A novel Daf-12 coactivator

DANIEL HOCHBAUM; YUE ZHANG; ALFRED L. FISHER

Encuentro; UPPDA meeting; 2009

The nematode C. elegans develops through four larval stages before becoming an adult. When environmental conditions are not favorable for reproduction, animals will arrest at the dauer diapause, which is a long-lived stress resistant alternate larval stage. When conditions are favorable, the dauer worms are able to rapidly resume development and become an adult. A key regulator of dauer formation is the nuclear receptor daf-12, which is the C elegans homolog of the vitamin D (VDR) and lipid X (LXR) receptors. daf-12 contains a DNA binding domain (DBD) and a ligand binding domain (LBD), which binds either co-activator or co-repressor complexes depending on the binding of ligand. Extensive work has shown that in a favorable environment, Insulin and TGF-β-like hormone secretion is required for the production of dafachronic acids (DA) which are bile acid-like ligands for daf-12. In unfavorable environments, neither the Insulin nor TGF-β-like hormones are present and DA are not produced. This leads the unliganded daf-12 receptor to bind to the din-1 co-repressor which drives dauer arrest. Favorable environments lead to the production of DA which disrupts this interaction and leads to adult development. While the hormones, receptor, and co-repressor involved in this process have been identified, the co-activators and target genes are unknown. In an attempt to find genes regulated by daf-12, a transgenic worm expressing a TAP-tagged daf-12 was generated. A chip on chip assay was performed, leading to the identification of >100 candidate target genes. To our surprise, daf-12 binds to its co-repressor din-1. This suggests that din-1 is both a target gene and a cofactor for daf-12. We also found that daf-12 binds to the cbp-1 promoter, which is the C elegans homolog of the co-activator CBP (CREBBP). By analogy with din-1, we hypothesized that cbp-1 could be both a target gene and co-activator for daf-12. For this, we are conducting yeast two hybrid experiments to test for interaction between these proteins. If an interaction is observed, we will also test if cbp-1 interacts with daf-12(rh284) which contains a point mutation within the C-terminal AF-2 domain that often binds co-activators. Together these results could establish a novel co-activator for daf-12, and lead to a model where environmental conditions establish different developmental programs, based on cbp-1 or din-1 association with daf-12.