School of Life Sciences, University of Warwick, Coventry, United Kingdom.
Department of Physics, Arizona State University, Tempe, United States.
Elife. 2023 Nov 14;12:RP89167. doi: 10.7554/eLife.89167.
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.
胆盐钠协同转运蛋白（BASS）家族能够跨膜转运多种分子，包括人类的胆汁酸和植物中的小分子代谢物。这些转运蛋白中，许多都是钠耦合的，已被证明采用电梯式运输机制，但底物结合如何与钠离子结合和运输耦联尚不清楚。在这里，我们解析了 2.3Å分辨率的来自 （ASBT）与泛酸结合的晶体结构，泛酸是 ASBT 的潜在底物。BASS 家族的特征是两条在蛋白质中心交叉的螺旋，这种交错的排列通过两个钠离子紧密结合在一起。我们观察到，泛酸特异性地结合在交叉区域两个相对螺旋的 N 末端之间。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，它的移动性更强。在有和没有泛酸的情况下比较结构表明，泛酸在一个交叉螺旋中引起构象变化。这改变了两个相对运动的结构域之间的界面，从而引发了电梯式机制。这些结果不仅对 ASBT，而且对整个 BASS 家族甚至其他通过电梯式机制工作的转运蛋白都具有重要意义。
