Ana M. Pajor, Ph.D., Professor
Molecular Mechanisms of Transporter Function:
The Na + /dicarboxylate cotransporters (NaDC) carry metabolic intermediates of the Tricarboxylic Acid Cycle, such as succinate, citrate, and a -ketoglutarate, into cells. Citrate is particularly important because its levels in the cell affect the activity of many key metabolic enzymes. The low-affinity NaDC-1 transporters absorb dietary Krebs cycle intermediates in the intestine and also help to conserve these metabolites in the kidney. The high-affinity NaDC-3 transporters have a broader distribution in organs such as the kidney, liver, placenta and brain. The NaDC transporters belong to a distinct gene family called SLC13 that also includes sodium-coupled sulfate transporters, NaSi. The members of this family have a similar transport mechanism and, for every transport cycle, 3 sodium ions and 1 divalent anion substrate are carried across the plasma membrane.
The main focus of our lab is to understand the transport mechanism of sodium-coupled transporters, particularly the members of the SLC13 family. Our group has cloned and characterized several members of this family. We have used a combination of mutagenesis and chimera formation to identify binding sites for sodium and substrates. The mutant transporters are expressed in Xenopus oocytes or cultured mammalian cells and the function can be assessed by measurement of radiotracers or substrate-dependent currents. Our results have shown that the C-terminal half of the transporters contains the substrate binding site. Furthermore, we have found that amino acids in transmembrane helices 7, 8 and 9 are alternately exposed and occluded from the outside of the cell during the transport cycle.
A new area of research in the lab is the involvement of the Na + /dicarboxylate cotransporters in aging. The Drosophila homolog of NaDC-1, called Indy , is a dicarboxylate transporter that appears to be involved in determining lifespan. Flies with mutations in one copy of the Indy gene live twice as long as controls. Interestingly, the only known intervention in mammals that increases lifespan is caloric restriction suggesting that the availability of substrates for NaDC-1 (which absorbs TCA cycle intermediates from the diet) may play a key role in controlling metabolism.
