Identification of a novel complex AβPP:Fe65:PP1 that regulates AβPP Thr668 phosphorylation levels

The amyloid-β protein precursor (AβPP) binds several proteins determining metabolism, processing, and the physiological fate of the former. Among these is Fe65, a protein with specific functional significance for AβPP, in particular conferring stability when the latter is dephosphorylated on Thr668. Thus, it follows that phosphatases like protein phosphatase 1 (PP1) are relevant to AβPP processing. Consequently, the identification of AβPP binding proteins, which can be modulated directly or indirectly by PP1, take on added relevance in terms of biological significance. Using the yeast tri-hybrid system and co-immunoprecipitation assays, we describe a novel tri-complex comprising AβPP, Fe65 and PP1. We show that the trimeric complex (AβPP:Fe65:PP1γ) occurs in COS-7 cells, rat hippocampal and cortical primary neurons, and in adult rat hippocampus and cortex. Using overlay assays, we demonstrate that Fe65 is in fact the bridging protein in the complex formed and thus we simultaneously describe another PP1 binding protein. This is singularly important given that PP1 binding proteins determine and confer subcellular localization, as well as substrate specificity, thus regulating the phosphatase activity and subsequent intracellular events. Additionally, we show that this interaction correlates with AβPP Thr668 phosphorylation state, consistent with the role of protein (de)phosphorylation as a key mechanism in regulating cellular events.