Becker Patrick, Naughton Fiona B, Brotherton Deborah H, Pacheco-Gomez Raul, Beckstein Oliver, Cameron Alexander D
School of Life Sciences, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, U.K.
Department of Physics, Arizona State University, Tempe, AZ 85287.
bioRxiv. 2023 Aug 16:2023.06.02.543391. doi: 10.1101/2023.06.02.543391.
The Bile Acid Sodium Symporter (BASS) family transports a wide array of molecules across membranes, including bile acids in humans, and small metabolites in plants. These transporters, many of which are sodium-coupled, have been shown to use an elevator mechanism of transport, but exactly how substrate binding is coupled to sodium ion binding and transport is not clear. Here we solve the crystal structure at 2.3 Å of a transporter from (ASBT) in complex with pantoate, a potential substrate of ASBT. The BASS family is characterised by two helices that cross-over in the centre of the protein in an arrangement that is intricately held together by two sodium ions. We observe that the pantoate binds, specifically, between the N-termini of two of the opposing helices in this cross-over region. During molecular dynamics simulations the pantoate remains in this position when sodium ions are present but is more mobile in their absence. Comparison of structures in the presence and absence of pantoate demonstrates that pantoate elicits a conformational change in one of the cross-over helices. This modifies the interface between the two domains that move relative to one another to elicit the elevator mechanism. These results have implications, not only for ASBT but for the BASS family as a whole and indeed other transporters that work through the elevator mechanism.
胆汁酸钠同向转运体（BASS）家族可跨膜转运多种分子，包括人类的胆汁酸以及植物中的小分子代谢物。这些转运体中的许多都是与钠偶联的，已被证明采用电梯式转运机制，但底物结合究竟如何与钠离子结合及转运偶联尚不清楚。在此，我们解析了来自[具体物种]的一种转运体（ASBT）与泛酸（ASBT的一种潜在底物）复合物在2.3埃分辨率下的晶体结构。BASS家族的特征是有两个螺旋在蛋白质中心交叉，这种排列由两个钠离子紧密维系在一起。我们观察到泛酸特异性地结合在这个交叉区域中两个相对螺旋的N端之间。在分子动力学模拟中，当存在钠离子时泛酸保持在这个位置，但在没有钠离子时其移动性更强。有泛酸和没有泛酸时结构的比较表明，泛酸会引发其中一个交叉螺旋的构象变化。这改变了两个相对移动以引发电梯机制的结构域之间的界面。这些结果不仅对ASBT有影响，对整个BASS家族以及实际上通过电梯机制工作的其他转运体也有影响。
