Importance of the Extracellular Loop 4 in the Human Serotonin Transporter for Inhibitor Binding and Substrate Translocation

Abstract

Background:The role of extracellular loop regions during substrate translocation in the serotonin transporter (SERT) is not well understood. Results: A mutation in the extracellular loop 4 of SERT disrupts the conformational equilibrium by favoring an outward-facing conformation. Conclusion: Extracellular loop 4 is important for conformational transitions in SERT.Significance: New insights are gained into the coordinated conformational changes associated with substrate translocation in SERT.

Monoamine transporters: structure, intrinsic dynamics and allosteric regulation

“…5h), depending on the transporter and the drug, but the coordination by the flexible (or rotatable) EL4a-EL4b linker residue Gly402 (or Phe320) appears to be essential for allosteric action. Notably, the mutation L406E in the immediate vicinity of Gly402 induces a remarkable gain of potency for a range of SERT inhibitors, in further support of the proposal of a role for EL4 in allosterically modulating inhibitor actions 82 . Finally, R104K is another SERT mutation found to cause a drastic decrease in potency 81 .…”

“… a Substrate, sodium, chloride and CHOL site I for MATs are based on the equivalent positions in the crystal structures resolved for dDAT and hSERT, and computational and mutational studies 111 . b Reported for SERT only 81,82 and computationally predicted for LeuT 55 . c Experimentally validated, compiled from previous reviews or publications. …”

Extracellular loops of the serotonin transporter act as a selectivity filter for drug binding

“…Thus EL2 and EL4 can be conceptualized as part of a selectivity filter in the extracellular gate, which limits access of inhibitors. Site-directed mutagenesis studies also point to a role of EL4 in determining the affinity for at least some inhibitors: substitution of L406 in EL4b by glutamate accelerates the association rate constant for [ 3 H]citalopram but not [ 125 I]RTI-55 (3β-(4-iodophenyl)tropan-2 beta-carboxylic acid methyl ester) ( 20 ).…”

“…The apparent movement of EL4 is more subtle in the inward-facing state of SERT ( 4 ). However, a mutation in EL4 (L406E), which is located in the vicinity of the tip of the wedge, enhances the binding of inhibitors and reduces substrate turnover rate ( 20 ). This is consistent with a role of EL4 in controlling access to the central binding site.…”

Depressive disorders: Treatment failures and poor prognosis over the last 50 years

“…The majority of antidepressants in clinical use today act by enhancing the neurotransmission of monoamines, serotonin (5‐HT), NE, DA, or all 3, either directly or indirectly . This is done by either blocking reuptake via monoamine transporters or blocking the metabolism of monoamines by inhibiting the major catalytic degradation enzymes, monoamine oxidase, and catechol‐o‐methyltransferase (COMT) . Other modes of action include direct or indirect modulation of receptors or signal transduction mechanism.…”