title: Functional roles of cationic amino acid residues in the sodium-dicarboxylate cotransporter 3 (NaDC-3) from winter flounder.
abstract: In the present study, we determined the functional role of 15 positively charged amino acid residues at or within 1 of the predicted 11 transmembrane helixes of the flounder renal sodium-dicarboxylate cotransporter fNaDC-3. Using site-directed mutagenesis, histidine (H), lysine (K), and arginine (R) residues of fNaDC-3 were replaced by alanine (A), isoleucine (I), or leucine (L). Most mutants showed sodium-dependent, lithium-inhibitable [14C]succinate uptake and, in two-electrode voltage-clamp (TEVC) experiments, Km and DeltaI(max) values comparable to wild-type (WT) fNaDC-3. The replacement of R109 and R110 by alanine and isoleucine (RR109/110AI) prevented the expression of fNaDC-3 at the plasma membrane. When the lysines at positions 232 and 235 were replaced by isoleucine (KK232/235II), the transporter was expressed but showed small transport rates and succinate-induced currents. K114I, located within transmembrane helix 4, showed [14C]succinate uptake similar to WT but relatively small inward currents. When K114 was replaced by arginine, glutamic acid (E), or glutamine (Q), all mutants were expressed at the cell surface. In [14C]succinate uptake and TEVC experiments performed simultaneously on the same oocytes, uptake was similar to or higher than WT, whereas succinate-induced currents were either comparable (K114R) to, or considerably smaller (K114E, K114I, K114Q) than, those evoked by WT. These results suggest that a positively charged residue at position 114 is required for electrogenic sodium-dicarboxylate cotransport.
authors.affiliation: Zentrum Physiologie und Pathophysiologie, Abt. Vegetative Physiologie und Pathophysiologie Universität Göttingen, Humboldtallee 23, 37073 Göttingen, Germany. hagos@physiol.med.uni-goettingen.de
authors.forename: Yohannes
authors.initials: Y
authors.lastname: Hagos
authors.forename: Jürgen
authors.initials: J
authors.lastname: Steffgen
authors.forename: Ahsan N
authors.initials: AN
authors.lastname: Rizwan
authors.forename: Denis
authors.initials: D
authors.lastname: Langheit
authors.forename: Ariane
authors.initials: A
authors.lastname: Knoll
authors.forename: Gerhard
authors.initials: G
authors.lastname: Burckhardt
authors.forename: Birgitta C
authors.initials: BC
authors.lastname: Burckhardt
mesh_terms: D000818:Animals; D002250:Carbon Radioisotopes; D004252:DNA Mutational Analysis; D029382:Dicarboxylic Acid Transporters / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D005432:Flounder / Q000235:genetics* / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000502:physiology; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000378:metabolism; D027701:Organic Cation Transport Proteins / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D018408:Patch-Clamp Techniques; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012964:Sodium / Q000378:metabolism; D019802:Succinic Acid / Q000493:pharmacokinetics; D014982:Xenopus laevis
chemical_list: D002250:Carbon Radioisotopes; D029382:Dicarboxylic Acid Transporters; D027701:Organic Cation Transport Proteins; D012964:Sodium; D019802:Succinic Acid