14-3-3 connects glycogen synthase kinase-3β to tau within a brain microtubule-associated tau phosphorylation complex

In a recent study, we reported that in bovine brain extract, glycogen synthase kinase-3beta and tau are parts of an similar to400-500 kDa microtubule-associated tau phosphorylation complex (Sun, W., Qureshi, H. Y., Cafferty, P. W., Sobue, K., Agarwal-Mawal, A., Neufield, K. D., and Paudel, H. K. (2002) J. Biol. Chem. 277,11933-11940). In this study, we find that when purified brain microtubules are subjected to Superose 12 gel filtration column chromatography, the dimeric scaffold protein 14-3-3xi coelutes with the tau phosphorylation complex components tau and GSK3beta. From gel filtration fractions containing the tau phosphorylation complex, 14-3-3xi, GSK3beta, and tau co-immunoprecipitate with each other. From extracts of bovine brain, COS-7 cells, and HEK-293 cells transfected with GSK3beta, 14-3-3xi co-precipitates with GSK3beta, indicating that GSK3beta binds to 14-3-3xi. From HEK-293 cells transfected with tau, GSK3beta, and 14-3-3xi in different combinations, tau co-immunoprecipitates with GSK3beta only in the presence of 14-3-3xi. In vitro, similar to10-fold more tau binds to GSK3beta in the presence of than in the absence of 14-3-3xi. In transfected HEK-293 cells, 14-3-3xi stimulates GSK3beta-catalyzed tau phosphorylation in a dose-dependent manner. These data indicate that in brain, the 14-3-3xi dimer simultaneously binds and bridges tau and GSK3beta and stimulates GSK3beta-catalyzed tau phosphorylation.