I-Rel: a novel rel-related protein that inhibits NF-kappa B transcriptional activity.

The NF-kappa B transcription factor complex is comprised of two subunits, p50 and p65, that share significant homology to the rel oncogene. We have isolated a cDNA encoding a novel 66-kD rel-related protein, designated I-Rel. Unlike other rel-related proteins, I-Rel does not interact with DNA. I-Rel forms heterodimers with p50, however, and greatly attenuates its DNA-binding activity--an effect probably resulting from the presence of a domain inhibitory to DNA binding present within the 121 amino-terminal residues of I-Rel. In contrast, I-Rel does not associate with p65. Transfection experiments demonstrate that I-Rel suppresses NF-kappa B-induced transcription, probably through its association with p50. Expression of I-Rel mRNA is induced by mitogenic stimulation and accumulates after the appearance of p50 transcripts. Our findings suggest that p50 and I-Rel are components of a feedback pathway where expression of I-Rel may modulate indirectly the expression of genes responsive to the NF-kappa B transcription factor complex.