Cysteine residues in the Na+/dicarboxylate co-transporter, NaDC-1.
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ABSTRACT:
SUBMITTER: Pajor AM
PROVIDER: S-EPMC1220632 | biostudies-other | 1999 Nov
REPOSITORIES: biostudies-other
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Project description:The Na+/dicarboxylate co-transporter, NaDC-1, from the kidney and small intestine, transports three sodium ions together with one divalent anion substrate, such as succinate2-. A previous study (Pajor, A. M. (2001) J. Biol. Chem. 276, 29961-29968), identified four amino acids, Ser-478, Ala-480, Ala-481, and Thr-482, near the extracellular end of transmembrane helix (TM) 9 that are likely to form part of the permeation pathway of the transporter. All four cysteine-substituted mutants were sensitive to inhibition by the membrane-impermeant reagent [2-(trimethylammonium)ethyl]-methanethiosulfonate (MTSET) and protected by substrate. In the present study, we continued the cysteine scan through extracellular loop 5 and TM10, from Thr-483 to Val-528. Most cysteine substitutions were well tolerated, although cysteine mutations of some residues, particularly within the TM, produced proteins that were not expressed on the plasma membrane. Six residues in the extracellular loop (Thr-483, Thr-484, Leu-485, Leu-487, Ile-489, and Met-493) were sensitive to chemical labeling by MTSET, depending on the conformational state of the protein. Transport inhibition by MTSET could be prevented by substrate regardless of temperature, suggesting that the likely mechanism of substrate protection is steric hindrance rather than large-scale conformational changes associated with translocation. We conclude that extracellular loop 5 in NaDC-1 appears to have a functional role, and it is likely to be located in or near the substrate translocation pore in the protein. Conformational changes in the protein affect the accessibility of the residues in extracellular loop 5 and provide further evidence of large-scale changes in the structure of NaDC-1 during the transport cycle.
| S-EPMC1224748 | biostudies-literature
Project description:Succinate transport by the rabbit Na+/dicarboxylate co-transporter, NaDC-1, expressed in Xenopus oocytes was inhibited by the histidyl-selective reagent diethyl pyrocarbonate (DEPC). Therefore the role of histidine residues in the function of NaDC-1 was examined by site-directed mutagenesis. All 11 histidine residues in NaDC-1 were converted to alanine, but only mutant H106A exhibited a decrease in succinate transport. Additional mutations of NaDC-1 at position 106 showed that aspartic acid and asparagine, but not arginine, can substitute for histidine. Examination of succinate and citrate kinetics of H106A revealed a decrease in Vmax with no change in Km. Cell surface biotinylation experiments showed that the transport activity of all four mutants at position 106 was correlated with the amount of cell surface expression, suggesting a role of His-106 in membrane expression rather than function. Two of the histidine mutants, H153A and H569A, exhibited insensitivity to inhibition by DEPC, indicating that these residues are involved in binding DEPC. Neither of these residues is required for transport activity; thus DEPC probably inhibits NaDC-1 function by hindrance of the mobility of the carrier. We conclude that histidine residues are not critical for transport function in NaDC-1, although His-106 might be involved in determining protein expression or stability in the membrane.
| S-EPMC1219347 | biostudies-other
Project description:The role of cationic amino acids in the Na(+)/dicarboxylate co-transporter NaDC-1 was investigated by site-directed mutagenesis and subsequent expression of mutant transporters in Xenopus oocytes. Of the ten residues chosen for mutagenesis, eight (Lys-34, Lys-107, Arg-108, Lys-333, Lys-390, Arg-368, Lys-414 and Arg-541) were found to be non-essential for function or targeting. Only two conserved residues, Lys-84 (at the cytoplasmic end of helix 3) and Arg-349 (at the extracellular end of helix 7), were found to be important for transport. Both mutant transporters were expressed at the plasma membrane. The mutation of Lys-84 to Ala resulted in an increased K(m) for succinate of 1.8 mM, compared with 0.3 mM in the wild-type NaDC-1. The R349A mutant had Na(+) and citrate kinetics that were similar to those of the wild type. However, succinate handling in the R349A mutant was altered, with evidence of inhibition at high succinate concentrations. In conclusion, charge neutralization of Lys-84 and Arg-349 in NaDC-1 affects succinate handling, suggesting that these residues might have roles in substrate binding.
