Pulmonary Hypertension Induces Serotonin Hyperreactivity and Metabolic Reprogramming in Coronary Arteries via NOX1/4-TRPM2 Signaling Pathway.

BACKGROUND: Clinical evidence revealed abnormal prevalence of coronary artery (CA) disease in patients with pulmonary hypertension (PH). The mechanistic connection between PH and CA disease is unclear. Serotonin (5-hydroxytryptamine), reactive oxygen species, and Ca2+ signaling have been implicated in both PH and CA disease. Our recent study indicates that NOXs (NADPH [nicotinamide adenine dinucleotide phosphate] oxidases) and TRPM2 (transient receptor potential cation channel subfamily M member 2) are key components of their interplay. We hypothesize that activation of the NOX-TRPM2 pathway facilitates the remodeling of CA in PH. METHODS: Left and right CAs from chronic hypoxia and monocrotaline-induced PH rats were collected to study vascular reactivity, gene expression, metabolism, and mitochondrial function. Inhibitors or specific siRNA were used to examine the pathological functions of NOX1/4-TRPM2 in CA smooth muscle cells. RESULTS: Significant CA remodeling and 5-hydroxytryptamine hyperreactivity in the right CA were observed in PH rats. NOX1/4-mediated reactive oxygen species production coupled with TRPM2-mediated Ca2+ influx contributed to 5-hydroxytryptamine hyperresponsiveness. CA smooth muscle cells from chronic hypoxia-PH rats exhibited increased proliferation, migration, apoptosis, and metabolic reprogramming in an NOX1/4-TRPM2-dependent manner. Furthermore, the NOX1/4-TRPM2 pathway participated in mitochondrial dysfunction, involving mitochondrial DNA damage, reactive oxygen species production, elevated mitochondrial membrane potential, mitochondrial Ca2+ accumulation, and mitochondrial fission. In vivo knockdown of NOX1/4 alleviated PH and suppressed CA remodeling in chronic hypoxia rats. CONCLUSIONS: PH triggers an increase in 5-hydroxytryptamine reactivity in the right CA and provokes metabolic reprogramming and mitochondrial disruption in CA smooth muscle cells via NOX1/4-TRPM2 activation. This signaling pathway may play an important role in CA remodeling and CA disease in PH.

Reduced CircSMOC1 Level Promotes Metabolic Reprogramming via PTBP1 (Polypyrimidine Tract-Binding Protein) and miR-329-3p in Pulmonary Arterial Hypertension Rats.

BACKGROUND: Pulmonary arterial hypertension maintains rapid cell proliferation and vascular remodeling through metabolic reprogramming. Recent studies suggested that circRNAs play important role in pulmonary vascular remodeling and pulmonary arterial smooth muscle cells proliferation. However, the relationship between circRNA, cell proliferation, and metabolic reprogramming in pulmonary arterial hypertension has not been investigated. METHODS: RNA-seq and qRT-PCR reveal the differential expression profile of circRNA in pulmonary arteries of pulmonary arterial hypertension rat models. Transfection was used to examine the effects of circSMOC1 on pulmonary artery smooth muscle cells, and the roles of circSMOC1 in vivo were investigated by adenoassociated virus. Mass spectrometry, RNA pull-down, RNA immunoprecipitation, and dual-luciferase reporter assay were performed to investigate the signaling pathway of circSMOC1 regulating the metabolic reprogramming. RESULTS: CircSMOC1 was significantly downregulated in pulmonary arteries of pulmonary arterial hypertension rats. CircSMOC1 knockdown promoted proliferation and migration and enhanced aerobic glycolysis of pulmonary artery smooth muscle cells. CircSMOC1 overexpression in vivo alleviates pulmonary vascular remodeling, right ventricular pressure, and right heart hypertrophy. In the nucleus, circSMOC1 directly binds to PTBP1 (polypyrimidine tract-binding protein), competitively inhibits the specific splicing of PKM (pyruvate kinase M) premRNA, resulting in the upregulation of PKM2 (pyruvate kinase M2), the key enzyme of aerobic glycolysis, to enhance glycolysis. In the cytoplasm, circSMOC1 acted as a miR-329-3p sponge, and its reduction in pulmonary arterial hypertension suppressed PDHB (pyruvate dehydrogenase E1 subunit beta) expression, leading to the impairment of mitochondrial oxidative phosphorylation. CONCLUSIONS: circSMOC1 is crucially involved in the metabolic reprogramming of pulmonary artery smooth muscle cells through PTBP1 and miR-329-3p to regulate pulmonary vascular remodeling in pulmonary arterial hypertension.

