1
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Bhatt M, Di Iacovo A, Romanazzi T, Roseti C, Cinquetti R, Bossi E. The "www" of Xenopus laevis Oocytes: The Why, When, What of Xenopus laevis Oocytes in Membrane Transporters Research. MEMBRANES 2022; 12:membranes12100927. [PMID: 36295686 PMCID: PMC9610376 DOI: 10.3390/membranes12100927] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/23/2022] [Revised: 09/22/2022] [Accepted: 09/23/2022] [Indexed: 05/16/2023]
Abstract
After 50 years, the heterologous expression of proteins in Xenopus laevis oocytes is still essential in many research fields. New approaches and revised protocols, but also classical methods, such as the two-electrode voltage clamp, are applied in studying membrane transporters. New and old methods for investigating the activity and the expression of Solute Carriers (SLC) are reviewed, and the kinds of experiment that are still useful to perform with this kind of cell are reported. Xenopus laevis oocytes at the full-grown stage have a highly efficient biosynthetic apparatus that correctly targets functional proteins at the defined compartment. This small protein factory can produce, fold, and localize almost any kind of wild-type or recombinant protein; some tricks are required to obtain high expression and to verify the functionality. The methodologies examined here are mainly related to research in the field of membrane transporters. This work is certainly not exhaustive; it has been carried out to be helpful to researchers who want to quickly find suggestions and detailed indications when investigating the functionality and expression of the different members of the solute carrier families.
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Affiliation(s)
- Manan Bhatt
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Via Dunant 3, 21100 Varese, Italy
- Experimental and Translational Medicine, University of Insubria, Via Ottorino Rossi 9, 21100 Varese, Italy
| | - Angela Di Iacovo
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Via Dunant 3, 21100 Varese, Italy
- Experimental and Translational Medicine, University of Insubria, Via Ottorino Rossi 9, 21100 Varese, Italy
| | - Tiziana Romanazzi
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Via Dunant 3, 21100 Varese, Italy
- Experimental and Translational Medicine, University of Insubria, Via Ottorino Rossi 9, 21100 Varese, Italy
| | - Cristina Roseti
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Via Dunant 3, 21100 Varese, Italy
- Centre for Neuroscience—Via Manara 7, University of Insubria, 21052 Busto Arsizio, Italy
| | - Raffaella Cinquetti
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Via Dunant 3, 21100 Varese, Italy
| | - Elena Bossi
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, Via Dunant 3, 21100 Varese, Italy
- Centre for Neuroscience—Via Manara 7, University of Insubria, 21052 Busto Arsizio, Italy
- Correspondence:
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2
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Castagna M, Cinquetti R, Verri T, Vacca F, Giovanola M, Barca A, Romanazzi T, Roseti C, Galli A, Bossi E. The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters. Neurochem Res 2021; 47:111-126. [PMID: 34304372 PMCID: PMC8310414 DOI: 10.1007/s11064-021-03410-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2021] [Revised: 04/18/2021] [Accepted: 07/20/2021] [Indexed: 11/16/2022]
Abstract
To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.
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Affiliation(s)
- Michela Castagna
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, 20134, Milan, Italy
| | - Raffaella Cinquetti
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, via Dunant 3, 21100, Varese, Italy
| | - Tiziano Verri
- Laboratory of Applied Physiology, Department of Biological and Environmental Sciences and Technologies, University of Salento, Via Provinciale Lecce-Monteroni, 73100, Lecce, Italy
| | - Francesca Vacca
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, via Dunant 3, 21100, Varese, Italy
| | - Matteo Giovanola
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, 20134, Milan, Italy
| | - Amilcare Barca
- Laboratory of Applied Physiology, Department of Biological and Environmental Sciences and Technologies, University of Salento, Via Provinciale Lecce-Monteroni, 73100, Lecce, Italy
| | - Tiziana Romanazzi
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, via Dunant 3, 21100, Varese, Italy
| | - Cristina Roseti
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, via Dunant 3, 21100, Varese, Italy.,Research Centre for Neuroscience, University of Insubria, Varese, Italy
| | - Alessandra Galli
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, 20134, Milan, Italy
| | - Elena Bossi
- Laboratory of Cellular and Molecular Physiology, Department of Biotechnology and Life Sciences, University of Insubria, via Dunant 3, 21100, Varese, Italy. .,Research Centre for Neuroscience, University of Insubria, Varese, Italy.
