Resolution of the nuclear localization mechanism of glycogen synthase kinase-3: functional effects in apoptosis

Mechanisms regulating the nuclear localization of glycogen synthase kinase-3beta (GSK3beta) remained enigmatic despite the crucial regulation by nuclear GSK3beta of important cellular functions. These include regulation of gene expression, cell cycle progression, and apoptosis, achieved by the phosphorylation by GSK3 of nuclear substrates (e.g. numerous transcription factors). We resolved this mechanism by identifying a bipartite nuclear localization sequence (NLS) that is necessary for the nuclear accumulation of GSK3beta and is sufficient to drive yellow fluorescent protein into the nucleus. Despite the NLS, most GSK3beta is cytosolic, sequestered in protein complexes that, although still mobile in the cytosol, block the NLS. Conditions promoting nuclear translocation of GSK3beta release it from cytosolic complexes, allowing the NLS to direct nuclear import. Using this information to prepare a nucleus-excluded active GSK3 construct, we found that the antiapoptotic effect of GSK3beta in tumor necrosis factor-induced apoptosis is mediated by cytosolic, not nuclear, GSK3beta. Identification of a GSK3beta NLS allows new strategies to decipher and manipulate its subcellular actions regulating gene expression and apoptosis and its involvement in diseases.