The sigma enigma: in vitro/in silico site-directed mutagenesis studies unveil σ1 receptor ligand binding

The σ1 receptor is an integral membrane protein that shares no homology with other receptor systems, has no unequivocally identified natural ligands, but appears to play critical roles in a wide variety of cell functions. While the number of reports of the possible functions of the σ1 receptor is increasing, almost no information about the three-dimensional structure of the receptor and/or possible modes of interaction of the σ1 protein with its ligands have been described. Here we performed an in vitro/in silico investigation to analyze the molecular interactions of the σ1 receptor with its prototypical agonist (+)-pentazocine. Accordingly, 23 mutant σ1 isoforms were generated, and their interactions with (+)-pentazocine were determined experimentally. All direct and/or indirect effects exerted by the mutant residues on the receptor-agonist interactions were reproduced and rationalized in silico, thus shining new light on the three-dimensional structure of the σ1 receptor and its ligand binding site.