Unique functional properties of a member of the Fushi Tarazu-Factor 1 family from Schistosoma mansoni

SmFtz-F1 (Schistosoma mansoni Fushi Tarazu-Factor 1) belongs to the Ftz-F1 subfamily of nuclear receptors, but displays marked structural differences compared with its mammalian homologues SF-1 (steroidogenic factor-1) or liver receptor homologue-1. These include a long F domain (104 amino acids), an unusually large hinge region (133 amino acids) and a poorly conserved E-domain. Here, using Gal4 constructs and a mammalian two-hybrid assay, we have characterized the roles of these specific regions both in the transcriptional activity of the receptor and in its interactions with cofactors. Our results have shown that, although the AF-2 (activation function-2) region is the major activation function of the receptor, both the F and D domains are essential for AF-2-dependent activity. Modelling of SmFtz-F1 LBD (ligand-binding domain) and structure-guided mutagenesis allowed us to show the important role of helix H1 in maintaining the structural conformation of the LBD, and suggested that its autonomous transactivation activity, also observed with SF-1, is fortuitous. This strategy also allowed us to study an eventual ligand dependence for this orphan receptor, the predicted three-dimensional models suggesting that the SmFtz-F1 LBD contains a large and well-defined ligand-binding pocket sealed by two arginine residues orientated towards the interior of the cavity. Mutation of these two residues provoked a loss of transcriptional activity of the receptor, and strongly reduced its interaction with SRC1 (steroid receptor cofactor-1), suggesting a ligand-dependent activity for SmFtz-F1. Taken together, our results argue for original and specific functional activities for this platyhelminth nuclear receptor.