Structure, function, and expression pattern of a novel sodium-coupled citrate transporter (NaCT) cloned from mammalian brain.
Citrate plays a pivotal role not only in the generation of metabolic energy but also in the synthesis of fatty acids, isoprenoids, and cholesterol in mammalian cells. Plasma levels of citrate are the highest ( approximately 135 microm) among the intermediates of the tricarboxylic acid cycle. Here we report on the cloning and functional characterization of a plasma membrane transporter (NaCT for Na+ -coupled citrate transporter) from rat brain that mediates uphill cellular uptake of citrate coupled to an electrochemical Na+ gradient. NaCT consists of 572 amino acids and exhibits structural similarity to the members of the Na+-dicarboxylate cotransporter/Na+ -sulfate cotransporter (NaDC/NaSi) gene family including the recently identified Drosophila Indy. In rat, the expression of NaCT is restricted to liver, testis, and brain. When expressed heterologously in mammalian cells, rat NaCT mediates the transport of citrate with high affinity (Michaelis-Menten constant, approximately 20 microm) and with a Na+:citrate stoichiometry of 4:1. The transporter does interact with other dicarboxylates and tricarboxylates but with considerably lower affinity. In mouse brain, the expression of NaCT mRNA is evident in the cerebral cortex, cerebellum, hippocampus, and olfactory bulb. NaCT represents the first transporter to be identified in mammalian cells that shows preference for citrate over dicarboxylates. This transporter is likely to play an important role in the cellular utilization of citrate in blood for the synthesis of fatty acids and cholesterol (liver) and for the generation of energy (liver and brain). NaCT thus constitutes a potential therapeutic target for the control of body weight, cholesterol levels, and energy homeostasis.
Inoue
Katsuhisa
K
Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, Georgia 30912, USA.
Zhuang
Lina
L
Maddox
Dennis M
DM
Smith
Sylvia B
SB
Ganapathy
Vadivel
V
D000199:Actins / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D015152:Blotting, Northern; D001835:Body Weight; D001921:Brain / Q000378:metabolism; D002531:Cerebellum / Q000378:metabolism; D002540:Cerebral Cortex / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D002951:Citrates / Q000737:chemistry*; D003001:Cloning, Molecular; D004305:Dose-Response Relationship, Drug; D006624:Hippocampus / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D017403:In Situ Hybridization; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008297:Male; D051379:Mice; D008969:Molecular Sequence Data; D009830:Olfactory Bulb / Q000378:metabolism; D011485:Protein Binding; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism; D000077559:Sodium Citrate; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D027981:Symporters / Q000096:biosynthesis* / Q000737:chemistry*; D013737:Testis / Q000378:metabolism; D013997:Time Factors; D014018:Tissue Distribution
D000199:Actins; D002951:Citrates; D012333:RNA, Messenger; C469354:Slc13a5 protein, rat; D027981:Symporters; D000077559:Sodium Citrate; D002784:Cholesterol; D012964:Sodium