P3–421: Identification of structural determinants on PS–1 and PS–2 that affect inhibitor potency

function in the molecular pathway of APP proteolytic processing at the level of -secretase. Knockdown of Fbxo11 expression by siRNA in 293 cells had no effect on the levels of APP full length, APP-CTFs, secreted APP, or presenilin 1 (PS1) fragments but increased AICD generation by 2-fold. Paradoxically, Fbxo11 siRNA decreased total A secretion, with A 42 showing a greater reduction than A 40, indicating shifts in the sites of and -cleavages in APP. Moreover, Fbxo11 siRNA transfection markedly reduced SDS-resistant high molecular weight species (100-130 kDa) of PS1 in multi-ubiquitin immune complexes, suggesting a role of Fbxo11 in the ubiquitination of PS1. A similar dose-dependent reduction in PS1 ubiquitination was seen by the treatment of ibuprofen, an A 42 lowering NSAID. In transient transfection experiments, myc-tagged Fbxo11 co-precipitated with PS1 in an expression-dependent manner. Conclusions: These results suggest that Fbxo11 alters A generation by modifying PS1 structure and function. Further studies are being carried out to decipher the precise manner in which Fbxo11 modifies presenilin-dependent -secretase activity.