PHOSPHORYLATION OF TAU BY PROLINE-DIRECTED PROTEIN-KINASE (P34(CDC2) P58(CYCLIN-A)) DECREASES TAU-INDUCED MICROTUBULE ASSEMBLY AND ANTIBODYSMI33 REACTIVITY/

Tau protein was evaluated as a substrate for a proline-directed protei n kinase (p34cdc2/p58cyclin A) which recognizes the phosphorylation si te 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 phosphorylatio n of both serine and threonine residues. Phosphorylation of recombinan t tau resulted in a decreased ability to induce microtubule assembly b ut had no effect on the final extent of microtubule formation or on th e rate of cold-induced microtubule disassembly. Phosphorylation of tau by the proline-directed protein kinase completely blocked immunoreact ivity 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 phos phatase, suggesting that the proline-directed protein kinase may phosp horylate tau at sites that are phosphorylated in Alzheimer's disease.