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3
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Giovanola M, Vollero A, Cinquetti R, Bossi E, Forrest LR, Di Cairano ES, Castagna M. Threonine 67 is a key component in the coupling of the NSS amino acid transporter KAAT1. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2018; 1860:1179-1186. [PMID: 29409909 DOI: 10.1016/j.bbamem.2018.01.020] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/12/2017] [Revised: 01/26/2018] [Accepted: 01/28/2018] [Indexed: 01/30/2023]
Affiliation(s)
- M Giovanola
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, 20134, Milano, Italy
| | - A Vollero
- Department of Biotechnology and Life Sciences, University of Insubria, Via J.H. Dunant 3, 21100, Varese, Italy
| | - R Cinquetti
- Department of Biotechnology and Life Sciences, University of Insubria, Via J.H. Dunant 3, 21100, Varese, Italy
| | - E Bossi
- Department of Biotechnology and Life Sciences, University of Insubria, Via J.H. Dunant 3, 21100, Varese, Italy
| | - L R Forrest
- Computational Structural Biology Section, NIH NINDS, 35 Convent Drive, Bethesda, MD 20892-3761, USA
| | - E S Di Cairano
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, 20134, Milano, Italy
| | - M Castagna
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, 20134, Milano, Italy.
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4
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Vollero A, Imperiali FG, Cinquetti R, Margheritis E, Peres A, Bossi E. The D-amino acid transport by the invertebrate SLC6 transporters KAAT1 and CAATCH1 from Manduca sexta. Physiol Rep 2016; 4:4/4/e12691. [PMID: 26884475 PMCID: PMC4759042 DOI: 10.14814/phy2.12691] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
The ability of the SLC6 family members, the insect neutral amino acid cotransporter KAAT1(K+‐coupled amino acid transporter 1) and its homologous CAATCH1(cation anion activated amino acid transporter/channel), to transport D‐amino acids has been investigated through heterologous expression in Xenopus laevis oocytes and electrophysiological techniques. In the presence of D‐isomers of leucine, serine, and proline, the msKAAT1 generates inward, transport‐associated, currents with variable relative potencies, depending on the driving ion Na+ or K+. Higher concentrations of D‐leucine (≥1 mmol/L) give rise to an anomalous response that suggests the existence of a second binding site with inhibitory action on the transport process. msCAATCH1 is also able to transport the D‐amino acids tested, including D‐leucine, whereas L‐leucine acts as a blocker. A similar behavior is exhibited by the KAAT1 mutant S308T, confirming the relevance of the residue in this position in L‐leucine binding and the different interaction of D‐leucine with residues involved in transport mechanism. D‐leucine and D‐serine on various vertebrate orthologs B0AT1 (SLC6A19) elicited only a very small current and singular behavior was not observed, indicating that it is specific of the insect neutral amino acid transporters. These findings highlight the relevance of D‐amino acid absorption in the insect nutrition and metabolism and may provide new evidences in the molecular transport mechanism of SLC6 family.
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Affiliation(s)
- Alessandra Vollero
- Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy
| | - Francesca G Imperiali
- Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy
| | - Raffaella Cinquetti
- Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy
| | - Eleonora Margheritis
- Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy
| | - Antonio Peres
- Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy
| | - Elena Bossi
- Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy Interuniversity Center "The ProteinFactory", Politecnico di Milano, ICRM-CNR Milano and Università dell'Insubria, Milan, Italy
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Buracco S, Peracino B, Cinquetti R, Signoretto E, Vollero A, Imperiali F, Castagna M, Bossi E, Bozzaro S. Dictyostelium Nramp1, which is structurally and functionally similar to mammalian DMT1 transporter, mediates phagosomal iron efflux. J Cell Sci 2015; 128:3304-16. [PMID: 26208637 PMCID: PMC4582194 DOI: 10.1242/jcs.173153] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2015] [Accepted: 07/21/2015] [Indexed: 01/01/2023] Open
Abstract
The Nramp (Slc11) protein family is widespread in bacteria and eukaryotes, and mediates transport of divalent metals across cellular membranes. The social amoeba Dictyostelium discoideum has two Nramp proteins. Nramp1, like its mammalian ortholog (SLC11A1), is recruited to phagosomal and macropinosomal membranes, and confers resistance to pathogenic bacteria. Nramp2 is located exclusively in the contractile vacuole membrane and controls, synergistically with Nramp1, iron homeostasis. It has long been debated whether mammalian Nramp1 mediates iron import or export from phagosomes. By selectively loading the iron-chelating fluorochrome calcein in macropinosomes, we show that Dictyostelium Nramp1 mediates iron efflux from macropinosomes in vivo. To gain insight in ion selectivity and the transport mechanism, the proteins were expressed in Xenopus oocytes. Using a novel assay with calcein, and electrophysiological and radiochemical assays, we show that Nramp1, similar to rat DMT1 (also known as SLC11A2), transports Fe(2+) and manganese, not Fe(3+) or copper. Metal ion transport is electrogenic and proton dependent. By contrast, Nramp2 transports only Fe(2+) in a non-electrogenic and proton-independent way. These differences reflect evolutionary divergence of the prototypical Nramp2 protein sequence compared to the archetypical Nramp1 and DMT1 proteins.
