Hyperphosphorylation of the cytoskeletal protein Tau by the MAP-kinase PK40erk2: regulation by prior phosphorylation with cAMP-dependent protein kinase A.

PK40erk2 is a MAP kinase which phosphorylates recombinant hTau40 up to 14 moles of phosphate/mole, markedly slowing its electrophoretic mobility. PK40erk2 acting on TAU is expected to cause the appearance of Alzheimer's disease-specific phosphoepitopes, detectable by specific antibodies. Maximal phosphorylation in vitro of hTau40 by PKAcat incorporates only 2-3 moles of phosphate/mole. Consequent, but smaller, reduction in electrophoretic mobility is seen, but not the formation of Alzheimer-specific or hyperphosphorylation-specific epitopes. Phosphorylation of hTau40 by PKAcat sharply reduces the number of phosphates that can now be introduced by PK40erk2 to 5-6 moles/mole, instead of the expected 11 moles/mole. Thus, prior phosphorylation by PKA, a non-proline-directed protein kinase, regulates the conformation of the protein substrate Tau so as to make some sites very much less accessible to phosphorylation by the proline-directed kinase, PK40erk2.