Phosphorylation sensitizes microtubule-associated protein τ to Al³⁺-induced aggregation

Abstract

In Alzheimer’s disease the microtubule-associated protein τ becomes hyperphosphorylated and aggregates into paired helical filaments (PHFs). Although the biochemical basis of the aggregation of τ into PHFs is not very clear, Al³⁺ has been suggested to play some role. Previous studies have shown that Al³⁺ alters the phosphorylation state and causes aggregation of τ in experimental animals and cultured neurons. In this study Al³⁺ inhibited phosphorylation of τ by neuronal cdc2-like kinase and dephosphorylation of phosphorylated τ by phosphatase 2B. These inhibitions are very likely due to Al³⁺-induced aggregations of various proteins present in phosphorylation/dephosphorylation assay mixtures since Al³⁺ caused aggregations of all proteins examined. Furthermore, compared to other proteins, τ displayed only an average sensitivity towards Al³⁺-induced aggregation. However upon phosphorylation, τ’s sensitivity towards Al³⁺ increased 3.5 fold. In the presence of the metal chelator EDTA, Al³⁺-induced aggregates of τ became soluble, whereas Al³⁺-induced phosphorylated τ aggregates were insoluble in the buffer containing EDTA and remained insensitive to proteolysis. Our data suggest that phosphorylation sensitizes τ to Al³⁺ and phosphorylated τ transforms irreversibly into a phosphatase and protease resistant aggregate in presence of this metal ion.