Essential role of the C148-C227 disulphide bridge in the human 5-HT_2A homodimeric receptor

The 5-HT_2A receptor is a homodimeric G protein-coupled receptor implied in multiple diseases, including schizophrenia. Recently, its co-crystallisation with the antipsychotic drugs zotepine and risperidone has revealed the importance of its extracellular domains in its pharmacology. Previous studies have shown that the non-specific disruption of extracellular disulphide bridges in the 5-HT_2A receptor decreases ligand binding and receptor activation. There is enough evidence to hypothesize that this decrease may be due to a reduction of the disulphide bridge that links transmembrane domain 3 (TM-3) and extracellular loop 2 (ECL-2) of the 5-HT_2A receptor via cysteine 148 (C148) and C227. Thus, to study the influence of the C148-C227 disulphide bridge on 5-HT_2A receptor pharmacology, we substituted C148 and C227 in the human 5-HT_2A receptor (WT) with alanines, to obtain two single mutants (C148A and C227A) and a double mutant (C148A/C227A), and the resultant DNA constructs were used to generate four stable cell lines. These substitutions reduced the binding of the 5-HT_2A receptor to [³H]lysergic acid diethylamide ([³H]LSD) and impeded the 5-HT_2A receptor-mediated activation of phospholipase C (PLC). Furthermore, bioluminescence resonance energy transfer (BRET) and western blotting analysis revealed that these mutations did not alter the homodimeric nature of the 5-HT_2A receptor. However, fluorescence microscopy showed that these mutations hindered receptor trafficking to the cell membrane. These results illustrate the importance of the disulphide bridge between TM-3 and ECL-2 in maintaining the correct 5-HT_2A receptor conformation to allow ligand binding and migration of the homodimeric receptor to the cell membrane.