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Affiliation(s)
- Simona Buracco
- Department of Clinical and Biological Sciences, University of Torino, AOU S. Luigi, Orbassano 10043, Italy
| | - Barbara Peracino
- Department of Clinical and Biological Sciences, University of Torino, AOU S. Luigi, Orbassano 10043, Italy
| | - Raffaella Cinquetti
- Department of Biotechnology and Life Sciences, University of Insubria, Via J. H. Dunant 3, Varese 21100, Italy
| | - Elena Signoretto
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, Milano 20133, Italy
| | - Alessandra Vollero
- Department of Biotechnology and Life Sciences, University of Insubria, Via J. H. Dunant 3, Varese 21100, Italy
| | - Francesca Imperiali
- Department of Biotechnology and Life Sciences, University of Insubria, Via J. H. Dunant 3, Varese 21100, Italy
| | - Michela Castagna
- Department of Pharmacological and Biomolecular Sciences, Università degli Studi di Milano, Via Trentacoste 2, Milano 20133, Italy
| | - Elena Bossi
- Department of Biotechnology and Life Sciences, University of Insubria, Via J. H. Dunant 3, Varese 21100, Italy
| | - Salvatore Bozzaro
- Department of Clinical and Biological Sciences, University of Torino, AOU S. Luigi, Orbassano 10043, Italy
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6
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Giovanola M, D'Antoni F, Santacroce M, Mari SA, Cherubino F, Bossi E, Sacchi VF, Castagna M. Role of a conserved glycine triplet in the NSS amino acid transporter KAAT1. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1818:1737-44. [PMID: 22402268 DOI: 10.1016/j.bbamem.2012.02.023] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/27/2011] [Revised: 02/15/2012] [Accepted: 02/20/2012] [Indexed: 10/28/2022]
Abstract
K+-coupled amino acid transporter 1 (KAAT1) belongs to the NSS family of solute transporters and it is expressed in the midgut and in salivary glands of Manduca sexta larvae. As more than 80% of family members, KAATI shows a stretch of three glycines (G85-G87) that according to the structure of the prototype transporter LeuT, is located close to the access of the permeation pathway. In this work the role of the triplet has been investigated by alanine and cysteine scanning methods in protein heterologously expressed in Xenopus laevis oocytes. All the mutants were functional but the surface expression level was reduced for G85A and G87A mutants and unaffected for G86A mutant. All presented altered amino acid uptake and transport associated currents in the presence of each of the cations (Na+, K+, Li+) that can be exploited by the wt. G87A mutant induced increased uncoupled fluxes in the presence of all the cations. Cross-linking studies, performed by the treatment of cysteine mutants with the oxidative complex Cu(Il)(l,10-phenanthroline)3, showed that limiting the flexibility of the region by covalent blockage of position 87, causes a significant reduction of amino acid uptake. Na+ protected G87C mutant from oxidation, both directly and indirectly. The conserved glycine triplet in KAAT1 plays therefore a complex role that allows initial steps of cation interaction with the transporter.
