Bc13, an IκB protein, as a novel transcription coactivator of the retinoid X receptor

We have recently shown that the IκB protein IκBβ interacted with the retinoid X receptor (RXR) and inhibited the 9-cis-retinoic acid (RA)- dependent transactivations (Na, S.-Y., Kim, H.-J., Lee, S.-K., Choi, H.-S., Na, D. S., Lee, M.-O., Chung, M., Moore, D. D., and Lee, J. W. (1998) J. Biol. Chem. 6, 3212-3215). Herein, we show that a distinct IκB protein Bc13 also interacts with RXR, as shown in the yeast two-hybrid tests and glutathione S-transferase pull-down assays. The Bc13 interaction involved two distinct subregions of RXR, i.e. constitutive interactions of the N-terminal ABC domains and 9-cis-RA-dependent interactions of the C-terminal DEF domains. In contrast to Īβ, Bc13 did not interact with the AF2 domain of RXR. Bc13 specifically interacted with the general transcription factors TFIIB, TBP, and TFIIA but not with TFIIEα in the GST pull-down assays. TBP and TFIIA, however, were not able to interact with IκBβ. Accordingly, Bc13 coactivated the 9-cis-RA-induced transactivations of RXR, in contrast to the inhibitory actions of IκBβ. In addition, coexpression of SRC-1 but not p300 further stimulated the Bc13-mediated enhancement of the 9-cis-RA-induced transactivations of RXR. These results suggest that distinct IκB proteins differentially modulate the 9-cis-RA-induced transactivations of RXR in vivo.