Nuclear transport is critical for maintenance of the levels and activities of transcription factors, nuclear kinases, and replication factors. We identified Ca+2/calmodulin as an activator of nuclear import and suggested a role for Ca+2 in the regulation of nuclear import during cell activation. We also demonstrated a role for the multifunctional lectin calreticulin in nuclear export. We suggest that bi-directional transport across the nuclear pore is controlled by GTP and Ca+2, thus providing coordinate regulation of nuclear transport and other signal transduction pathways. 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. O-linked GlcNAc transferase may mediate 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). The cytoplasmic O-GlcNAcase involved in GlcNAc removal has also been cloned and expressed in recombinant form. Using reverse genetics, knockout, and other transgenic models we are currently exploring the role of these essential proteins in signal transduction and the pathogenesis of diabetes mellitus.
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