Transcriptome and Metabolome Analyses Provide Insights Into the Composition and Biosynthesis of Grassy Aroma Volatiles in White-Fleshed Pitaya.

Aroma is one of the major inherent quality characteristics in fruits. Understanding the composition of aroma volatiles and their biosynthesis mechanism is crucial to improving fruit quality. However, the biosynthesis mechanism of aroma volatiles has not been characterized yet in white-fleshed pitaya (Hylocereus undatus). This study was performed to investigate aroma volatiles and related gene expression patterns in the pulp of "mild grassy" and "strong grassy" aroma cultivars. Analysis of volatile composition and concentration showed that aldehydes, alcohols, esters, and alkenes were predominant in both cultivars. However, comparative analysis revealed a significant difference in the concentration of several metabolites, particularly hexanal and 1-hexanol. The results of the comparative transcriptome identified a large number of aroma-related differentially expressed genes. The majority of these genes were enriched in fatty acid and isoleucine degradation pathways. According to integrative analyses, changes in the expression of lipoxygenase pathway genes, specifically FAD, LOXs, HPLs, and ADHs, probably lead to the difference in strength of "grassy" aroma between both cultivars. The qRT-PCR of 18 aroma-related genes was performed to validate the transcriptome analysis. Our results identified key genes and pathways connected with the biosynthesis of aroma volatiles in white-fleshed pitaya. These results will be useful to dissect the genetic mechanism of fruit aroma in white-fleshed pitaya.

A novel KCNT1 mutation in a Chinese family with severe autosomal-dominant nocturnal frontal lobe epilepsy.

We describe a Chinese family with severe autosomal-dominant nocturnal frontal lobe epilepsy (ADNFLE) and psychiatric problems in whom whole-exome family trio sequencing identified a heterozygous mutation in the potassium channel subfamily T, member 1 (KCNT1), a sodium-gated potassium channel gene, which was a novel missense mutation c.2153A>T (p. Asp718Val). The typical characteristics of the three patients in the family were refractory epilepsy, acquired cognitive impairment, and psychiatric problems, which include hallucinations and suicidal thoughts and behaviors. The age at onset was found to be earlier in son and daughter of the proband than that of the proband, as proven by the proband's history of an epileptic seizure at the age of 16 years and her son's and daughter's history of seizures at the age of 8 years. Magnetic resonance imaging findings were negative for any abnormalities. Because of psychiatric symptoms, these three patients were administered risperidone at different times during their illness. The protestor's son had tried fenofibrate treatment, but clinical remission was unclear. In summary, our findings broadened the mutation database in relation to KCNT1 and implicated the sodium-gated potassium channel complex in ADNFLE, more broadly, in the pathogenesis of focal epilepsies.

The emerging role of the MiR-1272-ADAM9-CDCP1 signaling pathway in the progression of glioma.

Glioma is a primary, malignant, and aggressive brain tumor in adults. To develop new therapeutic strategies for glioma, we must determine its underlying mechanisms. In the present study, we aimed to investigate the potential role of miR-1272-ADAM9-CDCP1 signaling in the progression of glioma. We found that ectopic expression of miR-1272 produced significant inhibitory effects on cell proliferation and migration and was associated with cell cycle G0/G1 arrest in A172 and SHG44 glioma cells. Using the luciferase reporter assay, we identified ADAM9 as a target of miR-1272. The expression of ADAM9 was markedly decreased or increased after overexpression or inhibition, respectively, of miR-1272 in glioma cells. Moreover, overexpression of ADAM9 reversed the inhibitory effects of miR-1272 on glioma cell progression. Furthermore, CDCP1 served as a potential downstream molecule of miR-1272/ADAM9 signaling in glioma and promoted the proliferation and migration of glioma. Results derived from clinical samples and online databases confirmed correlations between the expression of ADAM9 and CDCP1 and both the severity and prognosis of glioma. In conclusion, these results suggest that miR-1272 and CDCP1 may act as novel regulators in glioma. The miR-1272/ADAM9/CDCP1 pathway may serve as a potential candidate pathway for the prevention of glioma.

ErbB4 Receptors in the Medial Habenula Regulate Contextual Fear Memory.

