Nuclear glycogen and glycogen synthase kinase 3.

Glycogen is the principal storage form of glucose in animal cells. It accumulates in electron-dense cytoplasmic granules and is synthesized by glycogen synthase (GS), the rate-limiting enzyme of glycogen deposition. Glycogen synthase kinase-3 (GSK-3) is a protein kinase that phosphorylates GS. Two nearly identical forms of GSK-3 exist: GSK-3 alpha and GSK-3 beta. Both are constitutively active in resting cells and their activity can be modulated by hormones and growth factors. GSK-3 is implicated in the regulation of many physiological responses in mammalian cells by phosphorylating substrates including neuronal cell adhesion molecule, neurofilaments, synapsin I, and tau. Recent observations point to functions for glycogen and glycogen metabolism in the nucleus. GSK-3 phosphorylates several transcription factors, and we have recently shown that it modifies the major nuclear pore protein p62. It also regulates PK1, a protein kinase required for maintaining the interphase state and for DNA replication in cycling Xenopus egg extracts. Recently, glycogen was shown to be required for nuclear reformation in vitro using ovulated Xenopus laevis egg lysates. Because neither glycogen nor GSK-3 has been localized to the nuclear envelope or intranuclear sites, glycogen and GSK-3 activites were measured in rat liver nuclei and nuclear reformation extracts. Significant quantities of glycogen-like material co-purified with the rat-liver nuclear envelope. GSK-3 is also highly enriched in the glycogen pellet of egg extracts of Xenopus that is required for nuclear assembly in vitro. Based on the finding that enzymes of glycogen metabolism copurify with glycogen, we propose that glycogen may serve a structural role as a scaffold for nuclear assembly and sequestration of critical kinases and phosphatases in the nucleus.