Abstract
SUMO modification of nuclear receptors, including the constitutively active receptor steroidogenic factor 1 (SF-1; NR5A1), is proposed to repress their transcriptional activity. We examined the functional and structural consequences of SF-1 sumoylation at two conserved lysines (Lys119 and Lys194) that reside adjacent to the DNA-binding domain (DBD) and ligand-binding domain (LBD), respectively. Surprisingly, while previous loss-of-function studies predicted that sumoylation at Lys194 would greatly impact SF-1 function, the conformation and coregulator recruitment of fully sumoylated SF-1 LBD protein was either unchanged or modestly impaired. Sumoylation at Lys194 also modestly reduced Ser203 phosphorylation. In contrast to these findings, sumoylation of the DBD at Lys119 resulted in a marked and selective loss of DNA binding to noncanonical SF-1 targets, such as inhibinalpha; this binding deficit was extended to all sites when the sumoylated human mutant (R92Q) protein, which exhibits lower activity, was used. Consistent with this result, the K119R mutant, compared to wild-type SF-1, was selectively recruited to a "SUMO-sensitive" site in the endogenous inhibinalpha promoter, leading to increased transcription. DNA binding and sumoylation of Lys119 appeared to be mutually exclusive, suggesting that once SF-1 is bound to DNA, sumoylation may be less important in regulating SF-1 activity. We propose that sumoylation of nuclear receptors imposes an active posttranslational mark that dampens recognition of SUMO-sensitive target genes to restrain their expression.