Faculty Bibliography
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66
Structures of the human sodium-citrate cotransporter NaCT with and without substrates
The human sodium-citrate cotransporter NaCT imports various tri- and di-carboxylates into the cell as TCA cycle intermediates. This substrate uptake process is driven by an inward sodium gradient. The protein is a member of the divalent anion-sodium symporter (DASS) family. Whereas extensive biochemical and structural studies have been carried out for NaCT, how the substrate binding and translocation is coupled to the sodium gradient remains unclear. Here using single particle cryo-electron microscopy, we determined the structures of the human NaCT protein in three states: sodium-free, in the presence of sodium, and bound to sodium and a substrate mimicking inhibitor. These structures suggest a simultaneous binding mechanism for sodium-substrate coupling, distinct from the sequential binding, conformational selection mechanism previously observed for the bacterial DASS protein VcINDY.
PMID: 42673948
ISSN: 1878-4186
CID: 6071931
Functional role of a structural water in the elevator domain of dicarboxylate transporter VcINDY
The divalent anion sodium symporter (DASS) family mediates the uptake of Krebs cycle intermediates and sulfate and influences adiposity, insulin resistance, and metabolism in mammals. While Na+:substrate stoichiometry is known for several DASS transporters, the location of key Na+-binding sites remains elusive; important information for understanding the mechanism. In VcINDY, a bacterial DASS protein, we visualized a nonprotein cryo-electron microscopy (cryo-EM) density in the middle of the transport domain. Its size and coordination suggest that it may represent either a third Na+ ion or a structural water molecule. Using a combination of in vitro binding and transport assays, cryo-EM structural determination, and molecular dynamic simulations, we show that the density is not a Na+ ion. Instead, the data indicate that the density likely represents a structural water molecule critical for transport domain integrity. Sequence and structural similarities suggest this feature may be conserved across human DASS transporters such as NaCT and NaDC3.
PMCID:13394699
PMID: 42495692
ISSN: 2752-6542
CID: 6071685
Proceedings of the National Academy of Sciences of the United States of America (PNAS). 2026:123(2).DOI: 10.1073/pnas.2500723123
Elevator mechanism dynamics in a sodium-coupled dicarboxylate transporter
VcINDY, the sodium-dependent dicarboxylate transporter from
PMID: 41490488
ISSN: 1091-6490
CID: 5980652
Substrate translocation and inhibition in human dicarboxylate transporter NaDC3
The human high-affinity sodium-dicarboxylate cotransporter (NaDC3) imports various substrates into the cell as tricarboxylate acid cycle intermediates, lipid biosynthesis precursors and signaling molecules. Understanding the cellular signaling process and developing inhibitors require knowledge of the structural basis of the dicarboxylate specificity and inhibition mechanism of NaDC3. To this end, we determined the cryo-electron microscopy structures of NaDC3 in various dimers, revealing the protomer in three conformations: outward-open Co, outward-occluded Coo and inward-open Ci. A dicarboxylate is first bound and recognized in Co and how the substrate interacts with NaDC3 in Coo likely helps to further determine the substrate specificity. A phenylalanine from the scaffold domain interacts with the bound dicarboxylate in the Coo state and modulates the kinetic barrier to the transport domain movement. Structural comparison of an inhibitor-bound structure of NaDC3 to that of the sodium-dependent citrate transporter suggests ways for making an inhibitor that is specific for NaDC3.
PMID: 39622972
ISSN: 1545-9985
CID: 5780092
A fiducial-assisted strategy compatible with resolving small MFS transporter structures in multiple conformations using cryo-EM
Advancements in cryo-EM have stimulated a revolution in structural biology. Yet, for membrane proteins near the cryo-EM size threshold of approximately 40 kDa, including transporters and G-protein coupled receptors, the absence of distinguishable structural features makes image alignment and structure determination a significant challenge. Furthermore, resolving more than one protein conformation within a sample, a major advantage of cryo-EM, represents an even greater degree of difficulty. Here, we describe a strategy for introducing a rigid fiducial marker (BRIL domain) at the C-terminus of membrane transporters from the Major Facilitator Superfamily (MFS) with AlphaFold2. This approach involves fusion of the last transmembrane domain helix of the target protein with the first helix of BRIL through a short poly-alanine linker to promote helicity. Combining this strategy with a BRIL-specific Fab, we elucidated four cryo-EM structures of the 42 kDa Staphylococcus aureus transporter NorA, three of which were derived from a single sample corresponding to inward-open, inward-occluded, and occluded conformations. Hence, this fusion construct facilitated experiments to characterize the conformational landscape of NorA and validated our design to position the BRIL/antibody pair in an orientation that avoids steric clash with the transporter. The latter was enabled through AlphaFold2 predictions, which minimized guesswork and reduced the need for screening several constructs. We further validated the suitability of the method to three additional MFS transporters (GlpT, Bmr, and Blt), results that supported a rigid linker between the transporter and BRIL. The successful application to four MFS proteins, the largest family of secondary transporters in nature, and analysis of predicted structures for the family indicates this strategy will be a valuable tool for studying other MFS members using cryo-EM.
