DNA binding and IκB inhibition of the cloned p65 subunit of NF-κB, a rel-related polypeptide

The sequence and biochemical properties of the product of the cloned cDNA for the p65 subunit of nuclear factor kappa B (NF-kappa B) have been determined. The cDNA has an open reading frame of 549 amino acids capable of encoding a 60 kd protein. NF-kappa B p65 contains an amino-terminal region of 320 amino acids with extensive similarity to the oncogene c-rel and lesser similarity to NF-kappa B p50. In vitro translated p65 forms a DNA-binding complex with NF-kappa B p50, and the binding of this complex can be specifically inhibited by purified I kappa B. Progressive carboxy-terminal deletions of p65 show that, contrary to previous assumptions, p65 does include a DNA-binding domain that in vivo might become activated only through hetero-oligomerization with p50. DNA binding by truncated p65 is inhibited by I kappa B, thus mapping the I kappa B interaction domain to the rel-homologous region and suggesting that I kappa B exerts its inhibitory effect upon NF-kappa B primarily through interaction with p65.