A pincer-like configuration of TM2 in the human dopamine transporter is responsible for indirect effects on cocaine binding

The second transmembrane segment (TM2) of DAT and other neurotransmitter transporters has been proposed to play a role in oligomerization as well as in cocaine binding. In an attempt to determine whether TM2 contributes to the binding site and/or transport pathway of DAT, we mutated to cysteine, one at a time, 25 residues in TM2 - from Phe98 to Gln122 - in an appropriate DAT background construct. Four of the mutants, F98C, G110C, P112C, and E117C, did not express at the cell surface, and G121C was inactive, despite its presence on the cell surface. Of the 21 mutants that expressed, none of the substituted cysteines reacted with MTSEA biotin-CAP, and none of the 20 functional mutants was sensitive to MTSEA or MTSET. Thus, TM2 does not appear to be water-accessible, based both on the lack of functional effects of charged MTS derivatives, and on the biochemical determination of lack of reaction with a biotinylated MTS derivative. This leads to the conclusion that TM2 does not contribute directly to the substrate-binding site or the transport pathway, and suggests that the observed effect of mutations in this region on cocaine binding is indirect. Three mutants, M106C, V107C and I108C, were crosslinked by treatment with HgCl2. This crosslinking was inhibited by the presence of the cocaine analogue MFZ 2-12, likely due to a conformational rearrangement in TM2 upon inhibitor binding. However, the lack of crosslinking of cysteines substituted for Leu99, Leu113 and Leu120 - three of the residues that along with Met106 form a leucine heptad repeat in TM2 - makes it unlikely that this leucine repeat plays a role in symmetrical TM2 dimerization. Importantly, a high-resolution structure of LeuT, a sodium-dependent leucine transporter that is sufficiently homologous to DAT to suggest a high degree of structural similarity, became available while this manuscript was under review. We have taken advantage of this structure to explore further and interpret our experimental results in a rigorous structural context.