Hyperphosphorylation of the retinoid X receptor α 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 kinases, Mutants for each putative site were generated and overexpressed in transfected COS-1 cells. Constitutively phosphorylated residues identified by tryptic phosphopeptide mapping included serine 22 located in the A1 region that is specific to the RXR alpha 1 isoform, Overexpression and UV activation of the stress-activated kinases, c-Jun NH2-terminal kinases 1 and 2 (JNK1 and JNK2), hyperphosphorylated RXR alpha, resulting in a marked decrease in its electrophoretic mobility. This inducible hyperphosphorylation involved three residues (serines 61 and 75 and threonine 87) in the B region of RXR alpha and one residue (serine 265) in the ligand binding domain (E region). Binding assays performed in vitro with purified recombinant proteins demonstrated that JNKs did not interact with RXR alpha but bound to its heterodimeric partners, retinoic acid receptors alpha and gamma (RAR alpha and RAR gamma). Hyperphosphorylation by JNKs did not affect the transactivation properties of either RXR alpha homodimers or RXR alpha/RAR alpha heterodimers in transfected cultured cells.