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Affiliation(s)
- M Giovanola
- Department of Molecular Sciences Applied to Biosystems, Universita degli Studi di Milano, Via Trentacoste 2, 20134 Milano, Italy
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7
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Margheritis E, Terova G, Cinquetti R, Peres A, Bossi E. Functional properties of a newly cloned fish ortholog of the neutral amino acid transporter B0AT1 (SLC6A19). Comp Biochem Physiol A Mol Integr Physiol 2013; 166:285-92. [PMID: 23827520 DOI: 10.1016/j.cbpa.2013.06.027] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2013] [Revised: 06/20/2013] [Accepted: 06/26/2013] [Indexed: 11/27/2022]
Abstract
The functional properties of an ortholog of the B(0)AT1 (SLC6A19) amino acid transporter, cloned from the intestine of the sea bass Dicentrachus labrax, were investigated. The two-electrode voltage-clamp technique was applied to Xenopus laevis oocytes heterologously expressing the transporter in order to measure the currents associated with the transport process in different conditions. In particular the substrate specificity, the ionic requirements, and possible effects of pH were examined. Among the organic substrates, leucine, glycine, serine and valine generated the largest transport currents with apparent affinities in the lower millimolar range. The importance of Na(+) as the driver ion in the transport process is confirmed, although Li(+) is also capable to sustain transport, while K(+) is not. No evidence of a relevant role of Cl(-) in the transport activity was found. Concerning the other two kinds of currents commonly found in electrogenic transporters, very fast pre-steady-state currents were detected in the absence of organic substrate, while lithium-specific leak currents were not observed. The comparison of these properties with those of the mammalian and insect orthologs may give interesting indication for future structure-function studies in this transporter subfamily.
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Affiliation(s)
- Eleonora Margheritis
- Department of Biotechnology and Life Sciences, University of Insubria, Via J.H. Dunant 3 I-21100, Varese, Italy
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8
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Characterization of the transport of lysine-containing dipeptides by PepT1 orthologs expressed in Xenopus laevis oocytes. Comp Biochem Physiol A Mol Integr Physiol 2012; 164:520-8. [PMID: 23268205 DOI: 10.1016/j.cbpa.2012.12.016] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2012] [Revised: 12/13/2012] [Accepted: 12/17/2012] [Indexed: 11/23/2022]
Abstract
During digestion, dietary proteins cleaved in di and tri-peptides are translocated from the intestinal lumen into the enterocytes via PepT1 (SLC15A1) using an inwardly directed proton electrochemical gradient. The kinetic properties in various PepT1 orthologs (Dicentrarchus labrax, Oryctolagus cuniculus, Danio rerio) have been explored to determine the transport efficiency of different combinations of lysine, methionine, and glycine. Species-specific differences were observed. Lys-Met resulted the best substrate at all tested potentials in sea bass and rabbit PepT1, whereas in the zebrafish transporter all tested dipeptides (except Gly-Lys) elicited similar currents independently on the charge position or amino acid composition. For the sea bass and rabbit PepT1, kinetic parameters, K(0.5) and I(max) and their ratio, show the importance of the position of the charged lysine in the peptide. The PepT1 transporter of these species has very low affinity for Lys-Lys and Gly-Lys; this reduces the transport efficiency which is instead higher for Lys-Met and Lys-Gly. PepT1 from zebrafish showed relatively high affinity and excellent transport efficiency for Met-Lys and Lys-Met. These data led us to speculate about the structural determinants involved in substrate interaction according to the model proposed for this transporter.
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9
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An inverse relationship links temperature and substrate apparent affinity in the ion-coupled cotransporters rGAT1 and KAAT1. Int J Mol Sci 2012; 13:15565-74. [PMID: 23443081 PMCID: PMC3546649 DOI: 10.3390/ijms131215565] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2012] [Revised: 11/14/2012] [Accepted: 11/15/2012] [Indexed: 01/20/2023] Open
Abstract
The effects of temperature on the operation of two ion-coupled cotransporters of the SLC6A family, namely rat GAT1 (SLC6A1) and KAAT1 (SLC6A19) from Manduca sexta, have been studied by electrophysiological means in Xenopus laevis oocytes expressing these proteins. The maximal transport-associated current (Imax) and the apparent substrate affinity (K05) were measured. In addition to the expected increase in transport rate (Q10 = 3–6), both transporters showed greater K05 values (i.e., a decrease in apparent affinity) at higher temperatures. The transport efficiency, estimated as Imax/K05, increased at negative potentials in both transporters, but did not show statistically significant differences with temperature. The observation that the apparent substrate affinity is inversely related to the transport rate suggests a kinetic regulation of this parameter. Furthermore, the present results indicate that the affinities estimated at room temperature for mammalian cotransporters may not be simply extrapolated to their physiological operating conditions.
