PROTEIN KINASE-F(A) GSK-3 PHOSPHORYLATES-TAU ON SER(235)-PRO AND SER(404)-PRO THAT ARE ABNORMALLY PHOSPHORYLATED IN ALZHEIMERS-DISEASE BRAIN

Previously, we identified protein kinase F(A)/glycogen synthase kinase-3 (GSK-3) as a microtubule-associated protein tau kinase that can incorporate 4 mol of phosphates into 1 mol of tau protein and cause its electrophoretic mobility shift in sodium dodecyl sulfate gels, a unique property characteristic of paired helical filament-associated pathological tau (PHF-tau) in Alzheimer's disease brains. In this report, we identified TPPKS(p)PSAAK and SPVVSGDTS(p)PR as two phosphorylation site sequences phosphorylated by kinase F(A)/GSK-3 in tau using peptide sequence analysis and sequential manual Edman degradation for radiosequencing. When mapping with human brain tau sequence, we further identified Ser235-Pro and Ser404-Pro as the two major phosphorylation sites according to the numbering of the longest tau isoform. Ser235 and Ser404 have been reported as two of the major abnormal phosphorylation sites in PHF-tau. Taken together, the results provide initial evidence that protein kinase F(A)/GSK-3 may represent one of the Ser-Pro motif-directed tau kinases involved in the abnormal phosphorylation of pathological PHF-tau in Alzheimer's disease brain.