AUTOREGULATION OF THE NF-KAPPA-B TRANSACTIVATOR RELA (P65) BY MULTIPLE CYTOPLASMIC INHIBITORS CONTAINING ANKYRIN MOTIFS

RelA (p65) functions as the critical transactivating component of the heterodimeric p50-p65 NF-kappa B complex and contains a high-affinity binding site for its cytoplasmic inhibitor, I kappa B alpha. After cel lular activation, I kappa B alpha is rapidly degraded in concert with the induced nuclear translocation of NF-kappa B. The present study dem onstrates that tumor necrosis factor alpha-induced degradation of I ka ppa B alpha in human T cells is preceded by its rapid phosphorylation in vivo. However, these effects on I kappa B alpha result in nuclear m obilization of only a fraction of the entire cytoplasmic pool of RelA. Subsequent studies have revealed that (i) cytoplasmic RelA is stably associated not only with I kappa B alpha but also with other ankyrin m otif-rich proteins including the products of the NF-kappa B2 (p100) an d NF-kappa B1 (p105) genes; (ii) in contrast to RelA-I kappa B alpha, RelA-p100 cytoplasmic complexes are not dissociated following tumor ne crosis factor alpha activation; (iii) p100 functions as a potent inhib itor of RelA-mediated transcription in vivo; (iv) the interaction of R elA and p100 involves the conserved Rel homology domain of both protei ns but not the nuclear localization signal of RelA, which is required for I kappa B alpha binding; (v) p100 inhibition of RelA function requ ires the C-terminal ankyrin motif domain, which mediates cytoplasmic r etention of RelA; and (vi) as observed with I kappa B alpha, nuclear R elA stimulates p100 mRNA and protein expression. These findings thus r eveal the presence of a second inducible autoregulated inhibitory path way that helps ensure the rapid but transient action of nuclear NF-kap pa B.