Successive cleavage of beta-amyloid precursor protein by gamma-secretase

gamma-Secretase is a multimeric aspartyl protease that cleaves the membrane-spanning region of the beta-carboxyl terminal fragment (beta CTF) generated from beta-amyloid precursor protein. gamma-Secretase defines the generated molecular species of amyloid beta-protein (A beta), a critical molecule in the pathogenesis of Alzheimer's disease (AD). Many therapeutic trials for AD have targeted gamma-secretase. However, in contrast to the great efforts in drug discovery, the enzymatic features and cleavage mechanism of gamma-secretase are poorly understood. Here we review our protein-chemical analyses of the cleavage products generated from beta CTF by gamma-secretase, which revealed that A beta was produced by gamma-secretase through successive cleavages of beta CTF, mainly at three-residue intervals. Two representative product lines were identified. epsilon-Cleavages occur first at Leu49-Val50 and Thr48-Leu49 of beta CTF (in accordance with A beta numbering). Longer generated A beta s, A beta 49 and A beta 48, are precursors to the majority of A beta 40 and A beta 42, concomitantly releasing the tripeptides, ITL, VIV, and IAT; and VIT and TVI, respectively. A portion of A beta 42 is processed further to A beta 38, releasing a tetrapeptide, VVIA. The presence of additional multiple minor pathways may reflect labile cleavage activities derived from the conformational flexibility of gamma-secretase through molecular interactions. Because these peptide byproducts are not secreted and remain within the cells, they may serve as an indicator that reflects gamma-secretase activity more directly than secreted A beta.