Urate transport function of rat sodium-dependent nucleobase transporter 1

Removal of substrate inhibition of Acinetobacter baumannii xanthine oxidase by point mutation at Gln-201 enables efficient reduction of purine content in fish sauce

Xanthine oxidase is an oxidase that has a molybdopterin structure with substrate inhibition. Here, we show that a single point mutation (Q201) in the Acinetobacter baumannii xanthine oxidase (AbXOD) obtained mutant Q201E (k _cat =799.44 s^-1, no inhibition) with high enzyme activity and decrease of substrate inhibition in 5 mmol/L high substrate model, and which cause two loops structure change at active center, characterized by complete loss of substrate inhibition without reduction of enzymatic activity. Molecular docking results showed that the change of flexible loop increased the affinity between substrate and enzyme, and the formation of a π-π bond and two hydrogen bonds made the substrate more stable in the active center. Ultimately, Q201E can still maintain better enzyme activity under high purine content (an approximately 7-fold improvement over the wild-type), indicating a broader application prospect in the manufacture of low-purine food.

Current Understanding of the Intestinal Absorption of Nucleobases and Analogs

It has long been suggested that a Na⁺-dependent carrier-mediated transport system is involved in the absorption of nucleobases and analogs, including some drugs currently in therapeutic use, for their uptake at the brush border membrane of epithelial cells in the small intestine, mainly based on studies in non-primate experimental animals. The presence of this transport system was indeed proved by the recent identification of sodium-dependent nucleobase transporter 1 (SNBT1/Slc23a4) as its molecular entity in rats. However, this transporter has been found to be genetically deficient in humans and higher primates. Aware of this deficiency, we need to revisit the issue of the absorption of these compounds in the human small intestine so that we can understand the mechanisms and gain information to assure the more rational use and development of drugs analogous to nucleobases. Here, we review the current understanding of the intestinal absorption of nucleobases and analogs. This includes recent knowledge about the efflux transport of those compounds across the basolateral membrane when exiting epithelial cells, following brush border uptake, in order to complete the overall absorption process; the facilitative transporters of equilibrative nucleoside transporter 1 (ENT1/SLC29A1) and equilibrative nucleobase transporter 1 (ENBT1/SLC43A3) may be involved in that in many animal species, including human and rat, without any major species differences.

Comparative transcriptomic analysis of the l-4i silkworm (Lepidoptera: Bombyx mori) mutants and its wild-type strain P33 by RNA-Seq

The silkworm (Bombyx mori) is a domesticated holometabolous insect, and more than 400 Mendelian mutations have been identified. Investigating the mechanism behind these silkworm mutants is essential for understanding the development of silkworms and other lepidopterans, and lethal genes could be used for pest control. The lethal silkworm mutant in the fourth instar (l-4i) has been recently found; however, the underlying mechanism is not yet clear. Herein, we studied the l-4i mutant and its wild-type strain P33 using RNA sequencing (RNA-seq). Our results revealed that 2013 genes were significantly downregulated, and 20 biological processes, including spliceosomal snRNP assembly, protein folding and protein catabolic process, were significantly enriched in these downregulated genes. Moreover, 2405 genes were significantly upregulated in the l-4i mutant, and 20 biological processes, including purine nucleobase metabolic process, nucleoside metabolic process and de novo IMP biosynthetic process, were significantly enriched in these upregulated genes. The study suggests that the imbalance of multiple biological processes and pathways and abnormal protein generation from RNA alternative splicing may cause the death of the l-4i mutant.

Species-dependent patterns of incorporation of purine mononucleotides and nucleosides by lactic acid bacteria

Although most lactic acid bacteria do not directly incorporate purine nucleotides, the strain Lactobacillus gasseri PA-3 was found to incorporate purine mononucleotides. To determine whether the direct uptake of purine mononucleotides is dependent on the species or strain of lactic acid bacteria, incorporation of purine mononucleotides was assessed in L. gasseri, Lactcoccus lactis sbsp. lactis, Streptococcus thermophilus and other species of lactic acid bacteria. Each bacterial strain was incubated with ³²P-AMP or ¹⁴C-adenosine and the incorporation of each purine was evaluated by measuring their radioactivity. All investigated strains of L. gasseri incorporated ³²P-AMP, whereas strains of S. thermophilus and most strains of L. lactis did not. Incorporation of ³²P-AMP into strains of Pediococcus was dependent on the strain or species of that genus of bacteria. All investigated strains, except for one strain of L. gasseri, incorporated ¹⁴C-adenosine, with S. thermophilus, L. lactis and Pediococcus generally displaying greater incorporation of ¹⁴C-adenosine than L. gasseri. Although most lactic acid bacteria such as S. thermophiles and L. lactis do not incorporate purine mononucleotides, some species such as L. gasseri directly incorporate purine mononucleotides. These findings indicate that the preferential incorporation of purine mononucleotides or nucleosides by lactic acid bacteria is dependent on the species or strain.