The Medial Habenular (MHb) and the Lateral Habenular nuclei are 2 main parts of the habenular complex (Hb). Recent studies showed that MHb plays an important role in memory, and in the expression of ErbB4. However, the expression of MHb ErbB4 receptor and its role in fear memory is not well understood. In this study, western blotting and quantitative real-time polymerase chain reaction were used to assess the protein and mRNA levels of ErbB4 in the process of contextual fear conditioning. A pharmacological approach was used to block and stimulate the ErbB4 receptor. Contextual fear conditioning tests induced a significant increase on the expression of ErbB4 at various times in the Hb and the MHb. Moreover, the blockade and stimulation of MHb ErbB4 receptors did not affect the fear formation but impaired and improved the contextual-dependent fear expression. Furthermore, in vitro electrophysiological recordings showed that the blockade of the MHb ErbB4 receptor reduced the presynaptic gamma-amino butyric acid release. ErbB4 is a susceptible gene for schizophrenia and the above findings may provide new insights into the mechanisms of fear-related responses.

MicroRNA-6807-3p promotes the tumorigenesis of glioma by targeting downstream DACH1.

Accumulated evidence reveals that microRNAs play vital roles in various tumors, including gliomas. MiRNAs have been shown to participate in multiple cellular functions, including cell proliferation, migration and apoptosis. Here, we investigate the potential role of a novel miRNA, miR-6807-3p, in glioma. A quantitative reverse transcription-polymerase chain reaction (RT-qPCR) and western blot were applied to detect the expression of miR-6807-3p and its target molecule in glioma specimens and cultured cells. The direct targets of miR-6807-3p were predicted by bioinformatics software and were further verified by a luciferase reporter assay. The effects of miR-6807-3p on glioma cell proliferation, migration, cell apoptosis and the cell cycle of glioma cells were analyzed by the Cell-Counting Kit-8 (CCK-8) assay, a cell migration assay and flow cytometry assays. MiR-6807-3p was found to promote tumor growth and migration and inhibits apoptosis and cell cycle arrest in vitro, thus playing a tumor oncogenic role in the progression of glioma. Expression levels of miR-6807-3p were greatly upregulated in glioma specimens, and dachshund homolog 1 (DACH1) was ascertained as a direct target of miR-6807-3p, modulated by the expression of miR-6807-3p in glioma cells. Aberrant expression of DACH1 was associated with the clinical survival of glioma patients. Furthermore, overexpression of DACH1 rescued the promotive effects of miR-6807-3p in glioma. Based on these findings, a novel miR-6807-3p may act as a glioma enhancer by targeting DACH1.

MicroRNA-26b-3p/ANTXR1 signaling modulates proliferation, migration, and apoptosis of glioma.

Glioma is a common malignant human brain tumor. The progression of glioma is associated with dysregulation of various microRNAs. Previous studies have demonstrated that microRNA-26b-3p (miR-26b-3p) is correlated with the pathogenesis of various tumors, but the functional role of miR-26b-3p and its underlying mechanisms in glioma are not clear. Here, we found that overexpression of miR-26b-3p repressed cell migration and proliferation and promoted apoptosis. In contrast, the opposite effects were observed when miR-26b-3p was inhibited in glioma cells. Anthrax toxin receptor 1 (ANTXR1) was confirmed to be a downstream molecule of miR-26b-3p. Reintroduction of ANTXR1 with an ORF region rescued the suppressive effects of miR-26b-3p on proliferation and migration, and inhibited the apoptosis of glioma cells. Moreover, the downstream target of miR-26b-3p, ANTXR1, was increased in glioma tissues and was inversely correlated with miR-26b-3p. MiR-26b-3p and ANTXR1 were correlated with the severity of glioma. Taken together, these results demonstrate that miR-26b-3p is a critical modulator of glioma via its downstream molecule, ANTXR1. Further, the miR-26b-3p/ANTXR1 axis may serve as a treatment or diagnostic target in glioma.

MicroRNA-671-3p promotes proliferation and migration of glioma cells via targeting CKAP4.

