1月20日，清华大学生命科学学院施一公教授领导的研究组在《自然》在线发表论文，阐述在毒性大肠杆菌肠胃耐酸性保护机制中起重要作用的AdiC转运蛋白的底物识别和转运机理。这是继他们在2009年于《科学》发表首个APC家族转运蛋白晶体结构之后针对AdiC研究的又一重要突破。
AdiC是逆向转运arginine及其脱羧产物agmatine的转运蛋白。2009年5月，施一公教授的研究组发表了AdiC在没有底物状况下的3.6埃的分子结构，揭示了AdiC以二聚体形式存在，同时还发现APC家族的转运蛋白与其他几类没有序列同源性的钠离子依赖性转运蛋白具有相似的结构骨架。根据生化实验结果，他们推测出在转运过程中起重要作用的氨基酸残基以及可能的转运模型。
在《自然》最新发表的论文中，施一公研究组解析了AdiC在arginine结合状态下的3.0埃的晶体结构。这个较高分辨率的结构显示，过去报道的AdiC由于分辨率偏低、数据质量有限，因而尽管基本结论正确，但在部分跨膜螺旋区域，施一公的研究组以及美国另一研究组均存在结构修正不妥之处。所以在这一篇文章里，他们首先纠正了两个组以前的偏差，然后比较了AdiC在没有底物和结合arginine的状态下的结构。这一比较显示，在结合arginine时，AdiC的跨膜螺旋TM6旋转了近40°，覆盖于底物之上，使底物处于一个完全封闭的表面呈负电势的腔中。在这一观测的基础上，结合以往的生化数据，他们进一步修正了AdiC的转运模型，提出了逆向转运蛋白双结合位点的模型。（生物谷Bioon.com）
施一公近期科研成果：
PNAS：膜蛋白酶S2P活性调控研究
NSMB：解析TIPE2晶体结构
Cell：PSPs的结构以及生化功能
Molecular Cell：PP2A调节Tau蛋白脱磷酸
传说人物施一公——生物谷盘点2009
生物谷推荐原始出处：
Nature advance online publication 20 January 2010 | doi:10.1038/nature08741
Mechanism of substrate recognition and transport by an amino acid antiporter
Xiang Gao1,4, Lijun Zhou2,4, Xuyao Jiao2,3, Feiran Lu2, Chuangye Yan2, Xin Zeng1, Jiawei Wang2 & Yigong Shi1
1 Ministry of Education Protein Science Laboratory,
2 State Key Laboratory of Biomembrane and Membrane Biotechnology, Center for Structural Biology, School of Life Sciences and School of Medicine, Tsinghua University, Beijing 100084, China
3 School of Life Sciences, Shandong University, Jinan, Shandong 250100, China
4 These authors contributed equally to this work.
5 Correspondence to: Yigong Shi1 Correspondence and requests for materials should be addressed to Y.S.
In extremely acidic environments, enteric bacteria such as Escherichia coli rely on the amino acid antiporter AdiC to expel protons by exchanging intracellular agmatine (Agm2+) for extracellular arginine (Arg+)1, 2, 3. AdiC is a representative member of the amino acid-polyamine-organocation (APC) superfamily of membrane transporters4, 5. The structure of substrate-free AdiC revealed a homodimeric assembly, with each protomer containing 12 transmembrane segments and existing in an outward-open conformation6, 7. The overall folding of AdiC is similar to that of the Na+-coupled symporters8, 9, 10, 11. Despite these advances, it remains unclear how the substrate (arginine or agmatine) is recognized and transported by AdiC. Here we report the crystal structure of an E. coli AdiC variant bound to Arg at 3.0?? resolution. The positively charged Arg is enclosed in an acidic binding chamber, with the head groups of Arg hydrogen-bonded to main chain atoms of AdiC and the aliphatic portion of Arg stacked by hydrophobic side chains of highly conserved residues. Arg binding induces pronounced structural rearrangement in transmembrane helix 6 (TM6) and, to a lesser extent, TM2 and TM10, resulting in an occluded conformation. Structural analysis identified three potential gates, involving four aromatic residues and Glu?208, which may work in concert to differentially regulate the upload and release of Arg and Agm.
