Tau, one of the best characterized microtubule-associated proteins (MAPs), is a phosphoprotein, the biological activity of which is regulated by its degree of phosphorylation. The objective of the present study was to evaluate the regulation, phosphorylation and the biological activity of tau during differentiation. On differentiation, the tau/tubulin ratio increased about 3-fold regardless whether cells were optimally differentiated with retinoic acid and aphidicolin or with retinoic acid alone which does not inhibit proliferation. The phosphorylation at the Tau-1 (Ser-195/Ser-198/Ser-199/Ser-202) and PHF-1 (Ser-396/Ser-404) sites was increased, mostly in the retinoic acid treated cells, whereas phosphorylation of tau at the 12E8 (Ser-262/Ser-356) epitope was decreased in both groups by approximately 60%. Phosphorylation at the 12E8 site is thought to be one of the most prominent factors affecting the biological activity of tau. However, the microtubule binding activity of tau increased only slightly upon differentiation. Furthermore, a large part of the tau that bound to taxol-stabilized microtubules was phosphorylated at all three sites indicating that these sites are not major sites determining the biological activity of tau. These data show that differentiation of SY5Y cells results in increased tau levels rather than dephosphorylation of tau to meet the additional need in tau's biological activity.
Copyright 1999 Elsevier Science B.V.