Alanine substitutions in the GXXXG motif alter C99 cleavage by [gamma]-secretase but not its dimerization

The amyloid [beta] (A[beta]) protein is a major component of senile plaques, one of the neuropathological hallmarks of Alzheimer's disease. Amyloidogenic processing of amyloid precursor protein (APP) by [beta]- and [gamma]-secretases leads to production of A[beta]. APP contains tandem triple repeats of the GXXXG motif in its extracellular juxtamembrane and transmembrane regions. It is reported that the GXXXG motif is related to protein-protein interactions, but it remains controversial whether the GXXXG motif in APP is involved in substrate dimerization and whether dimerization affects [gamma]-secretase-dependent cleavage. Therefore, the relationship between the GXXXG motifs, substrate dimerization, and [gamma]-secretase-dependent cleavage sites remains unclear. Here, we applied blue native poly acrylamide gel electrophoresis to examine the effect of alanine substitutions within the GXXXG motifs of APP carboxyl terminal fragment (C99) on its dimerization and A[beta] production. Surprisingly, alanine substitutions in the motif failed to alter C99 dimerization in detergent soluble state. Cell-based and solubilized [gamma]-secretase assays demonstrated that increasing alanine substitutions in the motif tended to decrease long A[beta] species such as A[beta]42 and A[beta]43 and to increase in short A[beta] species concomitantly. Our data suggest that the GXXXG motif is crucial for A[beta] production, but not for C99 dimerization.