Characterization of steroid receptor coactivator in sea urchin, Strongylocentrotus nudus, and its involvement in embryonic development

Ligand-bound nuclear receptors (NRs) recruit coactivators such as members of the p160 steroid receptor coactivator (SRC) family and cyclic AMP responsive element binding protein (CREB)- binding protein (CBP) to specific enhancer elements and activate target gene transcription. In the present study, we isolated a novel SRC from the sea urchin Strongylocentrotus nudus (SnSRC) by using the ligand-binding domain of retinoid X receptor as a bait in a yeast two-hybrid screening. The SnSRC and vertebrate SRCs are different in size but share the overall characteristic domains, such as NR interacting domain (NID), CBP-binding and glutamine-rich regions. SnSRC mRNA showed highest expression levels at the 32-cell, 64-cell and pluteus larval stages. Full-length SnSRC (1992 amino acids) interacted with several NRs, including sea urchin estrogen receptor- related receptor (ERR), human and masu salmon estrogen receptors (ERalpha), mouse ERRgamma, rat glucocorticoid receptor alpha, and rat thyroid receptor beta. The SnSRC possesses two functional NIDs, both of which are dependent on their core LxxLL motifs. Furthermore, preferential interacting domains for ERalpha in the SnSRC are located in the central LxxLL motifs, revealed by the truncation and mutagenesis studies. Strikingly, the SnSRC has a single transcription activation domain, which interacts with CBP, a transcriptional integrator. In addition, transient knockdown of the SnSRC gene in the sea urchin embryo using morpoholino antisense RNA induced abnormal phenotypes at gastrulation stage such as the lack of primary invagitation and exogastrulation. These results suggest that the SnSRC is a new member of the SRC family and plays an important role during early embryonic development.