Forty Four Years With Baruch Kanner and The Chloride Ion

Abstract

Baruch Kanner and this author have had parallel careers investigating neurotransmitter transporters. At multiple times during their careers, they have found themselves collaborating or competing, but always learning from each other. This commentary elaborates on the interactions between the Kanner and Rudnick laboratories, with a focus on transporters in the Neurotransmitter: Sodium Symporter (NSS) family of amino acid and amine transporters. A key focus of these interactions is the mechanism by which chloride ions activate and drive transport.

Keywords: Chloride; GABA; Neurotransmitter; Serotonin; Transport.

Fig. 1

The author (left) with Shimon Schuldiner (center) and Baruch Kanner (right) in Erice, Sicily in 2005

Fig. 2

Reaction cycles for GAT-1 (a) and SERT (b). These diagrams are meant to show the differences between GAT-1 and SERT stoichiometry. GABA is transported together with 2 Na⁺ ions and one Cl⁻ ion while serotonin (5-HT) is transported together with one Na⁺ ion and in exchange for one K⁺ ion, which is transported in the opposite direction. Both transporters require Cl⁻ for function. Transporters, represented by T, are shown in outward- (T_o) and inward-facing (T_i) conformations with substrates (GABA or 5-HT), and ions Na⁺, K⁺ and Cl⁻ bound as indicated. For SERT (B) Na⁺ is shown bound at the Na2 site. The Na1 site (like the Cl⁻ site) is apparently always occupied unless Na⁺ (or Cl⁻) is removed from the extracellular medium

Fig. 3

Comparison of ion binding sites in outward occluded states of Cl⁻-independent LeuT (PDB 2A65) and Cl⁻-dependent SERT. The ionized γ-carboxyl of Glu290 in LeuT (top) is replaced in Cl⁻-dependent NSS transporters by the bound Cl⁻ ion, as shown in a model of SERT in an outward-occluded conformation [43] (bottom). In addition to Ser372, which corresponds to LeuT Glu290, the Cl ion is coordinated by residues conserved within the family, including Tyr121, Ser336 and Gln332. Gln332 in SERT, or Gln250 in LeuT is believed to interact with both the bound Cl⁻ in Cl⁻-dependent NSS transporters (or the negatively charged Glu290 in LeuT) and the ion pair represented by Arg30-Asp404 in LeuT and Arg104-Glu493 in SERT. Kanner first proposed that this interaction might influence transporter conformation [40] and this influence was recently demonstrated for the GlyT1 glycine transporter [42]. Transmembrane (TM) helices are color-coded as follows: TM1, raspberry; TM2, salmon; TM5, limon; TM6, forest (shown as ribbon for clarity); TM7, light teal; TM10, blue (ribbon)