Proline-directed phosphorylation of human Tau protein.

The primary sequence of the microtubule-associated protein tau contains multiple repeats of the sequence -X-Ser/Thr-Pro-X-, the consensus sequence for the proline-directed protein kinase (p34cdc2/p58cyclin A). When phosphorylated by proline-directed protein kinase in vitro, tau was found to incorporate up to 4.4 mol of phosphate/mol of protein. Isoelectric focusing of the tryptic phosphopeptides demonstrated the presence of five distinct peptides with pI values of approximately 6.9, 6.5, 5.6-5.9, 4.7, and 3.6. Mapping of the tryptic phosphopeptides by high performance liquid chromatography techniques demonstrated three distinct peaks. Data from gas phase sequencing, amino acid analysis, and phosphoamino acid analysis suggest that proline-directed protein kinase phosphorylates tau at four sites. Each site demonstrates the presence of a proline residue on the carboxyl-terminal side of the phosphorylated residue. Two phosphorylation sites are located adjacent to the three-repeat microtubule-binding domain that has been found to be required for the in vivo co-localization of tau protein to microtubules. Two other putative phosphorylation sites are located within the identified epitope of the monoclonal antibody Tau-1. Phosphorylation of these sites altered the immunoreactivity of tau to Tau-1 antibody. Since the neuronal microtubule-associated protein tau is multiply phosphorylated in Alzheimer's disease, and Tau-1 immunoreactivity is similarly reduced in neurofibrillary tangles and enhanced after dephosphorylation, phosphorylation at one or more of these sites may correlate with abnormally phosphorylated sites in tau protein in Alzheimer's disease.