Interaction of protein kinases stimulates an Alzheimer-like phosphorylation of tau

Prephosphorylation of tau by cAMP-dependent protein kinase (PKA) significantly stimulated its consecutive phosphorylation catalyzed by glycogen synthetase kinase-3 (GSK-3). After digestion of phosphorylated tau with trypsin, P-32-labeled tau peptides were purified by a sequential FeCl3 micro affinity column and a C-18 reverse phase high performance liquid chromatography. The results from combined techniques of high voltage electrophoresis, manual Edman degration and auto gas phase amino acid sequence analysis demonstrated that the phosphorylation sites of tau (pretreated with PKA) by GSK-3 were Ser (serine) -195, Ser-198, Ser-199, Ser-202, Ser-235, Ser-262 Ser-404, Thr ( threonine) -205 Among them, Ser-198, Ser-199, Ser-202, Ser-235, Ser-262, Ser-404, Thr-205 and Thr-231 are abnormal phosphorylation sites of tau found in Alzheimer disease. As the above phosphorylation potently inhibited the biological activity of tau, it was concluded that the phosphorylation of tau at above mentioned Alzheimer sites is critical to the inhibition of its biofunction, and that the interaction of PKA and GSK-3 might be a potential system responsible for the neurofibrillary degeneration seen in Alzheimer disease.