Purine and pyrimidine transporters of pathogenic protozoa - conduits for therapeutic agents

Purines and pyrimidines are essential nutrients for any cell. Most organisms are able to synthesize their own purines and pyrimidines, but this ability was lost in protozoans that adapted to parasitism, leading to a great diversification in transporter activities in these organisms, especially for the acquisition of amino acids and nucleosides from their hosts throughout their life cycles. Many of these transporters have been shown to have sufficiently different substrate affinities from mammalian transporters, making them good carriers for therapeutic agents. In this review, we summarize the knowledge obtained on purine and pyrimidine activities identified in protozoan parasites to date and discuss their importance for the survival of these parasites and as drug carriers, as well as the perspectives of developments in the field.

Functional characterization of monocarboxylate transporter 12 (SLC16A12/MCT12) as a facilitative creatine transporter

SLC16A12/MCT12 has been recently identified as a creatine transporter in a Xenopus oocyte expression system; however, the mechanism, by which MCT12 transports creatine, remains unclear. This study was performed to determine the functional and molecular characteristics of MCT12 in mammalian cells. The results showed that the uptake of [¹⁴C]creatine was not significantly increased in HEK293 cells transiently expressing MCT12 with or without CD147, a molecular chaperone, compared with mock cells. When [¹⁴C]creatine was accumulated in the cells with the aid of SLC6A8/CRT1, a concentrative creatine transporter, followed by assessing the remaining intracellular [¹⁴C]creatine after initiating efflux, coexpression of MCT12 resulted in a decrease in the intracellular [¹⁴C]creatine and remarkably enhanced the efflux of [¹⁴C]creatine from the cells in a time-dependent manner. This activity was not affected by extracellular pH. The creatine efflux activity involved dissipation by the mutations of conserved charged amino acids such as Arg37, Asp65 and Asp299 in the transmembrane domains, indicating direct involvement of MCT12 in the creatine efflux. These results suggest that MCT12 mediates facilitative diffusion of creatine, depending on the concentration gradient across the plasma membrane in mammalian cells. The finding may provide important clues to understanding the disposition kinetics of creatine and its derivatives.

Specific Residues in a Purine Transporter Are Critical for Dimerization, ER Exit, and Function

Transporters are transmembrane proteins that mediate the selective translocation of solutes across biological membranes. Recently, we have shown that specific interactions with plasma membrane phospholipids are essential for the formation and/or stability of functional dimers of the purine transporter UapA, a prototypic eukaryotic member of the ubiquitous nucleobase ascorbate transporter (NAT) family. Here, we provide strong evidence that distinct interactions of UapA with membrane lipids are essential for ab initio formation of functional dimers in the ER, or ER exit and further subcellular trafficking. Through genetic screens, we identify mutations that restore defects in dimer formation and/or trafficking. Suppressors of defective dimerization restore ab initio formation of UapA dimers in the ER. Most of these suppressors are located in the movable core domain, but also in the core-dimerization interface and in residues of the dimerization domain exposed to lipids. Molecular dynamics suggest that the majority of suppressors stabilize interhelical interactions in the core domain and thus assist the formation of functional UapA dimers. Among suppressors restoring dimerization, a specific mutation, T401P, was also isolated independently as a suppressor restoring trafficking, suggesting that stabilization of the core domain restores function by sustaining structural defects caused by the abolishment of essential interactions with specific lipids. Importantly, the introduction of mutations topologically equivalent to T401P into a rat homolog of UapA, namely rSNBT1, permitted the functional expression of a mammalian NAT in Aspergillus nidulans Thus, our results provide a potential route for the functional expression and manipulation of mammalian transporters in the model Aspergillus system.

Heterologous expression of the mammalian sodium-nucleobase transporter rSNBT1 in Leishmania tarentolae

Recombinant expression systems for mammalian membrane transport proteins are often limited by insufficient yields to support structural studies, inadequate post-translational processing and problems related with improper membrane targeting or cytotoxicity. Use of alternative expression systems and optimization of expression/purification protocols are constantly needed. In this work, we explore the applicability of the laboratory strain LEXSY of the ancient eukaryotic microorganism Leishmania tarentolae as a new expression system for mammalian nucleobase permeases of the NAT/NCS2 (Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2) family. We achieved the heterologous expression of the purine-pyrimidine permease rSNBT1 from Rattus norvegicus (tagged at C-terminus with a red fluorescent protein), as confirmed by confocal microscopy and biochemical analysis of the subcellular fractions enriched in membrane proteins. The cDNA of rSNBT1 has been subcloned in a pLEXSY-sat-mrfp1vector and used to generate transgenic L. tarentolae-rsnbt1-mrfp1 strains carrying the pLEXSY-sat-rsnbt1-mrfp1 plasmid either episomally or integrated in the chromosomal DNA. The chimeric transporter rSNBT1-mRFP1 is targeted to the ER and the plasma membrane of the L. tarentolae promastigotes. The transgenic strains are capable of transporting nucleobases that are substrates of rSNBT1 but also of the endogenous L. tarentolae nucleoside/nucleobase transporters. A dipyridamole-resistant Na⁺-dependent fraction of uptake is attributed to the exogenously expressed rSNBT1.