Phosphorylation of tau by proline-directed protein kinase (p34cdc2/p58cyclin A) decreases tau-induced microtubule assembly and antibody SMI33 reactivity.

Tau protein was evaluated as a substrate for a proline-directed protein kinase (p34cdc2/p58cyclin A) which recognizes the phosphorylation site motif X-Ser/Thr-Pro-X. The shortest human tau isoform, expressed as a recombinant protein, was phosphorylated to a stoichiometry of 2 mol phosphate/mol tau. Phosphoamino acid analysis revealed phosphorylation of both serine and threonine residues. Phosphorylation of recombinant tau resulted in a decreased ability to induce microtubule assembly but had no effect on the final extent of microtubule formation or on the rate of cold-induced microtubule disassembly. Phosphorylation of tau by the proline-directed protein kinase completely blocked immunoreactivity with antibody SMI33. Phosphorylation did not create the epitopes for the phosphate-dependent antibodies SMI31 or SMI34. Antibody SMI33 recognizes neurofibrillary tangles after treatment with alkaline phosphatase, suggesting that the proline-directed protein kinase may phosphorylate tau at sites that are phosphorylated in Alzheimer's disease.