NF-AT activation requires suppression of Crm1-dependent export by calcineurin

Nuclear import of the NF-AT transcription factors during T-cell activation requires the calcium-activated phosphatase calcineurin, which unmasks nuclear-location signals on NF-AT (refs 1-5), We show here that the nuclear import of NF-ATs is not sufficient to activate NF-AT target genes, as NF-ATs are subject to a futile cycling across the nuclear em elope owing to engagement with the exportin protein Crm1 (refs 6-8), Calcineurin suppresses this futile cycling by a non-catalytic mechanism involving the masking of nuclear export signals on NF-AT targeted by Crm1, This clustering of binding sites for calcineurin and Crm1 on NF-AT establishes an inherent competition between these molecules that imparts exquisite calcium sensitivity to the shuttling dynamics of the NF-AT transcription factors. Such a balance between nuclear import and export may regulate the action of other transcription factors.