Citrate transporter

Aligned sequences of 16 members of the sugar transporter family. Residues which are identical in 5=50% of the 16 sugar-transporter sequences (excluding the quinate transporter (qa-y), the citrate transporter (CIT), the tetracycline transporter (pBR322) and lac permease (LacY)) are highlighted, and recorded below the sequences as CONSERVED . The locations of predicted membrane-spanning helices are indicated by horizontal bars. The sequences were taken from the references cited in the text. 

The majority of Fur-regulated gene products are involved in iron uptake. Genes for transport and biosynthesis of enterobactin have been studied in E. coli K-12 (Earhart, 1996). It is assumed that this system is found in nearly every E. coli strain. Also the ferrichrome transport system seems to have a very broad distribution. The ferric citrate transport system (fee), however, is only present in some E. coli strains and may be part of a pathogenicity island. The aerobactin and yersiniabactin biosynthesis and transport systems are not found in all E. coli strains and are integrated into pathogenicity islands (Schubert et al., 1999). The ability to utilize haem seems also to be a specific pathogenicity-related adaptation. Haem transport systems are used in the animal or human host, where transferrin and lactoferrin create an iron-poor environment for bacteria. 

The novel mechanism of transcriptional control of the ferric citrate transport system via transmembrane signalling is also observed in Pseudomonas putida and probably also occurs in Pseudomonas aeruginosa. Synthesis of the PupB outer... 

Uptake of small organic metal complexes over transport systems of organic metabolites may be possible, for example, of small organic acids like citrate or amino acids. However, only few examples of such processes have been studied so far. Increased uptake of cadmium by an alga has been observed in the presence of citrate and has been attributed to accidental transport of the metal-citrate complex over a citrate transporter [212]. Transport systems of inorganic anions may also play a role in metal transport. Silver uptake by algae was enhanced in the presence of thiosulfate. In this case, the silver thiosulfate complex was transported over a sulfate uptake system [213]. It remains to be demonstrated how widespread these processes may be for metal uptake in the aquatic environment [12]. 

Fluoroacetate undergoes a "lethal synthesis"(18) to 2-fluorocitrate which may reversibly inhibit aconitase and which irreversibly binds to a membrane-associated citrate transport protein(19,20). Insecticidal and other biocidal uses of fluoroacetate (or its metabolic precursors) received considerable attention twenty-five years ago( ) but most uses have been abandoned due to high nonspecific vertebrate toxicity of these compounds. Vfe have reported the use of o)-fluoro fatty acids and their derivatives as delayed-action toxicants for targeted... 

Potent metabolic inhibitors of the citric acid cycle. Fluo-roacetate (F-CH2COO ) must first be converted to flu-oroacetyl-S-CoA (by acetyl-CoA synthetase) and thence to fluorocitrate (by citrate synthase) before it can act as a potent metabohc inhibitor of the aconitase reaction as well as citrate transport. Submicromolar concentrations of ( )-erythro-Q iOTOcitTate can irreversibly inhibit citrate uptake by isolated brain mitochondria. 

Nevertheless, the toxicity of fluoroacetate seems to be only partially due to the inhibition of aconitase. The competitive nature of the inhibition, its Xj value (Xj = 20-60 pM)," and the time-dependent nature (but reversible) of the inhibition of aconitase seem to be poorly compatible with the sharp and irreversible toxicity of fluorocitrate. Thus, it has been suggested that fluorocitrate can covalently bind with the proteins that are involved in citrate transport through the mitochondrial membrane. ... 

Yamamoto, H. Murata, M. Sekiguchi, J. The CitST two-component system regulates the expression of the Mg-citrate transporter in Bacillus subtilis. Mol. Microbiol., 37, 898-912 (2000)... 

A high molecular weight, citrate-inducible, outer membrane polypeptide has been detected by slab gel electrophoresis (65). This is believed to be the receptor for the ferric citrate transport system earlier characterized by Frost and Rosenberg (60). 

Transfer of citrate through the inner membrane of MCh is provided by a tricarboxylate transporter (m.w. 32.5 kD), which also catalyzes transport of treo-Ds-isocitrate, cis-aconitate and other tricarboxylates (LaNoue and School-werth, 1979 Kaplan et al, 1990). This is electroneutral exchange for either another tricarboxylate or dicarboxylate (e.g. malate or succinate), or for phosphoenolpyruvate. Formation of glutathione-citryl thioester is irreversibly inhibited by (-)erythrofluorocitrate (IC50 = 25 pmol FC/mg protein), which makes a stable adduct with the synthase (Kun et al, 1977). However, the block of citrate transport... 

Kirsten, E., Sharma, M. L., Kun, E. (1978). Molecular toxicity of (-)-erythro-fluorocitrate selective inhibition of citrate transport in mitochondria and the binding of fluorocitrate to mitochondrial proteins. Mol. Pharmacol. 14 172-84. 

Several additional mitochondrial carrier systems have been reconstituted into active form in proteoliposomes, using as starting material a crude neutral detergent mixture of membrane proteins from submitochondrial particles. These include the citrate transporter [203], the dicarboxylate carrier [204], and the carnitine transporters [202]. These reconstitution activities could be used as a basis for further purification and structural studies, but such studies have not yet been reported. 

The acquisition of iron, copper, and zinc in plant roots has been described in Chapter 7. Once within the root epidermal cell, the iron must be transported through the roots to the xylem and thence to the leaves, and this intercellular metal transport is illustrated for dicots in Fig. 8.8 and for monocots in Fig. 8.9. In dicots, Fe, Zn, and Cu are taken up into the symplast by transporters in the epidermis. Reduction of Fe and possibly of Cu by FR02 and acidification of the soil by an Arabidopsis ATPase contribute to increased metal uptake. Metals can then travel through the symplastic space to the vasculature. Transport into the xylem is still not fully characterised. In the case of Fe, it is probably as citrate, and the citrate transporter FRD3 has been shown to efflux citrate into the xylem and is required for Fe transport to the shoot. Zn and Cu are thought to be effluxed into the xylem by... 

Cytoplasmic generation of acetyl-CoA via citrate transport and related reactions. PPP = Pentose phosphate pathway FAS = fatty acid synthase. — -> Negative allosteric modifier. — -> Positive allosteric modifier. 

Krom bp, Warner JB, Konings WN and Lolkema J S (2000) Complementary metal ion specificity of the metal-citrate transporters CitM and CitH of Bacillus subtilis. ] Bacteriol 182 6374—6381. 

---