Glycosylation of nuclear and cytoplasmic proteins by the addition of a single N- acetylglucosamine monosaccharide (O-GlcNAc) is a major posttranslational modification in higher eukaryotes (Hart 1997). O-GlcNAcylation is characterized by the addition of single GlcNAc residues from a UDP-GlcNAc sugar donor to the hydroxyl groups of serine/threonine residues via an O-glycosidic bond in a β-linkage. Unlike some other forms of O-glycosylation, this sugar is generally unmodified and is not usually further elongated. O-GlcNAcylation of proteins has been shown to be both highly abundant and dynamic, often in response to cellular stimuli (Hart 1997), and it has been suggested that it is a form of regulation alternative to, but working in concert with, serine/threonine phosphorylation. Proteins modified by O-GlcNAc are myriad in form and function, with classes which include nuclear pore proteins, transcription factors, RNA polymerase II, oncoproteins, protein kinases and phosphatases, steroid receptors, and cytoskeletal proteins such as MAPs, neurofilaments, and α- crystallins (Hart 1997). Recently, an enzyme that catalyzes this modification was purified to homogeneity from rat liver (Haltiwanger et al. 1992), and the rat and human cDNAs were cloned (Kreppel et al. 1997; Lubas et al. 1997). This chapter examines the characteristics of this enzyme in detail.
KeywordsAcceptor Substrate Tetratricopeptide Repeat GlcNAc Residue Nuclear Pore Protein Stem Cell Viability
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