition, PrP(C) is shed from the membrane by the action of a zinc metalloprotease. Members of the ADAM (a disintegrin and metalloproteinase) family of zinc metalloproteases, notably ADAM10 and TACE (ADAM17) display alpha-secretase activity towards APP and appear to be responsible for the alpha-cleavage of PrP(C). The amyloidogenic cleavage of APP by the beta- and gamma-secretases appears to occur preferentially in cholesterol-rich lipid rafts, while the conversion of PrP(C) into the infectious form PrP(Sc) also appears to occur in these membrane domains.