Serotonin transporter (SLC6A4) extracellular allosteric S2 site (SERT S2 site)

The serotonin transporter (SERT) extracellular allosteric S2 site is a secondary binding pocket located within the extracellular vestibule of the SLC6A4 protein, distinct from the primary orthosteric substrate-binding site (Coleman et al., Nature 2016). This site plays a crucial role in modulating the kinetics of the transporter; specifically, the binding of certain selective serotonin reuptake inhibitors (SSRIs) like escitalopram to the S2 site can sterically hinder the dissociation of the drug from the primary site, thereby prolonging the inhibition of serotonin reuptake (Plenge et al., J. Biol. Chem. 2012). In the context of neuropsychiatric disorders such as major depressive disorder and anxiety, targeting this allosteric site offers a mechanism to enhance the potency and duration of therapeutic effects compared to purely orthosteric inhibitors (Chen et al., Nat. Commun. 2013). Research indicates that the S2 site is involved in the high-affinity blockade of serotonin transport, making it a significant focus for developing next-generation antidepressants with improved efficacy profiles (UniProt P31645). Understanding the structural dynamics of the S2 site has been facilitated by X-ray crystallography, revealing its proximity to the extracellular gate of the transporter and its influence on the conformational transitions required for substrate translocation. The occupancy of this site is thought to explain the superior clinical efficacy and faster onset of action observed with escitalopram compared to other SSRIs (Zhong et al., J. Med. Chem. 2012). Furthermore, the S2 site represents a potential target for allosteric modulators that could fine-tune serotonin signaling without the side effects associated with complete transporter blockade.

Other names

S2 siteExtracellular allosteric site of SERTSecondary binding site of the serotonin transporterVestibular binding site

Allosteric modulation of the serotonin transporter by binding to a secondary site (S2) located in the extracellular vestibule, which stabilizes the binding of ligands at the primary (orthosteric) site and slows their dissociation rate (Plenge et al., J. Biol. Chem. 2012; Coleman et al., Nature 2016).