| S-EPMC1221297 | biostudies-other
Project description:The SLC13 transporter family, whose members play key physiological roles in the regulation of fatty acid synthesis, adiposity, insulin resistance, and other processes, catalyzes the transport of Krebs cycle intermediates and sulfate across the plasma membrane of mammalian cells. SLC13 transporters are part of the divalent anion:Na(+) symporter (DASS) family that includes several well-characterized bacterial members. Despite sharing significant sequence similarity, the functional characteristics of DASS family members differ with regard to their substrate and coupling ion dependence. The publication of a high resolution structure of dimer VcINDY, a bacterial DASS family member, provides crucial structural insight into this transporter family. However, marrying this structural insight to the current functional understanding of this family also demands a comprehensive analysis of the transporter's functional properties. To this end, we purified VcINDY, reconstituted it into liposomes, and determined its basic functional characteristics. Our data demonstrate that VcINDY is a high affinity, Na(+)-dependent transporter with a preference for C4- and C5-dicarboxylates. Transport of the model substrate, succinate, is highly pH dependent, consistent with VcINDY strongly preferring the substrate's dianionic form. VcINDY transport is electrogenic with succinate coupled to the transport of three or more Na(+) ions. In contrast to succinate, citrate, bound in the VcINDY crystal structure (in an inward-facing conformation), seems to interact only weakly with the transporter in vitro. These transport properties together provide a functional framework for future experimental and computational examinations of the VcINDY transport mechanism.
| S-EPMC4035743 | biostudies-literature
Project description:The Na(+)-coupled dicarboxylate transporter, SdcL, from Bacillus licheniformis is a member of the divalent anion/Na(+) symporter (DASS) family that includes the bacterial Na(+)/dicarboxylate cotransporter SdcS (from Staphyloccocus aureus) and the mammalian Na(+)/dicarboxylate cotransporters, NaDC1 and NaDC3. The transport properties of SdcL produced in Escherichia coli are similar to those of its prokaryotic and eukaryotic counterparts, involving the Na(+)-dependent transport of dicarboxylates such as succinate or malate across the cytoplasmic membrane with a K(m) of approximately 6 microM. SdcL may also transport aspartate, alpha-ketoglutarate and oxaloacetate with low affinity. The cotransport of Na(+) and dicarboxylate by SdcL has an apparent stoichiometry of 2:1, and a K(0.5) for Na(+) of 0.9 mM. Our findings represent the characterization of another prokaryotic protein of the DASS family with transport properties similar to its eukaryotic counterparts, but with a broader substrate specificity than other prokaryotic DASS family members. The broader range of substrates carried by SdcL may provide insight into domains of the protein that allow a more flexible or larger substrate binding pocket.
| S-EPMC2787743 | biostudies-literature
Project description: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. DASS carboxylate transporters linked to metabolic diseases couple the Na+ gradient to nutrient uptake. Here, the authors unravel the structural basis by which Na+ binding coordinates substrate interactions, revealing a distinct coupling mechanism.
| S-EPMC9098524 | biostudies-literature
Project description:Mouse organic anion transporter 1 (mOAT1) belongs to a family of organic anion transporters, which play critical roles in the body disposition of clinically important drugs, including anti-HIV therapeutics, anti-tumour drugs, antibiotics, anti-hypertensives and anti-inflammatories. mOAT1-mediated transport of organic anion PAH ( p -aminohippurate) in HeLa cells was inhibited by the cysteine-modifying reagent PCMBS (p-chloromercuribenzenesulphonate). Therefore the role of cysteine residues in the function of mOAT1 was examined by site-directed mutagenesis. All 13 cysteine residues in mOAT1 were replaced by alanine, singly or in combination. Single replacement of these residues had no significant effect on mOAT1-mediated PAH transport, indicating that no individual cysteine residue is necessary for function. Multiple replacements at a C-terminal region (C335/379/427/434A; Cys(335/379/427/434)-->Ala) resulted in a substantial decrease in transport activity. A simultaneous replacement of all 13 cysteine residues (C-less) led to a complete loss of transport function. The decreased or lack of transport activity of the mutants C335/379/427/434A and C-less was due to the impaired trafficking of the mutant transporters to the cell surface. These results suggest that although cysteine residues are not required for function in mOAT1, their presence appears to be important for the targeting of the transporter to the plasma membrane. We also showed that, although all cysteine mutants of mOAT1 were sensitive to the inhibition by PCMBS, C49A was less sensitive than the wild-type mOAT1, suggesting that the modification of Cys49 may play a role in the inhibition of mOAT1 by PCMBS.
