Abnormal phosphorylation of tau proteins associated with bovine brain microtubules: activation by excess ATP and tyrosine dephosphorylation.

Phosphorylation of tau protein has been suggested as a major mechanism regulating its functions. In assembled brain microtubules, tau is phosphorylated, but additional phosphorylation can be induced in vitro. Supply of excess ATP alone was sufficient to reduce migration of tau on SDS gels, diminish Tau-1 immunostaining, and induce the expression of epitopes recognized by the PHF-1 antibody, suggesting that Alzheimer-type phosphorylation may have occurred. Okadaic acid had no further effect. However, treatment with tyrosine kinase inhibitors modifies the phosphorylation profiles of tau proteins. Most evidently, migration of the largest tau isoform was further retarded on SDS gels and PHF-1 immunostaining was enhanced. The profound effect of tyrosine kinase inhibitors on tau phosphorylation was also demonstrated in living cells following microinjection of cultured hippocampal neurons. Identification of proline-directed protein kinases and their regulatory factors associated with assembled microtubules indicated the presence of multiple phosphorylation pathways in microtubules. Our results are consistent with the hypothesis that sequential phosphorylation of tau proteins is at least partially mediated through tyrosine phosphorylation/dephosphorylation mechanisms.