Coactivators for the orphan nuclear receptor ROR alpha

A mutation in the nuclear orphan receptor ROR alpha results in a severe imp airment of cerebellar development by unknown mechanisms. We have shown prev iously that ROR alpha contains a strong constitutive activation domain in i ts C terminus, We therefore searched for mammalian ROR alpha coactivators u sing the minimal activation domain as bait in a two-hybrid screen. Several known and putative coactivators were isolated, including glucocorticoid rec eptor-interacting protein-1 (GRIP-1) and peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP/TRAP220/DRIP205). These interactions w ere confirmed in vitro and require the intact activation domain of ROR alph a although different requirements for interaction with GRIP-1 and PBP were detected. Even in the absence of exogenous ligand, ROR alpha interacts with a complex or complexes of endogenous proteins, similar to those that bind to ligand-occupied thyroid hormone and vitamin D receptors, Both PBP and GR IP-1 were shown to be present in these complexes. Thus we have identified s everal potential ROR alpha coactivators that, in contrast to the interactio ns with hormone receptors, interact with ROR alpha in yeast, in bacterial e xtracts, and in mammalian cells in vivo and in vitro in the absence of exog enous ligand. GRIP-1 functioned as a coactivator for the ROR alpha both in yeast and in mammalian cells. Thus, GRIP-1 is the first proven coactivator for ROR alpha.