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10
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Santacroce M, Castagna M, Sacchi VF. Passive water permeability of some wild type and mutagenized amino acid cotransporters of the SLC6/NSS family expressed in Xenopus laevis oocytes. Comp Biochem Physiol A Mol Integr Physiol 2010; 156:509-17. [PMID: 20394832 DOI: 10.1016/j.cbpa.2010.04.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/26/2009] [Revised: 04/06/2010] [Accepted: 04/06/2010] [Indexed: 11/19/2022]
Abstract
In this paper passive water movement across the cell membrane mediated by wild type and mutagenized cotransporters was investigated. We evaluated water movement and, in parallel, amino acid uptake induced by some members of the SLC6/NSS family belonging to different kingdoms, namely the rat GABA transporter GAT1, the insect amino acid transporters KAAT1 and CAATCH1 and the bacterial leucine transporter LeuT, whose structure was recently solved. We also tested whether mutated proteins in which the solute translocation mechanism is altered or even abolished were able to induce water movement across cell membrane. The proteins of interest were expressed in Xenopus laevis oocytes and osmotic water permeabilities were estimated from the rate of cell volume change induced by an osmotic gradient in the absence of cotransported solutes. Under osmotic stress all the studied wild type amino acid cotransporters increased the water permeability of the membrane. The GABA transport inhibitor SKF 89976A inhibited both GABA transport and water movement induced by the expression of GAT1. Interestingly, the capacity of mutant proteins to induce water movement was not predictable on the basis of their substrate transport ability. In particular the GAT1 mutant Q291N, void of any transport activity, induced a water permeability similar to that induced by the wt protein. The KAAT1 mutant T339C, which showed a higher transport activity, induced a water permeability not significantly different from the wild type transporter. Interestingly, the bacterial leucine cotransporter LeuT, whose binding site for leucine and Na(+) is void of water, induced water movement through the plasma membrane.
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Affiliation(s)
- Massimo Santacroce
- Department of Molecular Sciences Applied to Biosystems, Università degli Studi di Milano, Milan, Italy.
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11
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Castagna M, Bossi E, Sacchi VF. Molecular physiology of the insect K-activated amino acid transporter 1 (KAAT1) and cation-anion activated amino acid transporter/channel 1 (CAATCH1) in the light of the structure of the homologous protein LeuT. INSECT MOLECULAR BIOLOGY 2009; 18:265-279. [PMID: 19389142 DOI: 10.1111/j.1365-2583.2009.00881.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
K-activated amino acid transporter 1 (KAAT1) and cation-anion-activated amino acid transporter/channel 1 (CAATCH1) are amino acid cotransporters, belonging to the Na/Cl-dependent neurotransmitter transporter family (also called SLC6/NSS), that have been cloned from Manduca sexta midgut. They have been thoroughly studied by expression in Xenopus laevis oocytes, and structure/function analyses have made it possible to identify the structural determinants of their cation and amino acid selectivity. About 40 mutants of these proteins have been studied by measuring amino acid uptake and current/voltage relationships. The results obtained since the cloning of KAAT1 and CAATCH1 are here discussed in the light of the 3D model of the first crystallized member of the family, the leucine transporter LeuT.
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Affiliation(s)
- M Castagna
- Institute of General Physiology and Biological Chemistry 'G. Esposito', University of Milan, Milano, Italy
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12
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Harvey WR, Boudko DY, Rheault MR, Okech BA. NHE(VNAT): an H+ V-ATPase electrically coupled to a Na+:nutrient amino acid transporter (NAT) forms an Na+/H+ exchanger (NHE). ACTA ACUST UNITED AC 2009; 212:347-57. [PMID: 19151209 DOI: 10.1242/jeb.026047] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Glycolysis, the citric acid cycle and other metabolic pathways of living organisms generate potentially toxic acids within all cells. One ubiquitous mechanism for ridding cells of the acids is to expel H(+) in exchange for extracellular Na(+), mediated by electroneutral transporters called Na(+)/H(+) exchangers (NHEs) that are driven by Na(+) concentration gradients. The exchange must be important because the human genome contains 10 NHEs along with two Na(+)/H(+) antiporters (NHAs). By contrast, the genomes of two principal disease vector mosquitoes, Anopheles gambiae and Aedes aegypti, contain only three NHEs along with the two NHAs. This shortfall may be explained by the presence of seven nutrient amino acid transporters (NATs) in the mosquito genomes. NATs transport Na(+) stoichiometrically linked to an amino acid into the cells by a process called symport or co-transport. Three of the mosquito NATs and two caterpillar NATs have previously been investigated after heterologous expression in Xenopus laevis oocytes and were found to be voltage driven (electrophoretic). Moreover, the NATs are present in the same membrane as the H(+) V-ATPase, which generates membrane potentials as high as 120 mV. We review evidence that the H(+) V-ATPase moves H(+) out of the cells and the resulting membrane potential (V(m)) drives Na(+) linked to an amino acid into the cells via a NAT. The H(+) efflux by the V-ATPase and Na(+) influx by the NAT comprise the same ion exchange as that mediated by an NHE; so the V and NAT working together constitute an NHE that we call NHE(VNAT). As the H(+) V-ATPase is widely distributed in mosquito epithelial cells and there are seven NATs in the mosquito genomes, there are potentially seven NHE(VNAT)s that could replace the missing NHEs. We review published evidence in support of this hypothesis and speculate about broader functions of NHE(VNAT)s.
