Differential ligand-dependent protein-protein interactions between nuclearreceptors and a neuronal-specific cofactor

Nuclear receptors are transcription factors that require multiple protein-p rotein interactions to regulate target gene expression. We have cloned a 27 -kDa protein, termed NIX1 (neuronal interacting factor X 1), that directly binds nuclear receptors in vitro and in vivo. Protein-protein interaction b etween NIX1 and ligand-activated or constitutive active nuclear receptors, including retinoid-related orphan receptor beta (ROR beta) (NR1F2), strictl y depends on the conserved receptor C-terminal activation function 2 (AF2-D ). NIX1 selectively binds retinoic acid receptor (RAR) (NR1A) and thyroid h ormone receptor (TR) (NR1B) in a ligand-dependent manner, but does not inte ract with retinoid X receptor (RXR) (NR2B) or steroid hormone receptors, In terestingly, NIX1 down-regulates transcriptional activation by binding to l igand-bound nuclear receptors, A 39-aa domain within NIX1 was found to be n ecessary and sufficient for protein-protein interactions with nuclear recep tors, Northern blot analysis demonstrates low-abundance RNA messages only i n brain and neuronal cells. In site hybridization and immunohistochemistry revealed that NIX1 expression is restricted to the central nervous system a nd could be confined to neurons in the dentate gyrus of the hippocampus, th e amygdala, thalamic, and hypothalamic regions. In summary, protein-protein interactions between the neuronal protein NIX1 and ligand-activated nuclea r receptors are both specific and selective. By suppressing receptor-mediat ed transcription, NIX1 implements coregulation of nuclear receptor function s in brain.