NRIF3 is a novel coactivator mediating functional specificity of nuclear hormone receptors

Many nuclear receptors are capable of recognizing similar DNA elements. The molecular event(s) underlying the functional specificities of these recept ors (in regulating the expression of their native target genes) is a very i mportant issue that remains poorly understood. Here we report the cloning a nd analysis of a novel nuclear receptor coactivator (designated NRIF3) that exhibits a distinct receptor specificity. Fluorescence microscopy shows th at NRIF3 localizes to the cell nucleus. The yeast two-hybrid and/or in vitr o binding assays indicated that NRIF3 specifically interacts with the thyro id hormone receptor (TR) and retinoid X receptor (RXR) in a ligand dependen t fashion but does not bind to the retinoic acid receptor, vitamin D recept or, progesterone receptor, glucocorticoid receptor, or estrogen receptor. F unctional experiments showed that NRIF3 significantly potentiates TR- and R XR-mediated transactivation in vivo but has little effect on other examined nuclear receptors. Domain and mutagenesis analyses indicated that a novel C-terminal domain in NRIF3 plays an essential role in its specific interact ion with liganded TR and RXR while the N-terminal LXXLL motif plays a minor role in allowing optimum interaction. Computer modeling and subsequent exp erimental analysis suggested that the C-terminal domain of NRIF3 directly m ediates interaction with liganded receptors through an LXXIL (a variant of the canonical LXXLL) module while the other part of the NRIF3 protein may s till play a role in conferring its receptor specificity. Identification of a coactivator with such a unique receptor specificity may provide new insig ht into the molecular mechanism(s) of receptor-mediated transcriptional act ivation as well as the functional specificities of nuclear receptors.