Borths Elizabeth L, Locher Kaspar P, Lee Allen T, Rees Douglas C
Howard Hughes Medical Institute and Division of Chemistry and Chemical Engineering, Mail Code 114-96, California Institute of Technology, Pasadena, CA 91125, USA.
Proc Natl Acad Sci U S A. 2002 Dec 24;99(26):16642-7. doi: 10.1073/pnas.262659699. Epub 2002 Dec 10.
Bacterial binding protein-dependent ATP binding cassette (ABC) transporters facilitate uptake of essential nutrients. The crystal structure of Escherichia coli BtuF, the protein that binds vitamin B12 and delivers it to the periplasmic surface of the ABC transporter BtuCD, reveals a bi-lobed fold resembling that of the ferrichrome binding protein FhuD. B12 is bound in the "base-on" conformation in a deep cleft formed at the interface between the two lobes of BtuF. A stable complex between BtuF and BtuCD (with the stoichiometry BtuC2D2F) is demonstrated to form in vitro and was modeled using the individual crystal structures. Two surface glutamates from BtuF may interact with arginine residues on the periplasmic surface of the BtuCD transporter. These glutamate and arginine residues are conserved among binding proteins and ABC transporters mediating iron and B12 uptake, suggesting that they may have a role in docking and the transmission of conformational changes.
细菌结合蛋白依赖性ATP结合盒（ABC）转运蛋白有助于摄取必需营养素。大肠杆菌BtuF的晶体结构显示出一种双叶折叠结构，类似于铁载体结合蛋白FhuD，BtuF负责结合维生素B12并将其递送至ABC转运蛋白BtuCD的周质表面。维生素B12以“碱基朝上”的构象结合在BtuF两叶之间界面处形成的深裂缝中。体外实验证明BtuF和BtuCD（化学计量比为BtuC2D2F）之间能形成稳定的复合物，并利用各自的晶体结构进行了建模。BtuF的两个表面谷氨酸残基可能与BtuCD转运蛋白周质表面的精氨酸残基相互作用。这些谷氨酸和精氨酸残基在介导铁和维生素B12摄取的结合蛋白和ABC转运蛋白中是保守的，这表明它们可能在对接和构象变化传递中发挥作用。
