THE ENDOGENOUS AND CELL CYCLE-DEPENDENT PHOSPHORYLATION OF TAU-PROTEIN IN LIVING CELLS - IMPLICATIONS FOR ALZHEIMERS-DISEASE

In Alzheimer's disease the neuronal microtubule-associated protein tau becomes highly phosphorylated, loses its binding properties, and aggr egates into paired helical filaments. There is increasing evidence tha t the events leading to this hyperphosphorylation are related to mitot ic mechanisms. Hence, we have analyzed the physiological phosphorylati on of endogenous tau protein in metabolically labeled human neuroblast oma cells and in Chinese hamster ovary cells stably transfected with t au. In nonsynchronized cultures the phosphorylation pattern was remark ably similar in both cell lines, suggesting a similar balance of kinas es and phosphatases with respect to tau. Using phosphopeptide mapping and sequencing we identified 17 phosphorylation sites comprising 80-90 % of the total phosphate incorporated. Most of these are in SP or TP m otifs, except S214 and S262. Since phosphorylation of microtubule-asso ciated proteins increases during mitosis, concomitant with increased m icrotubule dynamics, we analyzed cells mitotically arrested with nocod azole. This revealed that S214 is a prominent phosphorylation site in metaphase, but not in interphase. Phosphorylation of this residue stro ngly decreases the tau-microtubule interaction in vitro, suppresses mi crotubule assembly, and may be a key factor in the observed detachment of tau from microtubules during mitosis. Since S214 is also phosphory lated in Alzheimer's disease tau, our results support the view that re activation of the cell cycle machinery is involved in tau hyperphospho rylation.