A lymphoid cell-specific nuclear factor containing c-rel-like proteins preferentially interacts with interleukin-6 κB-related motifs whose activities are repressed in lymphoid cells

The poroto-oncoprotein c-Rel is a member of the nuclear factor κB transcription factor family, which includes the p50 and p65 subunits of nuclear factor κB. We show here that c-Rel binds to κB sites as homodimers as well as heterodimers with P50. These homodimers and heterodimers show distinct DNA-binding specificities and affinities for various κB motifs. In particular, the c-Rel homodimer has a high affinity for interleukin-6 (IL-6) and bete interferon κB sites. In spite of its association with p50 in vitro, however, we found a lymphoid cell-specific nuclear factor in vivo that contains c-Rel but not P50 epitopes; this factor, termed IL-6κB binding factor II, appears to contain the c-Rel homodimer and preferentially recognizes several IL-6κB-related κB motifs. Although it has been previously shown that the IL-6κB motif functions as a potent IL-1/tumor necrosis factor-responsive element in nonlymphoid cells, its activity was found to be repressed in lymphoid cells such as a Jurkat T-cell line. We also present evidence that IL-6κB binding factor II functions as a repressor specific for IL-6κB-related κB motifs in lymphoid cells.