Novel dicarboxylate selectivity in an insect glutamate transporter homolog.
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 α-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.
Vollum Institute, Oregon Health & Science University, Portland, Oregon, United States of America.
Hui
H
Wang
Avi M
AM
Rascoe
David C
DC
Holley
Eric
E
Gouaux
Michael P
MP
Kavanaugh
D000595:Amino Acid Sequence; D027322:Amino Acid Transport System X-AG / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D001692:Biological Transport; D003465:Culex / Q000235:genetics / Q000378:metabolism*; D029382:Dicarboxylic Acid Transporters / Q000145:classification / Q000235:genetics / Q000378:metabolism; D003998:Dicarboxylic Acids / Q000378:metabolism*; D050580:Excitatory Amino Acid Transporter 3 / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D005260:Female; D006801:Humans; D019476:Insect Proteins / Q000235:genetics / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D008969:Molecular Sequence Data; D009154:Mutation; D009865:Oocytes / Q000378:metabolism / Q000502:physiology; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid; D014982:Xenopus laevis
D027322:Amino Acid Transport System X-AG; D029382:Dicarboxylic Acid Transporters; D003998:Dicarboxylic Acids; D050580:Excitatory Amino Acid Transporter 3; D019476:Insect Proteins