Okadaic acid enhances abnormal phosphorylation on tau proteins.

Tau proteins are one of the microtubule-associated proteins (MAPs) and show promoting activity on microtubule assembly. Tau proves to be the major constituent of Alzheimer's paired helical filaments, in which tau is found to be different from normal tau in that it is abnormally phosphorylated. To examine the effect of the abnormal phosphorylation on microtubule assembly, we obtained abnormally phosphorylated tau that was made in vitro by hyperphosphorylation with ATP or with ATP and okadaic acid, a drug inhibiting phosphatase, mainly 1 and 2A. We confirmed the biochemical properties of abnormally phosphorylated tau based on its retarded gel mobility and immunoreactivity to anti-PHF. We found that abnormally phosphorylated tau was able to promote the polymerization of microtubules but showed less activity as compared with normally phosphorylated tau. This effect of ATP on abnormal phosphorylation of tau was enhanced when okadaic acid was added in the phosphorylation reaction mixture during microtubule assembly. It is of significance that phosphatase activity as well as kinase activity are involved in the formation of abnormal tau. The present evidence suggests the simultaneous occurrence of microtubule disassembly and the pathogenesis of paired helical filaments following the abnormal phosphorylation of tau.