THE ACTIVATION FUNCTION-2 HEXAMER OF STEROIDOGENIC FACTOR-I IS REQUIRED, BUT NOT SUFFICIENT FOR POTENTIATION BY SRC-1

The orphan receptor steroidogenic factor 1 (SF-1) plays a central role in development and differentiation of the adrenal gland and gonads. I t also regulates the expression of several pivotal steroidogenic enzym es and other proteins that are essential for reproductive function. It s mechanism of target gene activation that directs these intricate pro cesses has not been previously established. We demonstrate here that t he activation function-2 (AF-2) activation hexamer (AF-2-AH) of SF-1, located within its carboxy-terminal region, is required for reporter g ene activation by SF-1, as well as for SF-1-mediated induction of a st eroidogenic phenotype in embryonic stem cells. We further demonstrate that SF-1's AF-2-AH is not sufficient for gene activation, requiring a n additional, proximally located domain of SF-1, positioned between re sidues 187-245. Correspondingly, we show that the coactivator SRC-1 po tentiates the activity of SF-1 and that the interaction between SF-1 a nd SRC-1 requires both AF-2-AH and the proximal activation domain. We conclude that SF-1 harbors at least two activation domains within its carboxy terminus and that both are required for its transcriptional ac tivation function and for direct interaction with SRC-1. It is likely that SRC-1 plays a key role in gene regulation by SF-1.