BACKGROUND AND OBJECTIVE: Glioma is one of the most aggressive and malignant cancers originating from the human brain. Increasing evidence suggests that aberrant expression of microRNAs (miRNAs) frequently occurs in glioma and miRNAs are critical regulators of glioma. miR-671 has recently been revealed to be a novel miRNA that plays a vital role in human glioblastoma multiforme. However, the functional role and underlying mechanisms of miR-671-3p require further analysis. MATERIALS AND METHODS: Western blot and fluorescence quantitative PCR were used to assess the expression of cytoskeleton-associated protein 4 (CKAP4) and miR-671-3p, respectively. A Cell Counting Kit-8 (CCK-8) assay and a Boyden chamber assay were used to detect the proliferative and migratory abilities of glioma cells. A luciferase assay was used to determine the target gene of miR-671-3p. Apoptosis was analyzed by flow cytometry. RESULTS: Our results revealed that overexpression of miR-671-3p promoted cell proliferation and migration in vitro. Meanwhile, forced expression of miR-671-3p reduced apoptosis. In contrast, inhibition of miR-671-3p had the opposite effects. We also identified CKAP4 to be a direct target of miR-671-3p. The expression levels of CKAP4 were decreased in clinical samples and inversely correlated with miR-671-3p expression levels. Ectopic expression of CKAP4 reversed the promotive activity of miR-671-3p in the proliferation and migration and enhanced apoptosis. CONCLUSION: Our study demonstrates that miR-671-3p is a predominant positive regulator of glioma progression, thus providing new insights into the molecular mechanisms of glioma development. The findings suggest that the miR-6713p/CKAP4 axis may serve as a potential therapeutic target or biomarker in glioma.

Dorsomedial prefrontal cortex 5-HT6 receptors regulate anxiety-like behavior.

The dorsomedial prefrontal cortex (dmPFC) plays a very important role in decision-related and anxiety-related information processing. It has enriched 5-HT6 receptors; however, the precise role of dmPFC 5-HT6 receptors in anxiety remains to be fully investigated. In this study, we injected dmPFC with the 5-HT6 receptor agonist EMD 386088 and antagonist SB 271046 using stereotactic technology. 5-HT6 receptor activation in mice increased time spent in the center area on the open-field test, increased exploration of the open arms on the elevated plus maze test, and increased ratio on the social interaction test. 5-HT6 receptor inactivation induced the opposite effects. In brain slices, EMD 386088 decreased both spontaneous inhibitory postsynaptic currents (sIPSC) and spontaneous excitatory postsynaptic currents (sEPSC), while SB 271046 only increased sEPSC. These effects of EMD 386088 and SB 271046 could be reversed by the GABAA receptor antagonist bicuculline (BMI) and positive allosteric modulator clonazepam (CLZ), respectively. Our results suggest that neurotransmission in the dmPFC by 5-HT6 receptor activation and inhibition may play an important role in anxiety-like behavior, and may provide new insight into the pathological mechanism and potential target of anxiety disorders.

Dimeric structure of the uracil:proton symporter UraA provides mechanistic insights into the SLC4/23/26 transporters.

The Escherichia coli uracil:proton symporter UraA is a prototypical member of the nucleobase/ascorbate transporter (NAT) or nucleobase/cation symporter 2 (NCS2) family, which corresponds to the human solute carrier family SLC23. UraA consists of 14 transmembrane segments (TMs) that are organized into two distinct domains, the core domain and the gate domain, a structural fold that is also shared by the SLC4 and SLC26 transporters. Here we present the crystal structure of UraA bound to uracil in an occluded state at 2.5 Å resolution. Structural comparison with the previously reported inward-open UraA reveals pronounced relative motions between the core domain and the gate domain as well as intra-domain rearrangement of the gate domain. The occluded UraA forms a dimer in the structure wherein the gate domains are sandwiched by two core domains. In vitro and in vivo biochemical characterizations show that UraA is at equilibrium between dimer and monomer in all tested detergent micelles, while dimer formation is necessary for the transport activity. Structural comparison between the dimeric UraA and the recently reported inward-facing dimeric UapA provides important insight into the transport mechanism of SLC23 transporters.

MicroRNA-132 targets PEA-15 and suppresses the progression of astrocytoma in vitro.

Gliomas are highly malignant tumors, the most common of which are astrocytomas. A growing number of studies suggest that dysregulation of miRNAs is a frequent event contributing to the pathogenesis of gliomas. In this study, we found that over-expression of miR-132 inhibited cell proliferation and migration and triggered apoptosis, while knockdown of miR-132 showed opposite effects. PEA-15 was identified as a direct target of miR-132. Reintroduction of PEA-15 without 3'UTR region reversed the inhibitory effects of miR-132 on cell proliferation, migration, and apoptosis. MiR-132 was inversely correlated with the PEA-15 expression. CREB (cAMP response element binding protein) and KLF (Krüppel-like factor 8) were conformed as transcription factors of miR-132, which bidirectionally regulate the expression of miR-132. Our study suggests that miR-132 is an important tumor suppressor of astrocytoma progression by targeting PEA-15, while CREB and KLF can modulate the expression of miR-132, thus providing new insight into the molecular mechanisms underlying astrocytoma progression in vitro.

