Structural Analysis of a Periplasmic Binding Protein in the Tripartite ATP-independent Transporter Family Reveals a Tetrameric Assembly That May Have a Role in Ligand Transport
Structural Analysis of a Periplasmic Binding Protein in the Tripartite ATP-independent Transporter Family Reveals a Tetrameric Assembly That May Have a Role in Ligand Transport
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DOI：
10.1074/jbc.m803595200
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发表时间：
2008-11-21
影响因子：
4.8
通讯作者：
Hellinga, Homme W.
中科院分区：
文献类型：
--
作者：
Cuneo, Matthew J.;Changela, Anita;Hellinga, Homme W.
几种细菌溶质转运机制涉及外质结合蛋白（PBP）超家族的成员，这些成员结合并将配体递送到atp结合盒、三羧酸转运蛋白或三羧酸转运蛋白（TRAP）家族中的整体膜转运蛋白上。参与atp结合盒运输系统的PBPs已经被很好地表征，但只有少数参与TRAP运输的PBPs被研究过。我们测量了热稳定性，通过小角度x射线散射确定了寡聚化状态，并求解了来自超嗜热细菌Thermotoga maritima （tm0322）的TRAP-PBP（开放阅读框tm0322）的x射线晶体结构到1.9埃的分辨率。TM0322的整体褶皱与其他与TRAP输运相关的PBPs相似，尽管在同一基团的PBPs中，主链原子的结构相似性（2.5-3.1埃均方根偏差）异常低。TM0322的四聚体不对称单元内的单体由于其结合口袋的构象异质性，彼此之间的均方根偏差很大（0.9埃）。凝胶过滤洗脱谱和小角x射线散射分析表明，TM0322在溶液中以二聚体的形式组装，在晶体不对称单元中又以二聚体的形式组装。四聚体化已经在另一种TRAP-PBP（球形红杆菌α -酮酸结合蛋白）中被观察到，其中四级结构的形成被认为是跨膜运输过程的重要必要条件。
Several bacterial solute transport mechanisms involve members of the periplasmic binding protein (PBP) superfamily that bind and deliver ligand to integral membrane transport proteins in the ATP-binding cassette, tripartite tricarboxylate transporter, or tripartite ATP-independent (TRAP) families. PBPs involved in ATP-binding cassette transport systems have been well characterized, but only a few PBPs involved in TRAP transport have been studied. We have measured the thermal stability, determined the oligomerization state by small angle x-ray scattering, and solved the x-ray crystal structure to 1.9 angstrom resolution of a TRAP-PBP (open reading frame tm0322) from the hyperthermophilic bacterium Thermotoga maritima (TM0322). The overall fold of TM0322 is similar to other TRAP transport related PBPs, although the structural similarity of backbone atoms (2.5-3.1 angstrom root mean square deviation) is unusually low for PBPs within the same group. Individual monomers within the tetrameric asymmetric unit of TM0322 exhibit high root mean square deviation (0.9 angstrom) to each other as a consequence of conformational heterogeneity in their binding pockets. The gel filtration elution profile and the small angle x-ray scattering analysis indicate that TM0322 assembles as dimers in solution that in turn assemble into a dimer of dimers in the crystallographic asymmetric unit. Tetramerization has been previously observed in another TRAP-PBP (the Rhodobacter sphaeroides alpha-keto acid-binding protein) where quaternary structure formation is postulated to be an important requisite for the transmembrane transport process.