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Affiliation(s)
- William R Harvey
- Whitney Laboratory for Marine Bioscience, University of Florida, St Augustine, FL 32080, USA.
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13
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Forrest LR, Tavoulari S, Zhang YW, Rudnick G, Honig B. Identification of a chloride ion binding site in Na+/Cl -dependent transporters. Proc Natl Acad Sci U S A 2007; 104:12761-6. [PMID: 17652169 PMCID: PMC1937540 DOI: 10.1073/pnas.0705600104] [Citation(s) in RCA: 169] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The recent determination of the crystal structure of the leucine transporter from Aquifex aeolicus (aaLeuT) has provided significant insights into the function of neurotransmitter:sodium symporters. Transport by aaLeuT is Cl(-) independent, whereas many neurotransmitter:sodium symporters from higher organisms depend on Cl(-) ions. However, the only Cl(-) ion identified in the aaLeuT structure interacts with nonconserved residues in extracellular loops, and thus the relevance of this binding site is unclear. Here, we use calculations of pK(A)s and homology modeling to predict the location of a functionally important Cl(-) binding site in serotonin transporter and other Cl(-)-dependent transporters. We validate our model through the site-directed mutagenesis of residues predicted to coordinate the Cl(-) ion and through the observation of sequence conservation patterns in other Cl(-)-dependent transporters. The proposed site is located midway across the membrane and is formed by residues from transmembrane helices 2, 6, and 7. It is close to the Na1 sodium binding site, thus providing an explanation for the coupling of Cl(-) and Na(+) ions during transport. Other implications of the model are also discussed.
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Affiliation(s)
- Lucy R. Forrest
- *Howard Hughes Medical Institute, Center for Computational Biology and Bioinformatics, and Department of Biochemistry and Molecular Biophysics, Columbia University, 1130 St. Nicholas Avenue, Room 815, New York, NY 10032; and
| | - Sotiria Tavoulari
- Department of Pharmacology, Yale University, 333 Cedar Street, New Haven, CT 06520-8066
| | - Yuan-Wei Zhang
- Department of Pharmacology, Yale University, 333 Cedar Street, New Haven, CT 06520-8066
| | - Gary Rudnick
- Department of Pharmacology, Yale University, 333 Cedar Street, New Haven, CT 06520-8066
| | - Barry Honig
- *Howard Hughes Medical Institute, Center for Computational Biology and Bioinformatics, and Department of Biochemistry and Molecular Biophysics, Columbia University, 1130 St. Nicholas Avenue, Room 815, New York, NY 10032; and
- To whom correspondence should be addressed. E-mail:
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14
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Castagna M, Soragna A, Mari SA, Santacroce M, Betté S, Mandela PG, Rudnick G, Peres A, Sacchi VF. Interaction between lysine 102 and aspartate 338 in the insect amino acid cotransporter KAAT1. Am J Physiol Cell Physiol 2007; 293:C1286-95. [PMID: 17626242 DOI: 10.1152/ajpcell.00190.2007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
KAAT1 is a lepidopteran neutral amino acid transporter belonging to the NSS super family (SLC6), which has an unusual cation selectivity, being activated by K(+) and Li(+) in addition to Na(+). We have previously demonstrated that Asp338 is essential for KAAT1 activation by K(+) and for the coupling of amino acid and driver ion fluxes. By comparing sequences of NSS family members, site-directed mutagenesis, and expression in Xenopus laevis oocytes, we identified Lys102 as a residue likely to interact with Asp338. Compared with wild type, the single mutants K102V and D338E each showed altered leucine uptake and transport-associated currents in the presence of both Na(+) and K(+). However, in K102V/D338E double mutant, the K102V mutation reversed both the inhibition of Na(+)-dependent transport and the block in K(+)-dependent transport that characterize the D338E mutant. K(+)-dependent leucine currents were not observed in any mutants with D338E. In the presence of the oxidant Cu(II) (1,10-phenanthroline)(3), we observed specific and reversible inhibition of K102C/D338C mutant, but not of the corresponding single cysteine mutants, suggesting that these residues are sufficiently close to form a disulfide bond. Thus both structural and functional evidence suggests that these two residues interact. Similar results have been obtained mutating the bacterial transporter homolog TnaT. Asp338 corresponds to Asn286, a residue located in the Na(+) binding site in the recently solved crystal structure of the NSS transporter LeuT(Aa) (41). Our results suggest that Lys102, interacting with Asp338, could contribute to the spatial organization of KAAT1 cation binding site and permeation pathway.
