Presenilin-1 is an unprimed glycogen synthase kinase-3β substrate

Previously we described presenilin-I (PSI) as a GSK-3 beta substrate [Kirschenbaum, F., Hsu, S.C., Cordell, B. and McCarthy, J.V. (2001) Substitution of a glycogen synthase kinase-3beta phosphorylation site in presenilin I separates presenilin function from beta-catenin signalling. J. Biol. Chem. 276, 7366-7375; Kirschenbaum, F., Hsu, S.C., Cordell, B. and McCarthy, J.V. (2001) Glycogen synthase kinase-3beta regulates presenilin 1 C-terminal fragment levels. J. Biol. Chem. 276, 30701-30707], though it has not been determined whether PSI is a primed or unprimed GSK-3 beta substrate. A means of separating GSK-3 beta activity toward primed and unprimed substrates was identified in the GSK-3 beta-R96A phosphate binding pocket mutant [Frame, S., Cohen, P. and Biondi, R.M. (2001) A common phosphate binding site explains the unique substrate specificity of GSK3 and its inactivation by phosphorylation. Mol. Cell 7, 1321-1327], which is unable to phosphorylate primed but retains the ability to phosphorylate unprimed GSK-30 substrates. By using wild type GSK-3 beta, GSK-3 beta-R96A, and a pharmacological modulator of GSK-3 beta activity, we demonstrate that PSI is an unprimed GSK-3 beta substrate. These findings have important implications for regulation of PSI function and the pathogenesis of Alzheimer's disease. (c) 2006 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.