PHOSPHORYLATION OF TAU AT BOTH THR-231 AND SER-262 IS REQUIRED FOR MAXIMAL INHIBITION OF ITS BINDING TO MICROTUBULES

The paired helical filaments (PHFs) found in Alzheimer's disease (AD) brains are composed primarily of the microtubule-associated protein ta n. PHF-tau is in a hyperphosphorylated state and is unable to promote microtubule assembly. We investigated whether the inhibition of tau bi nding to microtubules is increased when tau is phosphorylated by diffe rent kinases in combination with GSK-3, We found that when tau was fir st phosphorylated by A-kinase, C-kinase, cdk5, or CaM kinase II and th en by GSK-3, its binding to microtubules was inhibited by 45, 61, 78, and 79%, respectively. Further, the kinase combinations cdk5/GSK-3 and CaM kinase II/GSK-3 rapidly phosphorylated the sites Thr 231 and Ser 235. When these sites were individually replaced by Ala and the phosph orylation experiments repeated, tan binding to microtubules was inhibi ted by 54 and 71%, respectively. By comparison, when Ser 262 was repla ced by Ala, tau binding to microtubules was inhibited by only 8% after phosphorylation by CaM kinase II. From these observations we estimate that the phosphorylation of Thr 231, Ser 235, and Ser 262 contributes similar to 26, similar to 9, and similar to 33%, respectively, of the overall inhibition of tau binding to microtubules. Together, our resu lts indicate that the binding of tau to microtubules is controlled by the phosphorylation of several sites, among which are Thr 231, Ser 235 , and Ser 262. (C) 1998 Academic Press.