PMCID:11695964
PMID: 39746942
ISSN: 2041-1723
CID: 5779252
Proton-coupled transport mechanism of the efflux pump NorA
Efflux pump antiporters confer drug resistance to bacteria by coupling proton import with the expulsion of antibiotics from the cytoplasm. Despite efforts there remains a lack of understanding as to how acid/base chemistry drives drug efflux. Here, we uncover the proton-coupling mechanism of the Staphylococcus aureus efflux pump NorA by elucidating structures in various protonation states of two essential acidic residues using cryo-EM. Protonation of Glu222 and Asp307 within the C-terminal domain stabilized the inward-occluded conformation by forming hydrogen bonds between the acidic residues and a single helix within the N-terminal domain responsible for occluding the substrate binding pocket. Remarkably, deprotonation of both Glu222 and Asp307 is needed to release interdomain tethering interactions, leading to opening of the pocket for antibiotic entry. Hence, the two acidic residues serve as a "belt and suspenders" protection mechanism to prevent simultaneous binding of protons and drug that enforce NorA coupling stoichiometry and confer antibiotic resistance.
PMCID:11130294
PMID: 38802368
ISSN: 2041-1723
CID: 5663352
Structural basis of ion - substrate coupling in the Na+-dependent dicarboxylate transporter VcINDY
The Na+-dependent dicarboxylate transporter from Vibrio cholerae (VcINDY) is a prototype for the divalent anion sodium symporter (DASS) family. While the utilization of an electrochemical Na+ gradient to power substrate transport is well established for VcINDY, the structural basis of this coupling between sodium and substrate binding is not currently understood. Here, using a combination of cryo-EM structure determination, succinate binding and site-directed cysteine alkylation assays, we demonstrate that the VcINDY protein couples sodium- and substrate-binding via a previously unseen cooperative mechanism by conformational selection. In the absence of sodium, substrate binding is abolished, with the succinate binding regions exhibiting increased flexibility, including HPinb, TM10b and the substrate clamshell motifs. Upon sodium binding, these regions become structurally ordered and create a proper binding site for the substrate. Taken together, these results provide strong evidence that VcINDY's conformational selection mechanism is a result of the sodium-dependent formation of the substrate binding site.
PMCID:9098524
PMID: 35551191
ISSN: 2041-1723
CID: 5214742
Structural basis for inhibition of the drug efflux pump NorA from Staphylococcus aureus
Membrane protein efflux pumps confer antibiotic resistance by extruding structurally distinct compounds and lowering their intracellular concentration. Yet, there are no clinically approved drugs to inhibit efflux pumps, which would potentiate the efficacy of existing antibiotics rendered ineffective by drug efflux. Here we identified synthetic antigen-binding fragments (Fabs) that inhibit the quinolone transporter NorA from methicillin-resistant Staphylococcus aureus (MRSA). Structures of two NorA-Fab complexes determined using cryo-electron microscopy reveal a Fab loop deeply inserted in the substrate-binding pocket of NorA. An arginine residue on this loop interacts with two neighboring aspartate and glutamate residues essential for NorA-mediated antibiotic resistance in MRSA. Peptide mimics of the Fab loop inhibit NorA with submicromolar potency and ablate MRSA growth in combination with the antibiotic norfloxacin. These findings establish a class of peptide inhibitors that block antibiotic efflux in MRSA by targeting indispensable residues in NorA without the need for membrane permeability.
PMID: 35361990
ISSN: 1552-4469
CID: 5201392
The ups and downs of elevator-type di-/tricarboxylate membrane transporters
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.
PMID: 34403567
ISSN: 1742-4658
CID: 5066842
Structure and inhibition mechanism of the human citrate transporter NaCT
Citrate is best known as an intermediate in the tricarboxylic acid cycle of the cell. In addition to this essential role in energy metabolism, the tricarboxylate anion also acts as both a precursor and a regulator of fatty acid synthesis1-3. Thus, the rate of fatty acid synthesis correlates directly with the cytosolic concentration of citrate4,5. Liver cells import citrate through the sodium-dependent citrate transporter NaCT (encoded by SLC13A5) and, as a consequence, this protein is a potential target for anti-obesity drugs. Here, to understand the structural basis of its inhibition mechanism, we determined cryo-electron microscopy structures of human NaCT in complexes with citrate or a small-molecule inhibitor. These structures reveal how the inhibitor-which binds to the same site as citrate-arrests the transport cycle of NaCT. The NaCT-inhibitor structure also explains why the compound selectively inhibits NaCT over two homologous human dicarboxylate transporters, and suggests ways to further improve the affinity and selectivity. Finally, the NaCT structures provide a framework for understanding how various mutations abolish the transport activity of NaCT in the brain and thereby cause epilepsy associated with mutations in SLC13A5 in newborns (which is known as SLC13A5-epilepsy)6-8.
PMID: 33597751
ISSN: 1476-4687
CID: 4788372
