Y95 and E444 interaction required for high-affinity S-citalopram binding in the human serotonin transporter

The human serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT) is responsible for the reuptake of 5-HT following synaptic release, as well as for import of the biogenic amine into several non-5-HT synthesizing cells including platelets. The antidepressant citalopram blocks SERT and thereby inhibits the transport of 5-HT. To identify key residues establishing high-affinity citalopram binding, we have built a comparative model of hSERT and Drosophila melanogaster SERT (dSERT) based on the Aquifex aeolicus leucine transporter (LeuT(Aa)) crystal structure. In this study, citalopram has been docked into the homology model of hSERT and dSERT using RosettaLigand. Our models reproduce the differential binding affinities for the R- and S-isomers of citalopram in hSERT and the impact of several hSERT mutants. Species-selective binding affinities for hSERT and dSERT also can be reproduced. Interestingly, the model predicts a hydrogen bond between E444 in transmembrane domain 8 (TM8) and Y95 in TM1 that places Y95 in a downward position, thereby removing Y95 from a direct interaction with S-citalopram. Mutation of E444D results in a 10-fold reduced binding affinity for S-citalopram, supporting the hypothesis that Y95 and E444 form a stabilizing interaction in the S-citalopram/hSERT complex.