Tau protein kinase I/GSK-3 beta/kinase FA in heparin phosphorylates tau on Ser199, Thr231, Ser235, Ser262, Ser369, and Ser400 sites phosphorylated in Alzheimer disease brain.

Previously, tau protein kinase I/glycogen synthase kinase-3 beta/kinase FA(TPKI/GSK-3 beta/FA) was identified as a brain microtubule-associated tau kinase possibly involved in the Alzheimer disease-like phosphorylation of tau. In this report, we find that the TPKI/GSK-3 beta/FA can be stimulated to phosphorylate brain tau up to 8.5 mol of phosphates per mol of protein by heparin, a polyanion compound. Tryptic digestion of 32P-labeled tau followed by high-performance liquid chromatography and high-voltage electrophoresis/thin-layer chromatography reveals 12 phosphopeptides. Phosphoamino acid analysis together with sequential manual Edman degradation and peptide sequence analysis further reveals that TPKI/GSK-3 beta/FA after heparin potentiation phosphorylates tau on sites of Ser199, Thr231, Ser235, Ser262, Ser396, and Ser400, which are potential sites abnormally phosphorylated in Alzheimer tau and potent sites responsible for reducing microtubule binding possibly involved in neuronal degeneration. The results provide initial evidence that TPKI/GSK-3 beta/FA after heparin potentiation may represent one of the most potent systems possibly involved in the abnormal phosphorylation of PHF-tau and neuronal degeneration in Alzheimer disease brains.