The predominant inducible form of the NF-KB transcription factor is a heteromeric complex containing two Rel-related DNA-binding subunits, termed p65 and p50. Prior transfection studies have shown that when these p65 and p50 subunits are expressed independently as stable homodimers, p65 stimulates KB-directed transcription, whereas p50 functions as a KB-specific repressor. While authentic p50 homodimers (previously termed KBF1) have been detected in nuclear extracts from nontransfected cells, experimental evidence supporting the existence of p65 homodimers in vivo was lacking. We now provide direct biochemical evidence for the presence of an endogenous pool of inducible p65 homodimers in intact human T cells. As with the prototypical NF-cB p50-p65 heterodimer, this novel p65 homodimeric form of NF-KB is functionally sequestered in the cytoplasm but rapidly appears in the nuclear compartment following cellular stimulation.Site-directed mutagenesis studies indicate that the homodimerization function of p65 is dependent upon the presence of cysteine 216 and a conserved recognition motif for protein kinase A (RRPS; amino acids 273 to 276), both of which reside within a 91-amino-acid segment of the Rel homology domain that mediates self-association. In contrast, mutations at these two sites do not affect heterodimerization of p65 with p50 or its functional interaction with IKB. These later findings indicate that neither homo-nor heterodimer formation is an absolute prerequisite for IKBc recognition of p65. Taken together with prior in vivo transcription studies, these results suggest that the biological activities of p65 and p50 homodimers are independently regulated, thereby providing an integrated and flexible control mechanism for the rapid activation and repression of NF-KB/Reldirected gene expression.Nuclear induction of the NF-KcB transcription factor complex is an important convergent step in several distinct receptor-mediated signaling pathways that potentiate the growth response of activated T lymphocytes (19,26,39,62). This specific inductive event is initiated upon cellular stimulation with a variety of soluble mediators, including T-cell receptor ligands (22,29,33,34), interleukin-1 (52, 63), interleukin-2 (3), tumor necrosis factor alpha (40, 52), and phorbol esters (5, 12,62). Prior studies have revealed that the major inducible form of NF-KB corresponds to a heterodimeric complex composed of 65-kDa (p65) and 50-kDa (p50) subunits (6). Each of these DNA-binding proteins has extensive amino acid sequence homology with the N-terminal halves of the v-Rel oncoprotein, its cellular homolog c-Rel, and the Drosophila morphogen Dorsal (13,24,36,42,50,55; reviewed in reference 25). The induced nuclear expression of this p50-p65 complex is believed to involve the phosphorylation, dissociation, and subsequent degradation of an associated cytoplasmic inhibitor now termed IKBa (4,5,17,23,27,65). The recent finding that NF-KB also specifically activates IKBot gene expression indicates the presence of an autoregulator...