Thurtle-Schmidt Bryan H, Stroud Robert M
Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94158.
Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94158
Proc Natl Acad Sci U S A. 2016 Sep 20;113(38):10542-6. doi: 10.1073/pnas.1612603113. Epub 2016 Sep 6.
Boron is essential for plant growth because of its incorporation into plant cell walls; however, in excess it is toxic to plants. Boron transport and homeostasis in plants is regulated in part by the borate efflux transporter Bor1, a member of the solute carrier (SLC) 4 transporter family with homology to the human bicarbonate transporter Band 3. Here, we present the 4.1-Å resolution crystal structure of Arabidopsis thaliana Bor1. The structure displays a dimeric architecture in which dimerization is mediated by centralized Gate domains. Comparisons with a structure of Band 3 in an outward-open state reveal that the Core domains of Bor1 have rotated inwards to achieve an occluded state. Further structural comparisons with UapA, a xanthine transporter from the nucleobase-ascorbate transporter family, show that the downward pivoting of the Core domains relative to the Gate domains may access an inward-open state. These results suggest that the SLC4, SLC26, and nucleobase-ascorbate transporter families all share an elevator transport mechanism in which alternating access is provided by Core domains that carry substrates across a membrane.
硼对植物生长至关重要，因为它会掺入植物细胞壁中；然而，过量的硼对植物有毒。植物中硼的运输和稳态部分受硼酸盐外流转运蛋白Bor1调节，Bor1是溶质载体（SLC）4转运蛋白家族的成员，与人类碳酸氢盐转运蛋白Band 3具有同源性。在此，我们展示了拟南芥Bor1的4.1埃分辨率晶体结构。该结构呈现出二聚体架构，其中二聚化由位于中央的门控结构域介导。与处于向外开放状态的Band 3结构进行比较发现，Bor1的核心结构域已向内旋转以达到封闭状态。与核碱基 - 抗坏血酸转运蛋白家族的黄嘌呤转运蛋白UapA进行进一步的结构比较表明，核心结构域相对于门控结构域向下枢转可能会进入向内开放状态。这些结果表明，SLC4、SLC26和核碱基 - 抗坏血酸转运蛋白家族都共享一种电梯运输机制，其中携带底物穿过膜的核心结构域提供交替通道。
