Nanavati Dhaval M, Thirangoon Kamolwan, Noll Kenneth M
Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269-3125, USA.
Appl Environ Microbiol. 2006 Feb;72(2):1336-45. doi: 10.1128/AEM.72.2.1336-1345.2006.
The hyperthermophilic bacterium Thermotoga maritima has shared many genes with archaea through horizontal gene transfer. Several of these encode putative oligopeptide ATP binding cassette (ABC) transporters. We sought to test the hypothesis that these transporters actually transport sugars by measuring the substrate affinities of their encoded substrate-binding proteins (SBPs). This information will increase our understanding of the selective pressures that allowed this organism to retain these archaeal homologs. By measuring changes in intrinsic fluorescence of these SBPs in response to exposure to various sugars, we found that five of the eight proteins examined bind to sugars. We could not identify the ligands of the SBPs TM0460, TM1150, and TM1199. The ligands for the archaeal SBPs are TM0031 (BglE), the beta-glucosides cellobiose and laminaribiose; TM0071 (XloE), xylobiose and xylotriose; TM0300 (GloE), large glucose oligosaccharides represented by xyloglucans; TM1223 (ManE), beta-1,4-mannobiose; and TM1226 (ManD), beta-1,4-mannobiose, beta-1,4-mannotriose, beta-1,4-mannotetraose, beta-1,4-galactosyl mannobiose, and cellobiose. For comparison, seven bacterial putative sugar-binding proteins were examined and ligands for three (TM0595, TM0810, and TM1855) were not identified. The ligands for these bacterial SBPs are TM0114 (XylE), xylose; TM0418 (InoE), myo-inositol; TM0432 (AguE), alpha-1,4-digalactouronic acid; and TM0958 (RbsB), ribose. We found that T. maritima does not grow on several complex polypeptide mixtures as sole sources of carbon and nitrogen, so it is unlikely that these archaeal ABC transporters are used primarily for oligopeptide transport. Since these SBPs bind oligosaccharides with micromolar to nanomolar affinities, we propose that they are used primarily for oligosaccharide transport.
嗜热栖热菌通过水平基因转移与古菌共享了许多基因。其中一些基因编码假定的寡肽ATP结合盒（ABC）转运蛋白。我们试图通过测量其编码的底物结合蛋白（SBP）的底物亲和力来检验这些转运蛋白实际上运输糖类的假说。这些信息将增进我们对使该生物体保留这些古菌同源物的选择压力的理解。通过测量这些SBP在暴露于各种糖类时内在荧光的变化，我们发现所检测的8种蛋白质中有5种与糖类结合。我们无法鉴定SBP TM0460、TM1150和TM1199的配体。古菌SBP的配体有TM0031（BglE），其配体为β-葡萄糖苷纤维二糖和昆布二糖；TM0071（XloE），配体为木二糖和木三糖；TM0300（GloE），以木葡聚糖为代表的大葡萄糖寡糖；TM1223（ManE），β-1,4-甘露二糖；以及TM1226（ManD），β-1,4-甘露二糖、β-1,4-甘露三糖、β-1,4-甘露四糖、β-1,4-半乳糖基甘露二糖和纤维二糖。作为比较，检测了7种细菌假定的糖结合蛋白，其中3种（TM0595、TM0810和TM1855）的配体未被鉴定。这些细菌SBP的配体有TM0114（XylE），配体为木糖；TM0418（InoE），配体为肌醇；TM0432（AguE），α-1,4-二半乳糖醛酸；以及TM0958（RbsB），配体为核糖。我们发现嗜热栖热菌不能在几种复杂的多肽混合物作为唯一碳源和氮源的培养基上生长，因此这些古菌ABC转运蛋白不太可能主要用于寡肽运输。由于这些SBP以微摩尔到纳摩尔的亲和力结合寡糖，我们推测它们主要用于寡糖运输。
