Helix XI contributes to the entrance of the serotonin transporter permeation pathway

The sodium‐dependent transporters for dopamine, norepinephrine, and serotonin that regulate neurotransmission, also translocate the neurotoxin 1‐methyl‐4‐phenylpyridinium (MPP + ). Previous studies implicated residues in transmembrane helix (TMH) XI of DAT as important sites for MPP + transport. We examined the importance of TMH XI residues F551 and F556 for MPP + translocation by human SERT. Mutations at hSERT F556, but not F551, reduced both 5‐HT and MPP + transport compared to wild type. However, F556S/hSERT showed a reduction in surface expression explaining the decrease of transport activity for 5‐HT, but did not account for the decrease in MPP + transport observed. Cysteine mutants at those positions confirmed the accessibility of hSERT/F556 to different methanethiosulfonate (MTS) reagents, suggesting its presence in a hydrophilic environment of the protein. In the presence of MTSET, current induced by 5‐HT and MPP + was inhibited at the F556C mutant. In agreement with our homology model of SERT, based on the leucine transporter (LeuT Aa ) from Aquifex aeolicus structure, these results are consistent with the hypothesis that a portion of TMH XI lines the entrance into the substrate permeation pathway.