Protein Kinase FA/GSK-3 Phosphorylates on Ser235-Pro and Ser404-Pro that Are Abnormally Phosphorylated in Alzheimer's Disease Brain

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