The Alzheimer's disease-associated Presenilins are differentially phosphorylated proteins located predominantly within the endoplasmic reticulum

Abstract

Background: Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the deposition of extracellular senile plaques composed of amyloid β-peptide (Aβ). Whereas most cases of AD occur sporadically, about 10% of AD cases are inherited as fully penetrant autosomal dominant trait. Mutations in the recently cloned Presenilin genes (PS-1 and PS-2) are by far the most common cause of early onset familial AD. Materials and Methods: Cellular expression of endogenous and overexpressed PS protein was analyzed by immunocytochemistry and metabolic labeling followed by immunoprecipitation. In vivo phosphorylation sites of PS protein were analyzed by extensive mutagenesis. Results: PS-1 as well as PS-2 proteins were localized predominantly within the endoplasmic reticulum (ER). However, small amounts of the PS proteins were detected within the Golgi compartment, where they colocalize with the β-amyloid precursor protein (βAPP). The PS- 2 protein was found to be highly phosphorylated, whereas very little phosphorylation was observed for PS-1. The selective phosphorylation of PS-2 occurs exclusively on serine residues. In vivo phosphorylation of PS-2 was mapped to serine residues 7, 9, and 19 within an acidic stretch at the N terminus, which is absent in PS-1. Casein kinase (CK)-1 and CK-2 were shown to phosphorylate the N terminus of PS-2 in vitro. Conclusions: The majority of PS proteins were detected in the ER where little if any proteolytic processing of βAPP was reported. ER retention of PS proteins might occur by intramolecular aggregation. Small amounts of PS proteins were also detected in the Golgi where they colocalized with βAPP. This might suggest that potential interactions between PS proteins and βAPP could occur within the Golgi. Selective phosphorylation of PS-2 proteins within the acidic domain missing in PS-1 indicates differences in the biological functions and regulation of the two highly homologous proteins.