Nucleoplasmic and cytoplasmic glycoproteins.
We have described a new form of protein glycosylation in which N-acetylglucosamine is glycosidically linked to the hydroxyl of serine or threonine (O-GlcNAc). Unlike most other forms of protein glycosylation, O-GlcNAc is predominantly localized in the nuclear and cytoplasmic compartments of cells, where it occurs on important nuclear pore glycoproteins, well-characterized cytoskeletal proteins, as well as on many chromatin proteins, including factors that regulate gene transcription. Gas-phase protein sequencing of three O-GlcNAc-modified proteins has identified a common structural feature at sites of O-GlcNAc addition. An assay for UDP-GlcNAc:polypeptide O-GlcNAc transferase has been developed. The enzyme appears to be membrane-associated, its active site is cytoplasmic, and it has an absolute requirement for Mn2+. We are now purifying this glycosyltransferase, characterizing its substrate specificity, and determining the extent of elongation of attached saccharide moieties. The functions of O-GlcNAc remain largely unknown, but it may be important in blocking phosphorylation sites, it may be required for the assembly of specific multiprotein complexes, it might serve as a nuclear transport signal, or it may be directly involved in the active transport of macromolecules across nuclear pores.
Hart
G W
GW
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Haltiwanger
R S
RS
Holt
G D
GD
Kelly
W G
WG
D000117:Acetylglucosamine / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002467:Cell Nucleus / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D005700:Galactosyltransferases / Q000378:metabolism; D006023:Glycoproteins / Q000096:biosynthesis*; D006031:Glycosylation; D006801:Humans; D008969:Molecular Sequence Data
D006023:Glycoproteins; D005700:Galactosyltransferases; D000117:Acetylglucosamine