THE DNA-BINDING DOMAIN OF HEPATOCYTE NUCLEAR FACTOR-4 MEDIATES COOPERATIVE, SPECIFIC BINDING TO DNA AND HETERODIMERIZATION WITH THE RETINOID-X RECEPTOR-ALPHA

We recently showed that hepatocyte nuclear factor 4 (HNF-4) defines a unique subclass of nuclear receptors that exist in solution and bind D NA elements as homodimers (Jiang, G., Nepomuceno, L., Hopkins, K., and Sladek, F. M. (1995) Mel. Cell. Biol. 15, 5131-5143). In this study, we show that the dimerization domains of HNF-4 map to both the DNA bin ding and the ligand binding domain, Whereas the latter is critical for dimerization in solution, the DNA binding domain mediates cooperative , specific binding to direct repeats of AGGTCA separated by one or two nucleotides. Whereas amino acid residues 117-125 (the T-box/third hel ix region) are insufficient for cooperative homodimerization and high affinity DNA binding, residues 126-142 (encompassing the A-box region) are required. Finally, in contrast to the full-length receptor, the D NA binding domain of HNF-4 is capable of heterodimerizing with that of the retinoid X receptor alpha but not with that of other receptors. T hese results indicate that the HNF-4 DNA binding domain is distinct fr om that of other receptors and that the determinants that prevent HNF- 4 from heterodimerizing with RXR lie outside the DNA binding domain, p resumably in the ligand binding domain.