The NFκB transcription factor was originally identified as a protein complex consisting of a DNA-binding subunit and an associated protein. The DNAbinding subunit is functionally related to c-Rel p75 and Rel B p68. The p50 subunit was initially believed to be a functionally unique protein derived from the amino-terminus of a precursor designated p105. A second protein designated p52 (previously referred to as p49) has been identified that can act as an alternative NFκB subunit. Rel B does not bind with high affinity to NFκB sites, but heterodimers between Rel B and p50 bind with an affinity comparable to that of p50 NFκB homodimers. However, Rel B/p50 heterodimers, in contrast to NFκB heterodimers, transactivates transcription of promotors containing κB binding sites.
