Evolution of Nuclear Retinoic Acid Receptor Alpha (RARα) Phosphorylation Sites. Serine Gain Provides Fine-Tuned Regulation

The human nuclear retinoic acid (RA) receptor alpha (hRAR alpha) is a ligand-dependent transcriptional regulator, which is controlled by a phosphorylation cascade. The cascade starts with the RA-induced phosphorylation of a serine residue located in the ligand-binding domain, S(LBD), allowing the recruitment of the cdk7/cyclin H/MAT1 subcomplex of TFIIH through the docking of cyclin H. It ends by the subsequent phosphorylation by cdk7 of an other serine located in the N-terminal domain, S(NTD). Here, we show that this cascade relies on an increase in the flexibility of the domain involved in cyclin H binding, subsequently to the phosphorylation of S(LBD). Owing to the functional importance of RAR alpha in several vertebrate species, we investigated whether the phosphorylation cascade was conserved in zebrafish (Danio rerio), which expresses two RAR alpha genes: RAR alpha-A and RAR alpha-B. We found that in zebrafish RAR alpha s, S(LBD) is absent, whereas S(NTD) is conserved and phosphorylated. Therefore, we analyzed the pattern of conservation of the phosphorylation sites and traced back their evolution. We found that S(LBD) is most often absent outside mammalian RAR alpha and appears late during vertebrate evolution. In contrast, S(NTD) is conserved, indicating that the phosphorylation of this functional site has been under ancient high selection constraint. This suggests that, during evolution, different regulatory circuits control RAR alpha activity.