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

Previously we described presenilin-1 (PS1) as a GSK-3β substrate [Kirschenbaum, F., Hsu, S.C., Cordell, B. and McCarthy, J.V. (2001) Substitution of a glycogen synthase kinase-3beta phosphorylation site in presenilin 1 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 PS1 is a primed or unprimed GSK-3β substrate. A means of separating GSK-3β activity toward primed and unprimed substrates was identified in the GSK-3β-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-3β substrates. By using wild type GSK-3β, GSK-3β-R96A, and a pharmacological modulator of GSK-3β activity, we demonstrate that PS1 is an unprimed GSK-3β substrate. These findings have important implications for regulation of PS1 function and the pathogenesis of Alzheimer’s disease.