Alternate RNA splicing of murine nfkb1 generates a nuclear isoform of the p50 precursor NF-κB1 that can function as a transactivator of NF-κB- regulated transcription

The NF-κB1 subunit of the transcription factor NF-κB is derived by proteolytic cleavage from the N terminus of a 105-kDa precursor protein. The C terminus of p105NF-κB1, like those of IκB proteins, contains ankyrin- related repeats that inhibit DNA binding and nuclear localization of the precursor and confer IκB-like properties upon p105NF-κB1. Here we report the characterization of two novel NF-κB1 precursor isoforms. p84NF-κB1 and p98NF-κB1, that arise by alternate splicing within the C-terminal coding region of murine nfkb1. p98NF-κB1, which lacks the 111 C-terminal amino acids (aa) of p105NF-κB1, has a novel 35-aa C terminus encoded by an alternate reading frame of the gene. p84NF-κB1 lacks the C-terminal 190 aa of p105NF-κB1, including part of ankyrin repeat 7. RNA and protein analyses indicated that the expression of p84NF-κB1 and p98NF-κB1 is restricted to certain tissues and that the phorbol myristate acetate-mediated induction of p84NF-κB1 and p105NF-κB1 differs in a cell-type-specific manner. Both p84NF-κB1 and p98NF-κB1 are found in the nuclei of transfected cells. Transient transfection analysis revealed that p98NF-κB1, but not p105NF- κB1 or p84NF-κB1, acts a transactivator of NF-κB-regulated gene expression and that this is dependent on sequences in the Rel homology domain required for DNA binding and on the novel 35 C-terminal aa of this isoform. In contrast to previous findings, which indicated that p105NF-κB1 does not bind DNA, all of the NF-κB1 precursors were found to specifically bind with low affinity to a highly restricted set of NF-κB sites in vitro, thereby raising the possibility that certain of the NF-κB1 precursor isoforms may directly modulate gene expression.