Fluolab【JACS】捕捉细菌“特洛伊木马”：新型光亲和探针揭秘抗生素分子的入胞路径
文章概要
文章标题：Covalent Chemical Tagging of Transmembrane Transport Proteins Illuminates the Internalization Pathways of Xenosiderophores
通讯作者：Eszter Boros
文章概要
铁元素是细菌生存和致病的核心营养素，但由于环境中的铁极难溶解，细菌进化出了释放螯铁蛋白（Siderophores） 捕获铁离子的复杂机制。科学家们利用这一特性，开发出将抗生素偶联在螯铁蛋白上的“特洛伊木马”策略，以诱骗细菌主动将药物主动吸入体内。然而，长期以来，由于非共价基质相互作用非常短暂且微弱，科学家极难精确识别究竟是哪些跨膜转运蛋白负责了这些合成偶联物的内吞。本篇研究成功开发出了首款基于去铁胺（DFO）的光亲和标记探针。该探针通过精密设计的共价光交叉联结策略，在活细胞中直接捕获并识别了外源螯铁蛋白偶联物的跨膜输送链条，不仅印证了已知的转运路径，更首次发现了全新的多药转运通道。这一蓝图为破译细菌的金属离子摄取机制及开发新型抗生素提供了强有力的技术支撑。
Figure 1. (A) Chemical structure of conjugate M-D1, with the desferrioxamine moiety highlighted in magenta and the ciprofloxacine moiety highlighted in green. (B) Cocrystal structure overlays of Fe-DFO (magenta) bound to FoxA (copper) and Fe-D1 (green) bound to FoxA (turquoise) of the holo protein and substrate binding region (right) demonstrate that Fe-DFO conjugates replicate the binding mode of the corresponding Fe-bound siderophore with high accuracy. (C) Analysis of polar side chain amino acids involved in binding and interaction with Fe-D1 (tan) indicates a prevalence of tyrosine and aspartic acid within the binding pocket, in addition to other polar amino acids (shown in blue).
引言
在抗生素耐药性日益严重的今天，如何让药物突破革兰氏阴性菌坚固的外膜屏障是一个巨大挑战。螯铁蛋白介导的“特洛伊木马”策略正是利用了细菌对铁的绝对依赖，能够强效提升抗生素的吸入效率。过去，识别这些跨膜转运蛋白主要依赖于基因敲除等间接手段，这往往只能捕捉到高亲和力的内源性转运系统，而对亲和力较低、具有多重 promiscuity（广谱通用性）的外源螯铁蛋白或抗生素偶联物转运系统束手无策。传统的亲和富集法也因为结合力的转瞬即逝而屡屡失败。为了填补这一跨膜转运蛋白识别领域的空白，研究团队受到近年胞质小分子相互作用组学研究的启发，决定开发一种能够在紫外光触发下与转运蛋白靶点产生永久共价结合的光亲和探针，从而直接把正在工作的跨膜转运桶状蛋白抓个正现。
Figure 2. M-DFO-azir-01 and 02 reactivity with 5 natural amino acids. (A) Structure of M-DFO-azir-01 and 02. (B) Diazirine reactivity of M-DFO-azir-01 with N- acetyl, O-Me protected amino acids (100 equiv) in 1:4 water acetonitrile. Yield was calculated by LC-MS following UV-absorbance at 425 nm for the Fe complex and 280 nm for the Ga complexes. (C) Diazirine reactivity of M-DFO-azir-02 with N- acetyl, O-Me protected amino acids (100 equiv) in 1:4 water acetonitrile. Yield was calculated by LC-MS following UV-absorbance at 425 nm for Fe complex and 280 nm for the Ga complexes. (D) Evaluation of the ability of M-DFO-azir-01 and 02 to label BSA and FoxA in vitro. Probes were incubated with purified protein 30 min followed by a 15 min irradiation under UV light (365 nm). Analysis was conducted by visualizing in-gel fluorescence on SDS-PAGE. Relative fluorescence intensity was observed in comparison to total protein staining. The full gel and corresponding total protein stain are provided in Figure S56.
