Regulated nuclear transport is critical for the 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. This evolutionarily ancient, calcium-dependent import pathway was proposed to facilitate uptake of a distinct subset of nuclear proteins during cell activation. The physiological significance of the Ca+2-calmodulin-regulated transport pathway became clear with the finding that defects in calmodulin-dependent nuclear import underlie certain forms of human sex reversal. The HMG-box architectural transcription factors SRY and SOX9 must enter the nucleus of Sertoli cells and bind tightly to target DNA for proper male gonad development. Thus, SRY acts as the primary trigger for maleness; nuclear transport of SRY is required to release this trigger. In a subset of human patients with autosomal sex reversal (Swyers syndrome, Campomelic dysplasia) nuclear transport of SRY and SOX9 is blocked. The mutations associated with these defects reside in a conserved calmodulin-binding motif near the amino terminus of SRY and SOX9. Thus, the SOX family of transcription factors appears to use Ca+2/calmodulin both as import receptor and molecular switch allowing for nuclear import and DNA binding. A similar autosomal sex reversal phenotype occurs when three insulin-related receptors are ablated in mice suggesting that intracellular signaling cascades may impact the normal functions of SRY and SOX9. More recently, the gene encoding TCF7L2 was shown to be a diabetes susceptibility locus in the Icelandic population. This transcription factor is a member of the HMG-box family containing a calmodulin-binding motif. The import of nuclear proteins by calmodulin, while functionally redundant with the canonic Ran-dependent pathway, is subject to independent regulation by intracellular Ca+2 mobilization. Abnormalities in calmodulin-dependent import emerge as the primary defect in patients with autosomal sex-reversal. These sex-reversal syndromes represent the first direct examples of a defect in a nuclear import pathway leading to human disease. Other human disorders involving SOX-family transcription factors may have similar defects in calmodulin dependent nuclear import. In addition to analysis of the import of the HMG-box transcription factors in mammals, we have recently shown that the calmodulin-dependent nuclear import pathway functions in yeast. We are currently taking genetic approaches to define additional components of this novel import pathway and seek to identify small molecule inhibitors of the process.
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