Activation of orphan receptor-mediated transcription by Ca2+/calmodulin-dependent protein kinase IV

Retinoid-related receptor alpha (ROR alpha) is an orphan nuclear receptor that constitutively activates transcription from its cognate response element. We show that ROR alpha is Ca2+ responsive, and a Ca2+/calmodulinin-dependent form of Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) potentiates ROR alpha-dependent transcription 20- to 30-fold, Other orphan receptors including ROR alpha 2, ROR gamma and COUP-TFI are also potentiated by CaMKIV. Transcriptional activation by CaMKIV is orphan receptor selective and does not occur with either the thyroid hormone or estrogen receptor, CaMKIV does not phosphorylate ROR alpha or its ligand-binding domain (LBD) in vitro, although the LED is essential for transactivation. Therefore, the ROR alpha LED was used in the mammalian two-hybrid assay to identify a single class of small peptide molecules containing LXXLL motifs that interacted with greater affinity in the presence of CaMKIV. This class of peptides antagonized activation of orphan receptor-mediated transcription by CaMKIV, These studies demonstrate a pivotal role for CaMKIV in the regulation of orphan receptor-mediated transcription.