Nuclear transport plays a key role in regulating the levels and activities of transcription factors, nuclear kinases,and replication factors. Although targeting sequences for nuclear import, export, and subnuclear targeting have been defined, these processes remain poorly understood. The nuclear pore complex (NPC) mediates transport across the nuclear envelope. This bidirectional transport process is regulated, in part, by the ubiquitous intracellular mediators GTP and Ca+2, suggesting a coordinated regulation of nuclear transport and other signal transduction pathways. We identified calmodulin as an activator of nuclear transport and suggested that it plays a role in the regulation of transport during cell activation. Thus, both the GTP dependent molecular switch, Ran, and the Ca+2 dependent switch, calmodulin, may function as regulators of nuclear transport. Nuclear transport proceeds through the nuclear pore. The Nuclear pore complex contains numerous proteins bearing both phosphate and O-GlcNAc attached to Ser and Thr residues. We have studied the glycosylation and phosphorylation of nuclear pore proteins extensively. This post-translational modification is also present on RNA polymerase II, and numerous polymerase II transcription factors. Addition and removal of O-GlcNAc are dynamic processes occurring in the cytoplasm and nucleoplasm. O-GlcNAc is transferred to proteins from UDP-GlcNAc, a sugar nucleotide whose levels are regulated by the hexosamine biosynthetic pathway. The hexosamine biosynthetic pathway is a cellular sensor of energy availability and has been suggested to be involved in the regulation of a number of gene products including leptin, the product of the ob gene. Current evidence suggests that the O-linked GlcNAc transferase mediates a novel glycan-dependent signal transduction pathway. We have molecularly cloned and characterized the human O-linked GlcNAc transferase responsible for glycosylating nuclear pore proteins and transcription factors. When expressed in E. coli, the human O-linked GlcNAc transferase is catalytically active. Although the enzyme is found in a number of target tissues, it is most highly expressed in human pancreatic beta cells, consistent with a role in glucose-sensing. Based on its substrate specificity and molecular features, we have proposed that O-linked GlcNAc transferase is the terminal step in a glucose-responsive pathway that becomes disregulated in diabetes mellitus (NIDDM). Using reverse genetics, knockout, and other transgenic models we are currently exploring the role of this essential gene in signal transduction and the pathogenesis of diabetes mellitus.
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