MUTATIONS THAT ALTER LIGAND-INDUCED SWITCHES AND DIMERIZATION ACTIVITIES IN THE RETINOID-X RECEPTOR

The retinoid X receptor (RXR) heterodimerizes with a variety of nuclea r receptors. In addition, RXR forms homodimers in the presence of its ligand, 9-cis-retinoic acid. From deletion and point mutation analysis we present evidence that a short region (amino acids 413 to 443) in t he carboxy terminus of RXR alpha is critical for both homo- and hetero dimeric interactions as well as for diverse functional activities. In addition, we present evidence that homo- and heterodimer functions can be separated. The deletion of 19 amino acids from the C-terminal end of RXR dramatically reduced the transcriptional activation function of RXR. The removal of 10 additional amino acids resulted in a receptor (Delta RXR3) that had completely lost its ligand-dependent homodimer f unction but retained its heterodimer activities. Heterodimer function was abolished by the deletion of an additional 20 amino acids. Single amino acid substitutions in the region generated receptors with altere d RXR homodimer DNA binding, while Simultaneous mutation of three Leu residues (Leu-418, -419 and -422) completely abolished both RXR homodi mer and heterodimer DNA binding activities. Mutation of Leu-430 to Phe (L430-F) resulted in a receptor that bound to DNA strongly as homodim ers in a ligand-independent manner, while another single amino acid ex change (L422-Q) led to a mutant that behaved in a manner exactly oppos ite to that of wild-type RXR in that the homodimerization of the mutan t occurred in the absence of ligand and was inhibited by 9-cis-retinoi c acid. In transfection assays, both L422-Q and L430-F failed to act a s homodimers but retained their heterodimer function. Our studies demo nstrate the unique properties of the RXR ligand binding domain and poi nt to specific residues that mediate homo- and heterodimer activities and ligand-induced conformational switches.