Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125.
Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA 91125.
Proc Natl Acad Sci U S A. 2018 Nov 6;115(45):E10596-E10604. doi: 10.1073/pnas.1811003115. Epub 2018 Oct 23.
The methionine ABC transporter MetNI exhibits both high-affinity transport toward l-methionine and broad specificity toward methionine derivatives, including d-methionine. In this work, we characterize the transport of d-methionine derivatives by the MetNI transporter. Unexpectedly, the N229A substrate-binding deficient variant of the cognate binding protein MetQ was found to support high MetNI transport activity toward d-selenomethionine. We determined the crystal structure at 2.95 Å resolution of the ATPγS-bound MetNIQ complex in the outward-facing conformation with the N229A apo MetQ variant. This structure revealed conformational changes in MetQ providing substrate access through the binding protein to the transmembrane translocation pathway. MetQ likely mediates uptake of methionine derivatives through two mechanisms: in the methionine-bound form delivering substrate from the periplasm to the transporter (the canonical mechanism) and in the apo form by facilitating ligand binding when complexed to the transporter (the noncanonical mechanism). This dual role for substrate-binding proteins is proposed to provide a kinetic strategy for ABC transporters to transport both high- and low-affinity substrates present in a physiological concentration range.
甲硫氨酸 ABC 转运蛋白 MetNI 对 L-甲硫氨酸具有高亲和力，对甲硫氨酸衍生物（包括 D-甲硫氨酸）具有广泛的特异性。在这项工作中，我们研究了 MetNI 转运蛋白对 D-甲硫氨酸衍生物的转运。出乎意料的是，我们发现同源结合蛋白 MetQ 的 N229A 底物结合缺陷变体能够支持 D-硒代甲硫氨酸的高 MetNI 转运活性。我们以 2.95Å 的分辨率确定了 ATPγS 结合的 MetNIQ 复合物的晶体结构，该结构处于外向构象，其中含有 N229A apo MetQ 变体。该结构揭示了 MetQ 的构象变化，通过结合蛋白为跨膜转运途径提供了底物进入的通道。MetQ 可能通过两种机制介导甲硫氨酸衍生物的摄取：在与甲硫氨酸结合的形式下，将底物从周质空间传递到转运体（经典机制），以及在与转运体结合时通过促进配体结合的无规卷曲形式（非经典机制）。这种底物结合蛋白的双重作用为 ABC 转运蛋白提供了一种动力学策略，使其能够在生理浓度范围内运输高亲和性和低亲和性的底物。
