Structural insight into the selective agonist ST1936 binding of serotonin receptor 5-HT6.

The serotonin receptor 5-HT6R is an important G-protein-coupled receptor (GPCR) that involved in essential functions within the central and peripheral nervous systems and is linked to various psychiatric disorders. Selective activation of 5-HT6R promotes neural stem cell regeneration activity. As a 5-HT6R selective agonist, 2-(5 chloro-2-methyl-1H-indol-3-yl)-N, N-dimethylethanolamine (ST1936) has been widely used to investigate the functions of the 5-HT6R. The molecular mechanism of how ST1936 is recognized by 5-HT6R and how it effectively couples with Gs remain unclear. Here, we reconstituted the ST1936-5-HT6R-Gs complex in vitro and solved its cryo-electron microscopy structure at 3.1 Å resolution. Further structural analysis and mutational studies facilitated us to identify the residues of the Y3107.43 and "toggle switch" W2816.48 of the 5-HT6R contributed to the higher efficacy of ST1936 compared with 5-HT. By uncovering the structural foundation of how 5-HT6R specifically recognizes agonists and elucidating the molecular process of G protein activation, our discoveries offer valuable insights and pave the way for the development of promising 5-HT6R agonists.