Modeling of hDAT ∆N336 predicts the formation of a new K337-D345 hydrogen bond. (A) Modeling using Rosetta shows that hDAT (blue) positions a free residue K337 toward the intracellular space. Modeling hDAT ∆N336 (red) reveals a repositioning of K337 that, in turn, forms a new K337-D345 hydrogen bond that is absent in hDAT. This K337-D345 bond may stabilize IL3 by linking it to TM7. (B) Cartoon representation of a homology model of hDAT embedded in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine lipid bilayer. The chloride (cyan) and Na+ (blue) ions; DA (orange); and side chains of K66 (green), K337 (yellow), and D345 (red) are highlighted. (C) hDAT at 50 ns: salt bridge between K66 and D345. (D) hDAT ∆N336 at 50 ns: salt bridge between K66 and D345 (replica 2). (E) hDAT ∆N336 at 100 ns: residue D345 forms two salt bridges, one to K66 and a second to K337 (replica 2) (also SI Appendix, Fig. S3).