| S-EPMC1224163 | biostudies-other
Project description:Mammals express seven transporters from the SLC1 (solute carrier 1) gene family, including five acidic amino acid transporters (EAAT1-5) and two neutral amino acid transporters (ASCT1-2). In contrast, insects of the order Diptera possess only two SLC1 genes. In this work we show that in the mosquito Culex quinquefasciatus, a carrier of West Nile virus, one of its two SLC1 EAAT-like genes encodes a transporter that displays an unusual selectivity for dicarboxylic acids over acidic amino acids. In eukaryotes, dicarboxylic acid uptake has been previously thought to be mediated exclusively by transporters outside the SLC1 family. The dicarboxylate selectivity was found to be associated with two residues in transmembrane domain 8, near the presumed substrate binding site. These residues appear to be conserved in all eukaryotic SLC1 transporters (Asp444 and Thr448, human EAAT3 numbering) with the exception of this novel C. quinquefasciatus transporter and an ortholog from the yellow fever mosquito Aedes aegypti, in which they are changed to Asn and Ala. In the prokaryotic EAAT-like SLC1 transporter DctA, a dicarboxylate transporter which was lost in the lineage leading to eukaryotes, the corresponding TMD8 residues are Ser and Ala. Functional analysis of engineered mutant mosquito and human transporters expressed in Xenopus laevis oocytes provide support for a model defining interactions of charged and polar transporter residues in TMD8 with ?-amino acids and ions. Together with the phylogenetic evidence, the functional data suggest that a novel route of dicarboxylic acid uptake evolved in these mosquitos by mutations in an ancestral glutamate transporter gene.
| S-EPMC3737229 | biostudies-literature
Project description:The divalent anion sodium symporter (DASS) family (SLC13) plays critical roles in metabolic homeostasis, influencing many processes, including fatty acid synthesis, insulin resistance, and adiposity. DASS transporters catalyze the Na+-driven concentrative uptake of Krebs cycle intermediates and sulfate into cells; disrupting their function can protect against age-related metabolic diseases and can extend lifespan. An inward-facing crystal structure and an outward-facing model of a bacterial DASS family member, VcINDY from Vibrio cholerae, predict an elevator-like transport mechanism involving a large rigid body movement of the substrate-binding site. How substrate binding influences the conformational state of VcINDY is currently unknown. Here, we probe the interaction between substrate binding and protein conformation by monitoring substrate-induced solvent accessibility changes of broadly distributed positions in VcINDY using a site-specific alkylation strategy. Our findings reveal that accessibility to all positions tested is modulated by the presence of substrates, with the majority becoming less accessible in the presence of saturating concentrations of both Na+ and succinate. We also observe separable effects of Na+ and succinate binding at several positions suggesting distinct effects of the two substrates. Furthermore, accessibility changes to a solely succinate-sensitive position suggests that substrate binding is a low-affinity, ordered process. Mapping these accessibility changes onto the structures of VcINDY suggests that Na+ binding drives the transporter into an as-yet-unidentified conformational state, involving rearrangement of the substrate-binding site-associated re-entrant hairpin loops. These findings provide insight into the mechanism of VcINDY, which is currently the only structurally characterized representative of the entire DASS family.
| S-EPMC7939474 | biostudies-literature
Project description:Na(+)-coupled ascorbic acid transporter-2 (SVCT2) activity is impaired at acid pH, but little is known about the molecular determinants that define the transporter pH sensitivity. SVCT2 contains six histidine residues in its primary sequence, three of which are exofacial in the transporter secondary structure model. We used site-directed mutagenesis and treatment with diethylpyrocarbonate to identify histidine residues responsible for SVCT2 pH sensitivity. We conclude that five histidine residues, His(109), His(203), His(206), His(269), and His(413), are central regulators of SVCT2 function, participating to different degrees in modulating pH sensitivity, transporter kinetics, Na(+) cooperativity, conformational stability, and subcellular localization. Our results are compatible with a model in which (i) a single exofacial histidine residue, His(413), localized in the exofacial loop IV that connects transmembrane helices VII-VIII defines the pH sensitivity of SVCT2 through a mechanism involving a marked attenuation of the activation by Na(+) and loss of Na(+) cooperativity, which leads to a decreased V(max) without altering the transport K(m); (ii) exofacial histidine residues His(203), His(206), and His(413) may be involved in maintaining a functional interaction between exofacial loops II and IV and influence the general folding of the transporter; (iii) histidines 203, 206, 269, and 413 affect the transporter kinetics by modulating the apparent transport K(m); and (iv) histidine 109, localized at the center of transmembrane helix I, might be fundamental for the interaction of SVCT2 with the transported substrate ascorbic acid. Thus, histidine residues are central regulators of SVCT2 function.
| S-EPMC2978576 | biostudies-literature