Fig. 1. Experimental evidence for allosteric binding between the S1 and S2 sites in SERT.

a Chemical structure of the tested drugs. Left: imipramine (IMI). Right: R(−)- and S(+)-citalopram (R-CIT and S-CIT, respectively). b Allosteric potency of S-CIT measured as concentration-dependent inhibition of either [³H]IMI or [³H]S-CIT dissociation, prebound to SERT prior to the addition of S-CIT in the indicated concentrations. Data plotted as inhibition of [³H]ligand dissociation rate by S-CIT relative to no S-CIT added (ctrl). Prebound [³H]S-CIT (triangles) results in a 29-fold increase in S-CIT allosteric potency relative to prebound [³H]IMI (circles), with IC₅₀ = 5.1 [4.6, 5.8] µM and 152 [125, 185] µM, respectively (mean [S.E. interval], n = 3). c Allosteric potency of R-CIT measured as in (b). Here R-CIT possesses a fourfold higher allosteric potency for prebound [³H]IMI relative to prebound [³H]S-CIT, with IC₅₀ = 5.3 [4.9; 5.8] µM and 21.5 [20.3; 22.7] µM, respectively (mean [S.E. interval], n = 3–6). d Allosteric potency of S-CIT (blue) and R-CIT (green) in SERT E494Q predicted to dissipate the allosteric interaction between S1 and S2 sites. The allosteric potency is now collapsed around the observed allosteric potency for S-CIT inhibition of [³H]IMI dissociation (right dotted line) in SERT WT. The allosteric potency of S-CIT (blue) for inhibition of [³H]S-CIT (triangles) and [³H]IMI (circles) dissociation in E494Q is 64.8 [56.6; 74.1] µM and 131 [123; 140] µM, respectively (mean [S.E. interval], n = 5–6). The same values for R-CIT are 137 [118; 158] µM and 199 [191; 207] µM, respectively (mean [S.E. interval], n = 5). Left dotted line is allosteric potency for S-CIT with prebound [³H]S-CIT from (b), shown for comparison. e Allosteric potency for S-CIT with prebound [³H]S-CIT (triangles) or [³H]IMI (circles) in SERT T497A. Thr497 is predicted to mediate the S1:S2 allosteric interaction (see Fig. 2). The IC₅₀ is collapsed, around the allosteric potency for [³H]S-CIT dissociation from SERT WT (dotted lines); IC₅₀ for S-CIT: 5.10 [3.20; 8.00] µM and 8.80 [5.90; 13.4] µM for inhibition of [³H]S-CIT and [³H]IMI dissociation, respectively. The same values for R-CIT are 17.8 [15.7; 20.2] and 11.6 [10.1; 13.3], respectively (mean [S.E. interval], n = 3). Source data are provided as a Source Data file.