I kappa B alpha-mediated inhibition of v-Rel DNA binding requires direct interaction with the RXXRXRXXC Rel/kappa B DNA-binding motif.

Rel family proteins bind to kappa B DNA sites, form heterodimers with one another, and modulate expression of genes linked to kappa B motifs. I kappa B factors associate with Rel proteins, inhibit Rel DNA binding in vitro, and displace DNA from DNA-bound Rel complexes. We have investigated the mechanism by which the p40/I kappa B alpha inhibitor interfers with Rel DNA-binding activity. Here, we report that p40 contacts the RXXRXRXXC DNA-binding motif conserved in all Rel family proteins, in addition to associating with the nuclear localizing sequence. Competition assays with a Rel-derived peptide comprising the DNA-binding region specifically alleviated p40-mediated inhibition of v-Rel DNA-binding activity, whereas a covalently modified Rel peptide was inactive. Combined, these results indicate that I kappa B alpha interaction with the RXXRXRXXC motif is required for inhibition of v-Rel DNA binding and suggest that nuclear I kappa B factors may be critical for regulating transcription by Rel family proteins. IMAGES: