Nuclear hormone receptors are transcription factors that are uniquely regulated by lipophilic ligands. Subfamily V contains two vertebrate proteins Steroidogenic Factor 1 (SF-1, NR5A1) and Liver Receptor Homolog 1 (LRH-1, NR5A2) and a Drosophila homolog Ftz-F1 involved in larval segmentation. SF-1 controls male sexual differentiation, endocrine organ formation, and adult adrenal function. LRH-1 regulates genes important in bile acid metabolism, cholesterol transport, and ovarian physiology. Because Subfamily V receptors govern such diverse and vital transcriptional programs, it is essential to understand how these receptors are regulated. In addition to a putative phospholipid ligand, numerous posttranslational modifications such as phosphorylation and acetylation are known to modulate SF-1 and LRH-1 activity. Sumoylation or posttranslational modification with a small ubiquitin-like protein (SUMO) is a novel mechanism that potently represses Subfamily V receptors. In collaboration with Dr. Martin Lee, I examined the regulation of SF-1 sumoylation by SUMO E3 ligases, PIAS proteins, and showed that transcriptional repression of SF-1 by SUMO1 involves a DEAD-box helicase DP103, uncovering a novel role for ATPases/RNA helicases in transcriptional control. Additionally, I dissected structural and biochemical consequences of SF-1 sumoylation using in vitro sumoylation system. Sumoylation reduces in vitro MAP kinase phosphorylation of SF-1, but does not affect SF-1 LBD structure, as demonstrated by nuclear magnetic resonance spectroscopy. Moreover, I discovered a regulatory relationship between SF-1 DNA-binding and sumoylation. DNA-binding inhibits SF-1 sumoylation at the DBD, while SF-1 DBD sumoylation selectively inhibits SF-1 DNA- binding at low affinity sites. I propose that Subfamily V receptor sumoylation differentially represses target genes through combined action of the DEAD-box helicase DP103 and direct, but selective inhibition of DNA-binding. Collectively, these studies establish the importance of sumoylation in regulating transcription factor function.
