HIGH-RESOLUTION SOLUTION STRUCTURE OF THE RETINOID-X-RECEPTOR DNA-BINDING DOMAIN

The retinoid X receptor (RXR) is a member of the nuclear hormone recep tor superfamily of transcriptional regulators and plays a central role in the retinoid and, through its ability to heterodimerize with other nuclear hormone receptors, non-steroid signaling pathways. The DNA-bi nding and recognition functions of RXR are located in a conserved 83 a mino acid residue domain that recognizes the consensus sequence AGGTCA . in order to provide a detailed picture of its structure, we have cal culated a high-resolution solution structure of the C195A RXR alpha DN A-binding domain. Structures were calculated using 1131 distance and d ihedral angle constraints derived from H-1, C-13 and N-15 NMR spectra. The structures reveal a perpendicularly packed, ''loop-helix'' fold s imilar to other nuclear hormone receptor DNA-binding domains and confi rm the existence of the C-terminal helix, which was first observed in the low-resolution NMR structure. The C-terminal helix is well formed and is stabilized by packing interactions with residues in the hydroph obic core. The solution structure of RXR is very similar to that deter mined by X-ray crystallographic studies of the RXR-TR heterodimer comp lex with DNA, except that in the latter case no electron density was o bserved for residues corresponding to the C-terminal helix. Other diff erences between the X-ray and NMR structures occur in the second zinc- binding loop, which is disordered in solution. Heteronuclear N-15 NOE measurements suggest that this loop has enhanced flexibility in the fr ee protein. (C) 1998 Academic Press.