Structure and mechanism of a bacterial sodium-dependent dicarboxylate transporter

• Published in: Nature (London) Vol. 491; no. 7425; pp. 622 - 626
• Main Authors: Mancusso, Romina, Gregorio, G. Glenn, Liu, Qun, Wang, Da-Neng
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 22.11.2012; Nature Publishing Group
• Subjects: 631/326/41/2536; 631/57/2270/1140; 631/92/577; Amino Acid Motifs; Amino Acid Sequence; Amino acids; Analytical, structural and metabolic biochemistry; Bacteria; Binding Sites; Biological and medical sciences; Carboxylases; Carrier proteins; Citrates; Citric Acid - chemistry; Citric Acid - metabolism; Crystal structure; Crystallography, X-Ray; Dicarboxylic Acid Transporters - chemistry; Dicarboxylic Acid Transporters - metabolism; E coli; Energy balance; Fatty acids; Fingers & toes; Fundamental and applied biological sciences. Psychology; Humanities and Social Sciences; Ion Transport; letter; Microbiology; Models, Molecular; Molecular and cellular biology; Molecular Sequence Data; multidisciplinary; Oxidation; Protein Conformation; Proteins; Science; Sodium; Sodium - chemistry; Sodium - metabolism; Structural Homology, Protein; Structure-Activity Relationship; Symmetry; Translocation; Vibrio cholerae; Vibrio cholerae - chemistry; United States; Bacteria; Biological transport; Mechanism
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11542

The cytosolic concentration of citrate partially depends on its direct import across the plasma membrane by the Na + -dependent citrate transporter (NaCT); here the X-ray crystal structure of a bacterial homologue of NaCT is reported, which, along with transport-activity studies, suggests how specific conformational changes facilitate substrate translocation across the cellular membrane. Bacterial citrate transport Citrate is an important metabolite that serves as the starting material for the biosynthesis of fatty acids, triacylglycerols, cholesterol and low-density lipoprotein. The cytosolic concentration of citrate depends in part on direct import across the plasma membrane through the Na + -dependent citrate transporter, a member of the divalent anion/Na + symporter family. In this paper, the authors report the X-ray crystal structure and transport-activity studies of a bacterial Na + -dependent citrate transporter. The structural and biochemical studies suggest how specific conformational changes facilitate substrate translocation across the cell membrane. In human cells, cytosolic citrate is a chief precursor for the synthesis of fatty acids, triacylglycerols, cholesterol and low-density lipoprotein. Cytosolic citrate further regulates the energy balance of the cell by activating the fatty-acid-synthesis pathway while downregulating both the glycolysis and fatty-acid β-oxidation pathways 1 , 2 , 3 , 4 . The rate of fatty-acid synthesis in liver and adipose cells, the two main tissue types for such synthesis, correlates directly with the concentration of citrate in the cytosol 2 , 3 , 4 , 5 , with the cytosolic citrate concentration partially depending on direct import across the plasma membrane through the Na + -dependent citrate transporter (NaCT) 6 , 7 . Mutations of the homologous fly gene ( Indy ; I’m not dead yet ) result in reduced fat storage through calorie restriction 8 . More recently, Nact (also known as Slc13a5 )-knockout mice have been found to have increased hepatic mitochondrial biogenesis, higher lipid oxidation and energy expenditure, and reduced lipogenesis, which taken together protect the mice from obesity and insulin resistance 9 . To understand the transport mechanism of NaCT and INDY proteins, here we report the 3.2 Å crystal structure of a bacterial INDY homologue. One citrate molecule and one sodium ion are bound per protein, and their binding sites are defined by conserved amino acid motifs, forming the structural basis for understanding the specificity of the transporter. Comparison of the structures of the two symmetrical halves of the transporter suggests conformational changes that propel substrate translocation.