The NF-B/Rel family of transcription factors participates in the control of a wide array of genes, including genes involved in embryonic development and regulation of immune, inflammation, and stress responses. In most cells, inhibitory IB proteins sequester NF-B/Rel in the cytoplasm. Cellular stimulation results in the degradation of IB and modification of NF-B/Rel proteins, allowing NF-B/Rel to translocate to the nucleus and act on its target genes. Calmodulin (CaM) is a highly conserved,

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... uitously expressed Ca 2؉ binding protein that serves as a key mediator of intracellular Ca 2؉ signals. Here we report that two members of the NF-B/Rel family, c-Rel and RelA, interact directly with Ca 2؉ -loaded CaM. The interaction with CaM is greatly enhanced by cell stimulation, and this enhancement is blocked by addition of IB. c-Rel and RelA interact with CaM through a similar sequence near the nuclear localization signal. Compared to the wild-type protein, CaM binding-deficient mutants of c-Rel exhibit increases in both nuclear accumulation and transcriptional activity on the interleukin 2 and granulocyte macrophage colony-stimulating factor promoters in the presence of a Ca 2؉ signal. Conversely, for RelA neither nuclear accumulation nor transcriptional activity on these promoters is increased by mutation of the sequence interacting with CaM. Our results suggest that CaM binds c-Rel and RelA after their release from IB and can inhibit nuclear import of c-Rel while letting RelA translocate to the nucleus and act on its target genes. CaM can therefore differentially regulate the activation of NF-B/Rel proteins following stimulation.