DIFFERENTIAL PHOSPHORYLATION OF HUMAN TAU ISOFORMS CONTAINING 3 REPEATS BY SEVERAL PROTEIN-KINASES

The paired helical filaments (PHF) found in the brain of patients with Alzheimer disease (AD) are composed primarily of the microtubule-asso ciated protein tau. Six isoforms of tau have been recognized and all a re present in a hyperphosphorylated state in PHF. It is not known whet her all tau isoforms serve equally well as substrates for various kina ses. In this study we have compared the phosphorylation of human tau i soforms containing three microtubule-binding repeats and zero (tau 3), one (tau 3S), or two (tau 3L) N-terminal inserts. Four kinases (A-kin ase, CK-1, CaM kinase II, GSK-3) were used for this purpose. With A-ki nase, CK-1, and CaM kinase II the extent of phosphorylation was tau 3L > tau 3S > tau 3, With GSK-3 it was tau 3L similar or equal to tau 3S > tau 3. Tau 3 was a poor substrate for either CaM kinase II or CK-1, P-32 incorporation being only 5 and 11%, respectively, of that observ ed by these kinases when tau 3L was the substrate, After prephosphoryl ation of the three tau isoforms by A-kinase, a subsequent phosphorylat ion by GSK-3 was stimulated several fold over tau that was not prephos phorylated. Under these conditions the extent of P-32 incorporation wa s tau 3L > tau 3S > tau 3. Both CK-1 and GSK-3 phosphorylated ser 396 more rapidly in tau 3L compared to tau 3 or tau 3S. Our results sugges t that (1) the presence of N-terminal inserts in tau isoforms are impo rtant structural determinants that modulate the specificity of several kinases; (2) the different tau isoforms may be present at different s tates of phosphorylation in PHF. (C) 1996 Academic Press, Inc.