Molecular and phylogenetic characterization of a novel putative membrane transporter (SLC10A7), conserved in vertebrates and bacteria

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Abstract

The ‘Solute Carrier Family SLC10’ consists of six annotated members in humans, comprising two bile acid carriers (SLC10A1 and SLC10A2), one steroid sulfate transporter (SLC10A6), and three orphan carriers (SLC10A3 to SLC10A5). In this study we report molecular characterization and expression analysis of a novel member of the SLC10 family, SLC10A7, previously known as C4orf13. SLC10A7 proteins consist of 340–343 amino acids in humans, mice, rats, and frogs and show an overall amino acid sequence identity of >85%. SLC10A7 genes comprise 12 coding exons and show broad tissue expression pattern. When expressed in Xenopus laevis oocytes and HEK293 cells, SLC10A7 was detected in the plasma membrane but revealed no transport activity for bile acids and steroid sulfates. By immunofluorescence analysis of dual hemagglutinin (HA)- and FLAG-labeled SLC10A7 proteins in HEK293 cells, we established a topology of 10 transmembrane domains with an intracellular cis orientation of the N-terminal and C-terminal ends. This topology pattern is clearly different from the seven-transmembrane domain topology of the other SLC10 members but similar to hitherto uncharacterized non-vertebrate SLC10A7-related proteins. In contrast to the established SLC10 members, which are restricted to the taxonomic branch of vertebrates, SLC10A7-related proteins exist also in yeasts, plants, and bacteria, making SLC10A7 taxonomically the most widespread member of this carrier family. Vertebrate and bacterial SLC10A7 proteins exhibit >20% sequence identity, which is higher than the sequence identity of SLC10A7 to any other member of the SLC10 carrier family.

Introduction

The sodium bile acid symporter family (SBF) consists of more than 490 proteins from prokaryotes and eukaryotes, which all share the SBF domain (Protein family database (Pfam) Accession Number PF01758). All vertebrate SBF proteins belong to ‘Solute Carrier Family 10’ (SLC10 according to the Human Genome Organization Nomenclature Committee (HGNC), TC 2.A.28 according to the Transport Classification Database (TCDB)), i.e., SLC10A1 (NTCP), SLC10A2 (ASBT), SLC10A3 (P3), SLC10A4 (P4), SLC10A5 (P5), and SLC10A6 (SOAT) (Geyer et al., 2006). The Na+/taurocholate cotransporting polypeptide (NTCP) was identified by expression cloning from rat liver (Hagenbuch et al., 1990, Hagenbuch et al., 1991) as the first member of the SLC10 family. NTCP is exclusively expressed at the basolateral membrane of hepatocytes (Ananthanarayanan et al., 1994; Stieger et al., 1994), where it mediates sodium-coupled uptake of taurocholate and other bile acids (Boyer et al., 1994; Hagenbuch and Meier, 1994; Platte et al., 1996). The second bile acid transporter, the apical sodium-dependent bile acid transporter (ASBT), is expressed at the apical brush border membrane of ileal enterocytes (Shneider et al., 1995) and is involved in bile acid reabsorption from the intestine (Craddock et al., 1998; Kramer et al., 1999; Wong et al., 1994, Wong et al., 1995). For 10 years after the cloning of ASBT, no further member of this transporter family had been discovered; therefore, the SLC10 synonym “sodium bile acid cotransporter family” became established in the literature (Hagenbuch and Dawson, 2004). Recently, we cloned the sodium-dependent organic anion transporter (SOAT), which lacks transport of classical bile acids such as taurocholate and cholate but specifically transports sulfoconjugated steroid hormones and sulfoconjugated bile acids (Geyer et al., 2004, Geyer et al., 2007), indicating that the substrate pattern of SLC10 carriers is not restricted to bile acids. In contrast to NTCP, ASBT, and SOAT, little is known about the function and expression of SLC10A3, SLC10A4, and SLC10A5 (Geyer et al., 2006).

SBF proteins are common in bacteria and yeasts as well. These are referred to as ACR3 carriers (TC 2.A.59) and lack any ortholog in vertebrates. Members of this family, e.g., ACR3 from Saccharomyces cerevisiae and ArsB from Bacillus subtilis, confer resistance to arsenate and arsenite by an efflux-based transport mechanism (Bobrowicz et al., 1997; Sato and Kobayashi, 1998; Wysocki et al., 1997). Aside from the established SLC10 and ACR3 carriers, we have identified new orphan transporters in vertebrates, yeasts, plants, and bacteria, which also share the SBF domain and show an overall sequence identity of >18%. This group of SBF proteins here is referred to as SLC10A7 proteins. By large-scale sequencing analysis of a human fetal brain cDNA library Zou et al. (2005) previously isolated a C4orf13 cDNA which is identical to SLC10A7. They showed that C4orf13 is widely expressed in human tissues but no further experimental data were given. Here we report on the molecular characterization and expression analysis of SLC10A7 and on the phylogenetic relationship of SLC10A7 to the established SLC10 and ACR3 carriers.

Section snippets

Radiochemicals

[1,2,6,7-3H(N)]Dehydroepiandrosterone sulfate (60 Ci/mmol), [6,7-3H(N)]estrone-3-sulfate (57.3 Ci/mmol), and [3H]taurocholate (3.5 Ci/mmol) were purchased from PerkinElmer Life Sciences (Boston, MA, USA), and [14C]glycocholate (55 mCi/mmol) was obtained from Amersham Biosciences (Buckinghamshire, UK). [14C]Cholate (55 mCi/mmol), [14C]chenodeoxycholate (51.3 mCi/mmol), and [7-3H(N)]pregnenolone sulfate (20 Ci/mmol) were obtained from American Radiolabeled Chemicals (St. Louis, MO, USA).

Identification and cloning of human, rat, mouse, and Xenopus laevis SLC10A7 cDNAs

In order to

Cloning of the orphan carrier SLC10A7

In order to identify new SBF domain-containing eukaryotic proteins, we used the amino acid and cDNA sequences of NTCP and ASBT as queries for BLASTp and BLASTn analyses in the available protein and nucleotide databases, respectively. Besides the annotated SLC10 and ACR3 carriers, two hitherto uncharacterized sequences were identified (GenBank accession nos. XM_134313 from mouse and XM_214660 from rat), which were predicted by automated computational analysis from genomic sequences. These

Discussion

In this paper, we describe the molecular and functional characterization of a new member of the SLC10 carrier family. Along with the established SLC10 members SLC10A1 to A6, SLC10A7 contains the SBF domain and represents a membrane protein of 340–343 amino acids. Nonetheless, SLC10A7 is an atypical member of this carrier family concerning genomic organization, tissue expression, membrane topology, and transport function. Among the family of SLC10 carriers, SLC10A7 proteins have the highest

Acknowledgements

This study was supported by the German Research Foundation (DFG, Grant GE1921/1-1) and Graduate Research Program ‘Molecular Veterinary Medicine’ (Graduiertenkolleg 455), and KAAD (Katholischer Akademischer Ausländer-Dienst, Germany).

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