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Affiliation(s)
- M Castagna
- Institute of General Physiology and Biological Chemistry G Esposito, Via Trentacoste 2, 20134, Milan, Italy
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15
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Carroll JS, Ku CJ, Karunarathne W, Spence DM. Red blood cell stimulation of platelet nitric oxide production indicated by quantitative monitoring of the communication between cells in the bloodstream. Anal Chem 2007; 79:5133-8. [PMID: 17580956 DOI: 10.1021/ac0706271] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
ATP is a recognized stimulus of nitric oxide synthase and is released from red blood cells (RBCs) upon deformation. The objective of this work is to demonstrate that RBCs stimulate nitric oxide production in platelets by employing a continuous flow analysis system in which the stream contains both RBCs and platelets. Here, two drugs known to improve blood flow in vivo (pentoxyfilline and iloprost) are shown to increase both the release of RBC-derived ATP and the production of platelet-derived NO. A flow-based chemiluminescence assay (in vitro) was employed to quantitatively determine the amount of ATP released from erythrocytes subjected to flow-induced deformation. Prior to being subjected to flow, erythrocytes were incubated in the absence or presence of 4.8 microM pentoxyfilline or 80 nM iloprost. Erythrocytes obtained from rabbits (n=22) that were subjected to flow released 239 +/- 29 nM ATP. When treated with pentoxyfilline, the ATP released from the flowing RBCs increased to 450 +/- 94 nM ATP. An increase in RBC-derived ATP was also measured for iloprost-incubated RBCs in flow (362 +/- 45 nM ATP). Importantly, platelets that were loaded with diaminofluorofluorescein diacetate, an intracellular fluorescence probe for NO, exhibited increases in fluorescence intensity by 16% in the presence of RBCs treated with pentoxyfilline and a 10% increase when treated with iloprost. When the ATP release from the RBCs was inhibited with glybenclamide, the platelet fluorescence intensity decreased by 25 and 51% for RBCs incubated with pentoxyfilline and iloprost, respectively. In an experiment not involving the RBC, inhibition of the P2x receptor on the platelets (an ATP receptor) resulted in no increase in platelet NO production, suggesting that the NO production in the activated platelet is due to ATP.
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Affiliation(s)
- Jamie S Carroll
- Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA
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16
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Miszner A, Peres A, Castagna M, Bettè S, Giovannardi S, Cherubino F, Bossi E. Structural and functional basis of amino acid specificity in the invertebrate cotransporter KAAT1. J Physiol 2007; 581:899-913. [PMID: 17412764 PMCID: PMC2170845 DOI: 10.1113/jphysiol.2007.132555] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The substrate specificity of KAAT1, a Na+- and K+-dependent neutral amino acid cotransporter cloned from the larva of the invertebrate Manduca sexta and belonging to the SLC6A gene family has been investigated using electrophysiological and radiotracer methods. The specificity of KAAT1 was compared to that of CAATCH1, a strictly related transporter with different amino acid selectivity. Competition experiments between different substrates indicate that both transporters bind leucine more strongly than threonine and proline, the difference between KAAT1 and CAATCH1 residing in the incapacity of the latter to complete the transport cycle in presence of leucine. The behaviour of CAATCH1 is mimicked by the S308T mutant form of KAAT1, constructed on the basis of the atomic structure of a leucine-transporting bacterial member of the family, which indicates the participation of this residue in the leucine-binding site. The reverse mutation T308S in CAATCH1 conferred to this transporter the ability to transport leucine in presence of K+. These results may be interpreted by a kinetic scheme in which, in presence of Na+, the leucine-bound state of the transporter is relatively stable, while in presence of K+ and at negative potentials the progression of the leucine-bound form along the cycle is favoured. In this context serine 308 appears to be important in allowing the change to the inward-facing conformation of the transporter following substrate binding, rather than in determining the binding specificity.
