The N Terminus Specifies the Switch between Transport Modes of the Human Serotonin Transporter

J Biol Chem. 2017 Mar 3;292(9):3603-3613. doi: 10.1074/jbc.M116.771360. Epub 2017 Jan 19.


The serotonin transporter (SERT) and other monoamine transporters operate in either a forward transport mode where the transporter undergoes a full transport cycle or an exchange mode where the transporter seesaws through half-cycles. Amphetamines trigger the exchange mode, leading to substrate efflux. This efflux was proposed to rely on the N terminus, which was suggested to adopt different conformations in the inward facing, outward facing and amphetamine-bound states. This prediction was verified by tryptic digestion of SERT-expressing membranes: in the absence of Na+, the N terminus was rapidly digested. Amphetamine conferred protection against cleavage, suggesting a relay between the conformational states of the hydrophobic core and the N terminus. We searched for a candidate segment that supported the conformational switch by serial truncation removing 22 (ΔN22), 32 (ΔN32), or 42 (ΔN42) N-terminal residues. This did not affect surface expression, inhibitor binding, and substrate influx. However, amphetamine-induced efflux by SERT-ΔN32 or SERT-ΔN42 (but not by SERT-ΔN22) was markedly diminished. We examined the individual steps in the transport cycle by recording transporter-associated currents: the recovery rate of capacitive peak, but not of steady state, currents was significantly lower for SERT-ΔN32 than that of wild type SERT and SERT-ΔN22. Thus, the exchange mode of SERT-ΔN32 was selectively impaired. Our observations show that the N terminus affords the switch between transport modes. The findings are consistent with a model where the N terminus acts as a lever to support amphetamine-induced efflux by SERT.

Keywords: neurotransmitter release; neurotransmitter transport; patch clamp; serotonin; serotonin transporter.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amphetamines / chemistry*
  • Bacterial Proteins / chemistry
  • Biotinylation
  • HEK293 Cells
  • Humans
  • Hydrophobic and Hydrophilic Interactions
  • Inhibitory Concentration 50
  • Luminescent Proteins / chemistry
  • Microscopy, Confocal
  • Neurotransmitter Agents / chemistry
  • Patch-Clamp Techniques
  • Protein Conformation
  • Protein Domains
  • Serotonin / chemistry
  • Serotonin Plasma Membrane Transport Proteins / chemistry*
  • Sodium / chemistry
  • Trypsin / chemistry


  • Amphetamines
  • Bacterial Proteins
  • Luminescent Proteins
  • Neurotransmitter Agents
  • Serotonin Plasma Membrane Transport Proteins
  • yellow fluorescent protein, Bacteria
  • Serotonin
  • Sodium
  • Trypsin