Emerging roles of circRNA_001569 targeting miR-145 in the proliferation and invasion of colorectal cancer.

Circular RNAs (circRNAs), a large class of RNAs, have recently shown huge capabilities as gene regulators in mammals. Some of them bind with microRNAs (miRNAs) and act as natural miRNA sponges to inhibit related miRNAs' activities. Here we showed that hsa_circ_001569 acted as a positive regulator in cell proliferation and invasion of colorectal cancer (CRC). Moreover, hsa_circ_001569 was identified as a sponge of miR-145 and up-regulated miR-145 functional targets E2F5, BAG4 and FMNL2. In CRC tissues, circ_001569 negatively correlated with miR-145, and miR-145 correlated negatively with E2F5, BAG4 and FMNL2 expressions. Our study reveals a novel regulatory mechanism of circ_001569 in cell proliferation and invasion in CRC, provides a comprehensive landscape of circ_001569 that will facilitate further biomarker discoveries in the progression of CRC.

MicroRNA-34a targets FMNL2 and E2F5 and suppresses the progression of colorectal cancer.

Colorectal cancer (CRC) is one of the most common malignancies. Increasing evidences indicate that dysregulation of miRNAs is a frequent event in CRC and contributes to the pathogenesis of CRC. In this study, we found that over-expression of miR-34a inhibited cell proliferation and invasion, induced a cell cycle arrest and triggered apoptosis, while knockdown of miR-34a showed the opposite effects. Moreover, ectopic miR-34a suppressed tumor growth and metastasis of CRC cells in vivo. FMNL2 and E2F5 were identified as direct targets of miR-34a. Reintroduction of FMNL2 or E2F5 without 3'UTR region reversed the inhibitory effects of miR-34a on cell proliferation and invasion. MiR-34a was down-regulated in CRC cells and inversely correlated with FMNL2 and E2F5 expressions. Our study suggests that miR-34a is an important tumor suppressor of CRC progression by targeting FMNL2 and E2F5, thus providing new insight into the molecular mechanisms underlying CRC progression and establishing a strong potential for the application of miR-34a as a novel therapeutic marker against CRC.

Hypermethylation of FOXD3 suppresses cell proliferation, invasion and metastasis in hepatocellular carcinoma.

As a transcriptional repressor, forkhead box D3 (FOXD3) plays an important role in tumorigenesis and progression of several tumors. However, the function and methylation status of FOXD3 remain unknown in the progression of hepatocellular carcinoma (HCC). In this study, we found that FOXD3 was decreased in HCC tissues and correlated with differentiation, AFP and poor survival of HCC patients (p<0.05). Down-regulation of FOXD3 in HCC tissues was mainly due to promoter hypermethylation. In vitro and in vivo functional results showed that ectopic FOXD3 inhibited the proliferation, migration, epithelial-mesenchymal transition (EMT) and invasion in HepG2 and SMMC-7721 cells, and FOXD3 depletion in HepG2 and QGY-7701 cells showed the adverse effects (p<0.05). Moreover, FOXD3 was sufficient to suppress tumor growth and pulmonary metastatic potential in mice. Our findings suggest that down-regulation of FOXD3, due to promoter hypermethylation plays an important role in the progression of HCC and may be a promising prognostic biomarker for HCC patients.

The SOX17/miR-371-5p/SOX2 axis inhibits EMT, stem cell properties and metastasis in colorectal cancer.