主要实验及结论
研究团队首先对环丙沙星-去铁胺偶联物（Fe-D1）与铜绿假单胞菌外膜转运蛋白 FoxA 的共晶结构进行了深入的理性设计分析。他们注意到，该转运通道的口袋内部密集分布着大量的酪氨酸（Tyr）和天冬氨酸（Asp）残基。基于此，他们合成了第一代带有脂肪族重氮杂环（Diazirine）和炔基功能基团的探针，并通过改变连接链长度来测试其标记效率。质谱和计算模拟结果表明，连接链的长度对于保证探针在结合口袋内的特异性共价标记至关重要，过长或过短都会导致标记位点偏离。然而，第一代探针在后续的铜催化叠氮-炔基环加成（CuAAC）点击化学荧光染色中表现不佳，研究人员推测这是由于转运蛋白桶状结构限制了炔基的暴露，同时铁离子产生了光诱导电子转移的荧光淬灭效应。
Figure 3. Photo-cross-linking of protein isolates and sites of conjugation. (A) An annotated FoxA protein fragment shows conjugation with Fe-DFO-azir-02. In the FoxA cocrystal structure with Fe-DFO-azir-01, amino acids labeled by Fe-DFO-azir-02 are highlighted in green, and the protein fragment is shown in blue. (B) An annotated FoxA protein fragment shows conjugation with Ga-DFO-azir-02. In the FoxA cocrystal structure with Fe-DFO, amino acids labeled by Ga-DFO-azir-02 are shown in orange, and the protein fragment is shown in pink. (C) Computational results show that M-DFO-azir-02 can be recognized by FoxA and cross-link Y445, Y799, and E646 (highlighted in green). The distances (in Ångströms) between the carbon atom linked to the diazirine and the side chains of amino acids labeled by Fe-DFO-azir-02 and Ga-DFO-azir-02 are shown in the FoxA structure.
Figure 4. (A) Structure of M-DFO-azir-04, -05, and -06. (B) Diazirine reactivity of M-DFO-azir-04, -05, and -06 with N-acetyl, O-Me protected amino acids (100 equiv) in 1:4 water acetonitrile. Yield was calculated by LC-MS following UV-absorbance at 425 nm for Fe complexes and 280 nm for Ga complexes.
为了克服这一瓶颈，团队乘胜追击开发了直接内置荧光基团或生物素（Biotin）标签的第二代探针。在活体的大肠杆菌过表达模型中，利用生物素化的探针配合链霉亲和素富集技术，成功实现了活细胞共价标记。在随后的铜绿假单胞菌野生型菌株 PAO1 的活细胞实验中，探针成功捕获了已知的 ferrioxamine E 转运体 FoxA，以及负责次级转运路径的焦维啶受体 FpvB，完美重现了已知的摄取机制。令人兴奋的是，实验还出人意料地显著富集了由基因 PA1271 表达的 TonB 依赖型转运蛋白。该蛋白此前基于序列同源性一直被预测为钴胺素（维生素 B12）转运体，而本实验首次证实了它在介导去铁胺偶联物跨膜吸入中的全新跨界角色。
Figure 5. Validation of M-DFO-azir-05 and -06 in_E. coli_Lemo21 (DE3) cells overexpressing FoxA. (A) Workflow: combination of in-gel fluorescence and antibiotin Western blot to visualize proteins and conduct competition assays. Further analysis of in-gel digestion and streptavidin enrichment confirmed protein identification. (B) In-gel fluorescence of probes DFO-azir-05 and Ga-DFO-azir-05. (C) Competition assay on Ga-DFO-azir-05 using increasing concentrations of Fe-DFO. The decrease in band intensity of specific Fe-DFO-binding proteins, correlated with the increase of Fe-DFO, is observed by in-gel fluorescence. (D) Competition assay on Fe-DFO-azir-06 using increasing concentrations of Fe-DFO. The decrease in band intensity of specific Fe-DFO-binding proteins, correlated with the increase of Fe-DFO, is observed by Western blot using antibiotin antibodies. The Ctrl band corresponds to untreated (without probe or Fe-DFO) cells. (E) Competition assay on Ga-DFO-azir-06 using increasing concentrations of Fe-DFO. The decrease in band intensity of specific Fe-DFO-binding proteins, correlated with the increase of Fe-DFO, is observed by Western blot using antibiotin antibodies. The Ctrl band corresponds to untreated (without probe or Fe-DFO) cells. Complete gel images, along with total protein staining used to determine band intensity, can be found in Figures S58–S59.
