THE X11-ALPHA PROTEIN SLOWS CELLULAR AMYLOID PRECURSOR PROTEIN PROCESSING AND REDUCES A-BETA-40 AND A-BETA-42 SECRETION

Constitutive amyloid precursor protein (APP) metabolism results in the generation of soluble APP (APPs) and A beta peptides, including A bet a 40 and A beta 42-the major component of amyloid plaques in Alzheimer 's disease brain. The phosphotyrosine binding (PTB) domain of X11 bind s to a peptide containing a YENPTY motif found in the carboxyl terminu s of APP. We have cloned the full-length X11 gene now referred to as X 11 alpha. Coexpression of X11 alpha with APP results in comparatively greater levels of cellular APP and less APPs, A beta 40, and A beta 42 recovered in conditioned medium of transiently transfected HEK 293 ce lls. These effects are impaired by a single missense mutation of eithe r APP (Y682G within the YENPTY motif) or X11 alpha (F608V within the P TB domain), which diminishes their interaction, thus demonstrating spe cificity. The inhibitory effect of X11 alpha on A beta 40 and A beta 4 2 secretion is amplified by coexpression with the Swedish mutation of APP (K595N/M596L), which promotes its amyloidogenic processing. Pulse- chase analysis demonstrates that X11 alpha prolongs the half-life of A PP from similar to 2 h to similar to 4 h. The effects of X11 alpha on cellular APP and APPs recovery were confirmed in a 293 cell line stabl y transfected with APP. The specific binding of the PTB domain of X11 alpha to the YENPTY motif-containing peptide of APP appears to slow ce llular APP processing and thus reduces recovery of its soluble fragmen ts APPs, A beta 40, and A beta 42 in conditioned medium of transfected HEK 293 cells, X11 alpha may be involved in APP trafficking and metab olism in neurons and thus may be implicated in amyloidogenesis in norm al aging and Alzheimer's disease brain.