Phosphorylation and glycosylation of nucleoporins

The nuclear pore complex mediates macromolecular transport between the nucl eus and cytoplasm. Many nuclear pore components (nucleoporins) are modified by both phosphate and O-linked N-acetylglucosamine (O-GlcNAc). Among its m any functions, protein phosphorylation plays essential roles in cell cycle progression, The role of O-GlcNAc addition is unknown. Here, levels of nucl eoporin phosphorylation and glycosylation during cell cycle progression are examined. Whereas nuclear pore glycoproteins are phosphorylated in a cell- cycle-dependent manner, levels of O-GlcNAc remain constant. The major nucle oporin p62 can be phosphorylated in vitro by protein kinase A and glycogen synthase kinase (GSK)-3 alpha but not by cyclin B/cdc2 or GSK-3 beta. The c onsensus sites of these kinases resemble sites which can be glycosylated by O-GlcNAc transferase. These data are consistent with a model that O-GlcNAc limits nucleoporin hyperphosphorylation during M-phase and hastens the res umption of regulated nuclear transport at the completion of cell division. (C) 1999 Academic Press.