Figure 6. Ga and Fe-DFO-azir-06 enrichment in_P. aeruginosa_PAO1 cells. (A) Enrichment protocol: incubation of M-DFO-azir-06 probes or control azir-biotin, photoconjugation to interacting proteins, followed by biotin–streptavidin enrichment and MS/MS analysis. (B) Structure of M-DFO-azir-06 probes and control azir-biotin. (C) Quantification by spectral counts of transporters identified by MS/MS in enrichment assays using M-DFO-azir-06 and azir-biotin. (D) Heatmap representation of spectral counts of transporters identified by MS/MS in enrichment assays using M-DFO-azir-06 and azir-biotin. The results shown are representative of three independent experiments (Table S2).
随后，研究团队将该体系拓展至大肠杆菌 K-12 菌株。此前大肠杆菌中并没有明确的去铁胺专属受体，但其对螯铁蛋白抗生素非常敏感。通过该光亲和探针的高通量质谱定量分析，团队一举确证了 FhuA 和 FhuE 是大肠杆菌吸入该类偶联物的两条核心双位点通路。值得一提的是，实验结果表明使用镓（Ga）替代铁（Fe）作为螯合金属中心，能够维持细菌的缺铁应激状态，从而大幅提升靶点蛋白的表达丰度和标记的忠实度。同时，大肠杆菌中的钴胺素转运体 BtuB 也被强烈标记。两类不同革兰氏阴性菌中同源转运体的共同发现，强有力地证明了维生素 B12 转运通道在外源金属螯合物摄取中普遍存在着此前未被察觉的广谱底物包容性。
Figure 7. Schematic representation of DFO uptake pathways identified in_P. aeruginosa_using enrichment experiments with M-DFO-azir-06 (highlighted in red). Probable TonB-dependent receptor PA1271, named BtuB due to its homology with_E. coli_cobalamine transporter BtuB, is identified by our experiments as a putative DFO transporter.
Figure 8. Ga and Fe-DFO-azir-06 enrichment in E. coli K-12 cells. (A) Quantification by spectral counts of transporters identified by MS/MS in enrichment assays using M-DFO-azir-06 and azir-biotin. (B) Heatmap representation of spectral counts of transporters identified by MS/MS in enrichment assays using M-DFO-azir-06 and azir-biotin. (C) Schematic representation of DFO uptake pathways identified in_E. coli_using enrichment experiments with M-DFO-azir-06 (highlighted in red). The results shown are representative of three independent experiments (Table S3).
总结及展望
本研究展示了一套从结构理性设计、分子多代优化到活细胞原位标记与定量蛋白质组学分析的完整闭环方案。通过巧妙引入生物素预连接策略以及非螯铁蛋白控制探针，团队成功过滤了非特异性背景干扰，明确了药物偶联物在复杂细菌双层膜环境中的动态吸入全景。这不仅彻底理清了去铁胺类“特洛伊木马”抗生素在铜绿假单胞菌和大肠杆菌中的关键转运命脉，还挖掘出了以 BtuB 和 PA1271 为代表的全新潜在药物吸入靶通道。这项工作不仅是螯铁蛋白领域光亲和标记技术的首次成功跨越，也为未来针对更多不同种类的微生物螯合物开展靶点解析提供了标准的底层技术蓝图，对精准指导下一代广谱、高效抗生素的结构改造具有决定性的里程碑意义。