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Affiliation(s)
- Andreea Miszner
- Laboratory of Cellular and Molecular Physiology, Department of Structural and Functional Biology, University of Insubria, Via Dunant 3, 21100 Varese, Italy
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17
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Thimgan MS, Berg JS, Stuart AE. Comparative sequence analysis and tissue localization of members of the SLC6 family of transporters in adult Drosophila melanogaster. ACTA ACUST UNITED AC 2006; 209:3383-404. [PMID: 16916974 DOI: 10.1242/jeb.02328] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
The SLC6 family comprises proteins that move extracellular neurotransmitters, amino acids and osmolytes across the plasma membrane into the cytosol. In mammals, deletion of SLC6 family members has dramatic physiologic consequences, but in the model organism Drosophila melanogaster, little is known about this family of proteins. Therefore, in this study we carried out an initial analysis of 21 known or putative SLC6 family members from the Drosophila genome. Protein sequences from these genes segregated into either well-defined subfamilies, including the novel insect amino acid transporter subfamily, or into a group of weakly related sequences not affiliated with a recognized subfamily. Reverse transcription-polymerase chain reaction analysis and in situ hybridization showed that seven of these genes are expressed in the CNS. In situ hybridization revealed that two previously cloned SLC6 members, the serotonin and dopamine transporters, were localized to presumptive presynaptic neurons that previously immunolabelled for these transmitters. RNA for CG1732 (the putative GABA transporter) and CG15088 (a member of the novel insect amino acid transporter family) was localized in cells likely to be subtypes of glia, while RNA for CG5226, CG10804 (both members of the orphan neurotransmitter transporter subfamily) and CG5549 (a putative glycine transporter) were expressed broadly throughout the cellular cortex of the CNS. Eight of the 21 sequences were localized outside the CNS in the alimentary canal, Malpighian tubules and reproductive organs. Localization for six sequences was not found or not attempted in the adult fly. We used the Drosophila ortholog of the mammalian vesicular monoamine transporter 2, CG33528, to independently identify monoaminergic neurons in the adult fly. RNA for CG33528 was detected in a limited number of cells in the central brain and in a beaded stripe at the base of the photoreceptors in the position of glia, but not in the photoreceptors themselves. The SLC6 localization observations in conjunction with likely substrates based on phylogenetic inferences are a first step in defining the role of Na/Cl-dependent transporters in Drosophila physiology.
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Affiliation(s)
- Matthew S Thimgan
- Department of Cell and Molecular Physiology, University of North Carolina School of Medicine, Chapel Hill, 27599, USA.
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18
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Bossi E, Soragna A, Miszner A, Giovannardi S, Frangione V, Peres A. Oligomeric structure of the neutral amino acid transporters KAAT1 and CAATCH1. Am J Physiol Cell Physiol 2006; 292:C1379-87. [PMID: 17135296 DOI: 10.1152/ajpcell.00473.2006] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The highly homologous neutral amino acid transporters KAAT1 and CAATCH1, cloned from the midgut epithelium of the Manduca sexta larva, are members of the Na(+)/Cl(-)-dependent transporter family. Recent evidence indicates that transporters of this family form constitutive oligomers. CAATCH1 and KAAT1 give rise to specific kinds of current depending on the transported amino acid, cotransported ion, pH, and membrane voltage. Different substrates induce notably distinct transport-associated currents in the two proteins that represent useful tools in structural-functional studies. To determine whether KAAT1 and CAATCH1 form functional oligomers, we have constructed four concatameric proteins for electrophysiological analysis, consisting of one KAAT1 protein covalently linked to another KAAT1 (K-K concatamer) or to CAATCH1 (K-C concatamer) and vice versa (C-C concatamer and C-K concatamer), and eight constructs where the two transporters were linked to yellow or cyan fluorescent protein in the NH(2) or COOH terminus, to determine the oligomer formation and the relative distance between the different subunits by fluorescence resonance energy transfer (FRET) analysis. Heterologous expression of the concatenated constructs and coinjection of the original proteins in different proportions allowed us to compare the characteristics of the currents to those of the oocytes expressing only the wild-type proteins. All the constructs were fully active, and their electrophysiological behavior was consistent with the activity as monomeric proteins. However, the FRET studies indicate that these transporters form oligomers in agreement with the LeuT(Aa) atomic structure and confirm that the COOH termini of the adjacent subunits are closer than NH(2) termini.
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Affiliation(s)
- Elena Bossi
- Laboratory of Cellular and Molecular Physiology, Department of Structural and Functional Biology, University of Insubria, Via Dunant 3, 21100 Varese, Italy.
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