The orphan nuclear receptor NOR-1 interacts with the homeobox containing protein Six3

Neuron-derived orphan receptor 1 (NOR-1) is a member of the NGFI-B subfamil y within the nuclear receptor superfamily. In order to identify cofactors t hat associate with NOR-1 in the fetal forebrain, we tested a yeast two-hybr id system with the NOR-1 cDNA fragment lacking a transactivating domain as a bait. By screening of the rat fetal brain embryonic day 17 library, a rat homologue of Six3 was identified as an associated protein. We demonstrated that NOR-1 interacted with Six3 in yeast and in vitro, and the association was required for the DNA binding and AF2 domains of NOR-1. Regarding the o ther members of the family (NGFI-B and RNR-1), association with Six3 was no t observed in yeast. In addition, cotransfection experiments with Six3 and NOR-1 indicated that Six3 had a negative activity against the transactivati on by NOR-1 through the NBRE response element in a dose-dependent manner. T he overlap in expression of NOR-1 and Six3 was mainly detected in the rat f etal forebrain on embryonic day 18. Thereafter, the expression of both gene s diminished rapidly. These results suggest that a dimer consisting of a ho meobox containing protein Six3 and transcriptional factor NOR-1 might regul ate gene expression during the late stage of the fetal forebrain developmen t. This study provides, after the association of Ftz and Ftz-F1 in Drosophi la, another example of a dimer formation of a homeobox protein and an orpha n nuclear receptor. Copyright (C) 2001 S. Karger AG, Basel.