The nucleobase-ascorbate transporter (NAT) signature motif is a conserved 11-amino acid sequence of the ubiquitous NAT/ NCS2 family, essential for function and selectivity of both a bacterial (YgfO) and a fungal (UapA) purine-transporting homolog. We examined the role of NAT motif in more detail, using Cys-scanning and site-directed alkylation analysis of the YgfO xanthine permease of Escherichia coli. Analysis of single-Cys mutants in the sequence 315-339 for sensitivity to inactivation by 2-sulfonatoethyl methanethiosulfonate (MTSES ؊ ) and N-ethylmaleimide (NEM) showed a similar pattern: highly sensitive mutants clustering at the motif sequence (323-329) and a short ␣-helical face downstream (332, 333, 336). In the presence of substrate, N325C is protected from alkylation with either MTSES ؊ or NEM, whereas sensitivity of A323C to inactivation by NEM is enhanced, shifting IC 50 from 34 to 14 M. Alkylation or sensitivity of the other mutants is unaffected by substrate; the lack of an effect on Q324C is attributed to gross inability of this mutant for high affinity binding. Site-directed mutants G333R and S336N at the ␣-helical face downstream the motif display specific changes in ligand recognition relative to wild type; G333R allows binding of 7-methyl and 8-methylxanthine, whereas S336N disrupts affinity for 6-thioxanthine. Finally, all assayable motif-mutants are highly accessible to MTSES ؊ from the periplasmic side. The data suggest that the NAT motif region lines the solvent-and substrate-accessible inner cavity, Asn-325 is at the binding site, Ala-323 responds to binding with a specific conformational shift, and Gly-333 and Ser-336 form part of the purine permeation pathway.
Bacterial and fungal members of the ubiquitous nucleobaseascorbate transporter (NAT/NCS2) family use the NAT signature motif, a conserved 11-amino acid sequence between amphipathic helices TM9a and TM9b, to define function and selectivity of the purine binding site. To examine the role of flanking helices TM9a, TM9b, and TM8, we employed Cysscanning analysis of the xanthine-specific homolog YgfO from Escherichia coli. Using a functional mutant devoid of Cys residues (C-less), each amino acid residue in sequences 259 FLVVG-TIYLLSVLEAVGDITATAMVSRRPIQGEEYQSRLKGGVLAD-GLVSVIASAV 314 and 342 TIAVMLVILGLFP 354 including these TMs (underlined) was replaced individually with Cys, except the irreplaceable Glu-272 and Asp-304, which had been studied previously. Of 67 single Cys mutants, 55 accumulate xanthine to 35-140% of the steady state observed with C-less, five (I265C, D276C, I277C, G299C, L350C) accumulate to low levels (10 -20%) and seven (T278C, A279C, T280C, A281C, G305C, G351C, P354C) show negligible expression in the membrane. Extensive mutagenesis reveals that a carboxyl group is needed at Asp-276 for high activity and that D276E differs from wild type as it recognizes 8-methylxanthine (K i 79 M) but fails to recognize 2-thioxanthine, 3-methylxanthine or 6-thioxanthine; bulky replacements of Ala-279 or Thr-280 and replacements of Gly-305, Gly-351, or Pro-354 impair activity or expression. Single Cys mutants V261C, A273C, G275C, and S284C are sensitive to inactivation by N-ethylmaleimide and sensitivity of G275C (IC 50 15 M) is enhanced in the presence of substrate. The data suggest that residues crucial for the transport mechanism cluster in two conserved motifs, at the cytoplasmic end of TM8 (EXXG-DXXAT) and in TM9a (GXXXDG).
Cancer cells acquire unique secretome compositions that contribute to tumor development and metastasis. The aim of our study was to elucidate the biological processes involved in cervical cancer, by performing a proteomic analysis of the secretome from the following informative cervical cell lines: SiHa (HPV16+), HeLa (HPV18+), C33A (HPV−), and HCK1T (normal). Proteins were analyzed by 2D gel electrophoresis coupled to MALDI-TOF-MS. Enrichment of secreted proteins with characteristic profiles for each cell line was followed by the identification of differentially expressed proteins. Particularly, transforming growth factor-beta-induced protein ig-h3 (Beta ig-h3) and peroxiredoxin-2 (PRDX2) overexpression in the secretome of cancer cell lines was detected and confirmed by Western blot. Bioinformatics analysis identified the transcription factor NRF2 as a regulator of differentially expressed proteins in the cervical cancer secretome. NRF2 levels were measured by both Western blot and Multiple Reaction Monitoring (MRM) in the total cell extract of the four cell lines. NRF2 was upregulated in SiHa and C33A compared to HCK1T. In conclusion, the secreted proteins identified in cervical cancer cell lines indicate that aberrant NRF2-mediated oxidative stress response (OSR) is a prominent feature of cervical carcinogenesis.
Uncovering the molecular mechanisms of Mesial temporal lobe epilepsy (MTLE) is critical to identify therapeutic targets. In this study, we performed global protein expression analysis of a kainic acid (KA) MTLE mouse model at various time-points (1d, 3d, 30d post KA injection -dpi), representing specific stages of the syndrome.High resolution liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), in combination to label-free protein quantification, using three processing approaches for quantification, was applied. Following comparison of KA versus NaCl-injected mice, 22, 53 and 175 proteins were differentially (statistically significant) expressed at 1, 3 and 30dpi respectively, according to all 3 quantification approaches. Selected findings were confirmed by multiple reaction monitoring LC-MS/MS. As a positive control, the astrocyte marker GFAP was found to be upregulated (3dpi:1.9 fold; 30dpi:12.5 fold), also verified by IHC. The results collectively suggest that impairment in synaptic transmission occurs even right after initial status epilepticus (1dpi), with neurodegeneration becoming more extensive during epileptogenesis (3dpi) and sustained at the chronic phase (30dpi), where also extensive glial and astrocyte-mediated inflammation is evident. This molecular profile is in line with observed phenotypic changes in human MTLE, providing the basis for future studies on new molecular targets for the disease.
The composition of the salivary proteome is affected by pathological conditions. We analyzed by high resolution mass spectrometry approaches saliva samples collected from children and adolescents with type 1 diabetes and healthy controls. The list of more than 2000 high confidence protein identifications constitutes a comprehensive characterization of the salivary proteome. Patients with good glycemic regulation and healthy individuals have comparable proteomic profiles. In contrast, a significant number of differentially expressed proteins were identified in the saliva of patients with poor glycemic regulation compared to patients with good glycemic control and healthy children. These proteins are involved in biological processes relevant to diabetic pathology such as endothelial damage and inflammation. Moreover, a putative preventive therapeutic approach was identified based on bioinformatic analysis of the deregulated salivary proteins. Thus, thorough characterization of saliva proteins in diabetic pediatric patients established a connection between molecular changes and disease pathology. This proteomic and bioinformatic approach highlights the potential of salivary diagnostics in diabetes pathology and opens the way for preventive treatment of the disease.
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