Cancer stem cells (CSCs) and EMT-type cells, which share molecular characteristics with CSCs, have been believed to play critical roles in tumor metastasis. Although much progress has been garnered in elucidating the molecular pathways that trigger EMT, stemness and metastasis, a number of key mechanistic gaps remain elusive. In the study, miR-371-5p was obviously down-regulated in primary CRC tissues compared with matched adjacent normal mucosa and correlated significantly with differentiation, tumor size, lymphatic and liver metastases. MiR-371-5p could attenuate proliferation, invasion in vitro and metastasis in vivo in CRC cells. It also suppressed EMT by regulating Wnt/ß-catenin signaling and strongly decreased the CRC stemness phenotypes. Moreover, demethylation of SOX17 induced miR-371-5p expression and consequently suppressed its direct target SOX2 in CRC cells. MiR-371-5p was necessary for SOX17 mediated cancer-related traits and SOX2 was a functional target of miR-371-5p. A positive relationship between SOX17 and miR-371-5p expression and a negative one between miR-371-5p and SOX2 expression were observed in CRC cell lines and tissues. In conclusion, we identified miR-371-5p as an important "oncosuppressor" in CRC progression and elucidated a novel mechanism of the SOX17/miR-371-5p/SOX2 axis in the regulation of EMT, stemness and metastasis, which may be a potential therapeutic target.

Crystal structures of the Lsm complex bound to the 3' end sequence of U6 small nuclear RNA.

Splicing of precursor messenger RNA (pre-mRNA) in eukaryotic cells is carried out by the spliceosome, which consists of five small nuclear ribonucleoproteins (snRNPs) and a number of accessory factors and enzymes. Each snRNP contains a ring-shaped subcomplex of seven proteins and a specific RNA molecule. The U6 snRNP contains a unique heptameric Lsm protein complex, which specifically recognizes the U6 small nuclear RNA at its 3' end. Here we report the crystal structures of the heptameric Lsm complex, both by itself and in complex with a 3' fragment of U6 snRNA, at 2.8 Å resolution. Each of the seven Lsm proteins interacts with two neighbouring Lsm components to form a doughnut-shaped assembly, with the order Lsm3-2-8-4-7-5-6. The four uridine nucleotides at the 3' end of U6 snRNA are modularly recognized by Lsm3, Lsm2, Lsm8 and Lsm4, with the uracil base specificity conferred by a highly conserved asparagine residue. The uracil base at the extreme 3' end is sandwiched by His 36 and Arg 69 from Lsm3, through π-π and cation-π interactions, respectively. The distinctive end-recognition of U6 snRNA by the Lsm complex contrasts with RNA binding by the Sm complex in the other snRNPs. The structural features and associated biochemical analyses deepen mechanistic understanding of the U6 snRNP function in pre-mRNA splicing.

Structure of the mRNA splicing complex component Cwc2: insights into RNA recognition.

The Prp19-associated complex [NTC (nineteen complex)] plays a crucial role in intron removal during premature mRNA splicing in eukaryotes. Only one component of the NTC, Cwc2, is capable of binding RNA. In the present study we report the 1.9 Å (1 Å=0.1 nm) X-ray structure of the Cwc2 core domain, which is both necessary and sufficient for RNA binding. The Cwc2 core domain contains two sub-domains, a CCCH-type ZnF (zinc finger) and a RRM (RNA recognition motif). Unexpectedly, the ZnF domain and the RRM form a single folding unit, glued together by extensive hydrophobic interactions and hydrogen bonds. Structure-guided mutational analysis revealed that the intervening loop [known as the RB loop (RNA-binding loop)] between ZnF and RRM plays an essential role in RNA binding. In addition, a number of highly conserved positively charged residues on the ß-strands of RRM make an important contribution to RNA binding. Intriguingly, these residues and a portion of the RB loop constitute an extended basic surface strip that encircles Cwc2 halfway. The present study serves as a framework for understanding the regulatory function of the NTC in RNA splicing.

Structure and mechanism of the uracil transporter UraA.

The nucleobase/ascorbate transporter (NAT) proteins, also known as nucleobase/cation symporter 2 (NCS2) proteins, are responsible for the uptake of nucleobases in all kingdoms of life and for the transport of vitamin C in mammals. Despite functional characterization of the NAT family members in bacteria, fungi and mammals, detailed structural information remains unavailable. Here we report the crystal structure of a representative NAT protein, the Escherichia coli uracil/H(+) symporter UraA, in complex with uracil at a resolution of 2.8 Å. UraA has a novel structural fold, with 14 transmembrane segments (TMs) divided into two inverted repeats. A pair of antiparallel ß-strands is located between TM3 and TM10 and has an important role in structural organization and substrate recognition. The structure is spatially arranged into a core domain and a gate domain. Uracil, located at the interface between the two domains, is coordinated mainly by residues from the core domain. Structural analysis suggests that alternating access of the substrate may be achieved through conformational changes of the gate domain.