The ups and downs of elevator‐type di‐/tricarboxylate membrane transporters

Abstract

The divalent anion sodium symporter (DASS) family contains both sodium‐driven anion cotransporters and anion/anion exchangers. The family belongs to a broader ion transporter superfamily (ITS), which comprises 24 families of transporters, including those of AbgT antibiotic efflux transporters. The human proteins in the DASS family play major physiological roles and are drug targets. We recently determined multiple structures of the human sodium‐dependent citrate transporter (NaCT) and the succinate/dicarboxylate transporter from Lactobacillus acidophilus (LaINDY). Structures of both proteins show high degrees of structural similarity to the previously determined VcINDY fold. Conservation between these DASS protein structures and those from the AbgT family indicates that the VcINDY fold represents the overall protein structure for the entire ITS. The new structures of NaCT and LaINDY are captured in the inward‐ or outward‐facing conformations, respectively. The domain arrangements in these structures agree with a rigid body elevator‐type transport mechanism for substrate translocation across the membrane. Two separate NaCT structures in complex with a substrate or an inhibitor allowed us to explain the inhibition mechanism and propose a detailed classification scheme for grouping disease‐causing mutations in the human protein. Structural understanding of multiple kinetic states of DASS proteins is a first step toward the detailed characterization of their entire transport cycle.

Energy landscapes and dynamics of ion translocation through membrane transporters: a meeting ground for physics, chemistry, and biology

“…H + ion (bold line) subsequently translocates along its concentration gradient through the two mutually non-colinear half-access channels shown in the figure and binds to and unbinds from its conserved Glu/Asp-61 binding site on the c-subunit lying within the electrostatic field of the bound succinate anion [30]. These principles are similar to those uncovered recently from pioneering X-ray structural studies on coupled Na + -succinate cotransporters of the DASS family [83][84][85]. The change in local electrical potential, ΔΔ due to the binding and unbinding events and subsequent ion pair recombination causes electrostatic interaction between the charges on the a-and c-subunits that leads to mechanical rotation, and ultimately to torsional energy storage in the γ-subunit of F 1 to be utilized thereafter for formation of ATP, as detailed in Nath's torsional mechanism of energy transduction and ATP synthesis [30,34,37,49,[52][53][54][55]66] In Eq.…”

Thermostability-based binding assays reveal complex interplay of cation, substrate and lipid binding in the bacterial DASS transporter, VcINDY

“…Our understanding of the mechanism of transport by human DASS transporters, and effects of disease causing mutations, has recently been substantially advanced by the publication of the structure of the Na + /citrate transporter, NaCT, in the presence of its substrate, citrate, and in the presence of an inhibitor, PF2 [ 11 ]. The structure of NaCT revealed that it has the same overall architecture as VcINDY, and the locations of the critical binding site SNT motifs and the location of the Na1 and Na2 sites are conserved between NaCT and VcINDY [ 11 , 57 ]. Therefore, this demonstrates that VcINDY remains an excellent model for the DASS family from which to derive general mechanistic insight, and it is highly likely that the same length-dependent selectivity between the two carboxylate binding regions that we predict for VcINDY holds true for NaCT as well.…”

Role of Sodium dependent SLC13 transporter and its inhibitors in various metabolic disorders

“…The transport activity was strong in HgCl 2 treated cysteine less proteins [15]. The transport domain translocated to 15 Å downward rotating 43 o to the inward facing state releasing the substrate to the cytoplasm [16].…”

“…Figure 3: Substrate transport by elevator type mechanism. The substrate binds to the outward facing of the transport domain, translocation inhibited by the hydrophobic barrier (scaffold and oligomerization domain).The transport domain translocated to 15 Å downward rotating 43 o to the inward facing state releasing the substrate to the cytoplasm[16].…”