Hyperphosphorylation of the retinoid X receptor alpha by activated c-Jun NH2-terminal kinases

The nuclear receptor mouse retinoid X receptor alpha (mRXR alpha) was shown to be constitutively phosphorylated in its NH2-terminal A/B region, which contains potential phosphorylation sites for proline-directed Ser/Thr kinas es, Mutants for each putative site were generated and overexpressed in tran sfected COS-1 cells. Constitutively phosphorylated residues identified by t ryptic phosphopeptide mapping included serine 22 located in the A1 region t hat is specific to the RXR alpha 1 isoform, Overexpression and UV activatio n of the stress-activated kinases, c-Jun NH2-terminal kinases 1 and 2 (JNK1 and JNK2), hyperphosphorylated RXR alpha, resulting in a marked decrease i n its electrophoretic mobility. This inducible hyperphosphorylation involve d three residues (serines 61 and 75 and threonine 87) in the B region of RX R alpha and one residue (serine 265) in the ligand binding domain (E region ). Binding assays performed in vitro with purified recombinant proteins dem onstrated that JNKs did not interact with RXR alpha but bound to its hetero dimeric partners, retinoic acid receptors alpha and gamma (RAR alpha and RA R gamma). Hyperphosphorylation by JNKs did not affect the transactivation p roperties of either RXR alpha homodimers or RXR alpha/RAR alpha heterodimer s in transfected cultured cells.