Distinct molecular regulation of glycogen synthase kinase-3alpha isozyme controlled by its N-terminal region: functional role in calcium/calpain signaling.
Glycogen synthase kinase-3 (GSK-3) is expressed as two isozymes α and β. They share high similarity in their catalytic domains but differ in their N- and C-terminal regions, with GSK-3α having an extended glycine-rich N terminus. Here, we undertook live cell imaging combined with molecular and bioinformatic studies to understand the distinct functions of the GSK-3 isozymes focusing on GSK-3α N-terminal region. We found that unlike GSK-3β, which shuttles between the nucleus and cytoplasm, GSK-3α was excluded from the nucleus. Deletion of the N-terminal region of GSK-3α resulted in nuclear localization, and treatment with leptomycin B resulted in GSK-3α accumulation in the nucleus. GSK-3α rapidly accumulated in the nucleus in response to calcium or serum deprivation, and accumulation was strongly inhibited by the calpain inhibitor calpeptin. This nuclear accumulation was not mediated by cleavage of the N-terminal region or phosphorylation of GSK-3α. Rather, we show that calcium-induced GSK-3α nuclear accumulation was governed by GSK-3α binding with as yet unknown calpain-sensitive protein or proteins; this binding was mediated by the N-terminal region. Bioinformatic and experimental analyses indicated that nuclear exclusion of GSK-3α was likely an exclusive characteristic of mammalian GSK-3α. Finally, we show that nuclear localization of GSK-3α reduced the nuclear pool of β-catenin and its target cyclin D1. Taken together, these data suggest that the N-terminal region of GSK-3α is responsible for its nuclear exclusion and that binding with a calcium/calpain-sensitive product enables GSK-3α nuclear retention. We further uncovered a novel link between calcium and nuclear GSK-3α-mediated inhibition of the canonical Wnt/β-catenin pathway.
Department of Human Molecular Genetics and Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel.
Inbar
I
Azoulay-Alfaguter
Yakey
Y
Yaffe
Avital
A
Licht-Murava
Malgorzata
M
Urbanska
Jacek
J
Jaworski
Shmuel
S
Pietrokovski
Koret
K
Hirschberg
Hagit
H
Eldar-Finkelman
D021581:Active Transport, Cell Nucleus / Q000502:physiology; D000818:Animals; D019556:COS Cells; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D002154:Calpain / Q000235:genetics / Q000378:metabolism*; D002467:Cell Nucleus / Q000201:enzymology*; D002522:Chlorocebus aethiops; D038362:Glycogen Synthase Kinase 3 / Q000235:genetics / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D051381:Rats; D051153:Wnt Proteins / Q000235:genetics / Q000378:metabolism; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism
D007527:Isoenzymes; D051153:Wnt Proteins; D051176:beta Catenin; C000605506:GSK3B protein, human; D000071679:Glycogen Synthase Kinase 3 beta; C000605508:Gsk3b protein, rat; D038362:Glycogen Synthase Kinase 3; C448021:glycogen synthase kinase 3 alpha; D002154:Calpain; D002118:Calcium
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