Membrane transporter dimerization driven by differential lipid solvation energetics of dissociated and associated states.
Membrane transporter dimerization driven by differential lipid solvation energetics of dissociated and associated states.
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DOI：
10.7554/elife.63288
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发表时间：
2021-04-07
期刊：
影响因子：
7.7
通讯作者：
Robertson JL
中科院分区：
文献类型：
--
作者：
Chadda R;Bernhardt N;Kelley EG;Teixeira SC;Griffith K;Gil-Ley A;Öztürk TN;Hughes LE;Forsythe A;Krishnamani V;Faraldo-Gómez JD;Robertson JL
超过三分之二的已知结构的完整膜蛋白可以组装成低聚物。然而，驱动这些蛋白质结合的作用力仍然有待描述，因为脂质双层是一个结构和化学上都很复杂的溶剂环境。在这项研究中，我们揭示了脂类溶剂如何定义ClC-EC1Cl-/H+逆向转运体的二聚化平衡。结合实验和计算方法，我们表明，单体缔合避免了由于疏水失配而在其暴露的二聚界面形成的薄膜缺陷。在这一缺陷中，脂质强烈倾斜，密度低于大块，相对小叶之间有更大程度的缠绕，水更多地渗透到双层内部。二聚作用使膜恢复到接近自然的状态，因此，似乎是由解离的原体脂溶剂化的更大自由能成本驱动的。支持这一理论，我们证明了短链脂质的加入使二聚化平衡强烈地向单体状态移动，并表明这种影响的原因是这些脂类优先溶解缺陷。重要的是，我们证明了这种转变只需要最少量的短链脂质，对膜的宏观物理状态或蛋白质的生物功能都没有可测量的影响。基于这些观察，我们假设局部脂溶剂化的自由能差定义了膜-蛋白质结合平衡。因此，我们认为优先脂溶剂化是寡聚过程中脂调节的一种合理的细胞机制，因为它可以在低浓度下发生，并且不需要膜性质的全局改变。细胞的外膜是由被称为脂类的分子组成的，它们结合在一起形成一层柔性的薄膜，只有两个分子厚。这种膜上点缀着蛋白质，这些蛋白质负责将物质运进和运出细胞。这些膜蛋白中的大多数与其他蛋白质结合形成被称为寡聚体的结构。除了膜结合的蛋白质如何组装成低聚物--驱动这些分子形成的物理力量--尚不清楚。这在一定程度上是因为类脂脂膜的结构、物理和化学性质与细胞内部的水有根本的不同。因此，膜低聚物形成的条件与细胞内那些周围的蛋白质不同。膜蛋白也比细胞内的水溶性蛋白更难研究和表征，但许多治疗药物，如抗生素，都是专门针对膜蛋白的。总体而言，我们对脂膜的独特性质如何影响嵌入其中的蛋白质结构的形成缺乏了解，值得进一步研究。现在，Chadda，Bernhardt等人。聚焦于一种膜蛋白，称为ClC，它往往成对存在--或二聚体。为了了解这些蛋白质形成二聚体的原因(称为二聚化的过程)，Chadda，Bernhardt等人。首先使用了计算机模拟，然后在实验测试中验证了发现。这些互补的方法表明，CLC蛋白质“二聚化”的主要原因在于它们与脂膜的相互作用，而不是一种蛋白质对其伴侣的吸引。当CLC蛋白单独存在时，它们会使周围的膜变形，并产生结构缺陷，使膜处于紧张状态。但当两个CLC蛋白以二聚体的形式结合时，这种膜菌株就消失了--使二聚化成为更稳定和能量更有利的选择。Chadda，Bernhardt等人。研究还表明，随着添加一些特定的脂类，特别是较小的脂类，细胞膜对蛋白质诱导的结构变化变得更耐受。这可能解释了细胞如何通过控制寡聚体的形成来使用不同的脂类微调膜蛋白的活性。然而，这一理论还需要进一步检验。总之，这项工作提供了对塑造膜结合蛋白的物理力量的基本见解，与研究细胞生物学和药理学的研究人员相关。
Over two-thirds of integral membrane proteins of known structure assemble into oligomers. Yet, the forces that drive the association of these proteins remain to be delineated, as the lipid bilayer is a solvent environment that is both structurally and chemically complex. In this study, we reveal how the lipid solvent defines the dimerization equilibrium of the CLC-ec1 Cl-/H+ antiporter. Integrating experimental and computational approaches, we show that monomers associate to avoid a thinned-membrane defect formed by hydrophobic mismatch at their exposed dimerization interfaces. In this defect, lipids are strongly tilted and less densely packed than in the bulk, with a larger degree of entanglement between opposing leaflets and greater water penetration into the bilayer interior. Dimerization restores the membrane to a near-native state and therefore, appears to be driven by the larger free-energy cost of lipid solvation of the dissociated protomers. Supporting this theory, we demonstrate that addition of short-chain lipids strongly shifts the dimerization equilibrium toward the monomeric state, and show that the cause of this effect is that these lipids preferentially solvate the defect. Importantly, we show that this shift requires only minimal quantities of short-chain lipids, with no measurable impact on either the macroscopic physical state of the membrane or the protein's biological function. Based on these observations, we posit that free-energy differentials for local lipid solvation define membrane-protein association equilibria. With this, we argue that preferential lipid solvation is a plausible cellular mechanism for lipid regulation of oligomerization processes, as it can occur at low concentrations and does not require global changes in membrane properties. A cell’s outer membrane is made of molecules called lipids, which band together to form a flexible thin film, just two molecules thick. This membrane is dotted with proteins that transport materials in to and out of cells. Most of these membrane proteins join with other proteins to form structures known as oligomers. Except, how membrane-bound proteins assemble into oligomers – the physical forces driving these molecules to take shape – remains unclear. This is partly because the structural, physical and chemical properties of fat-like lipid membranes are radically different to the cell’s watery interior. Consequently, the conditions under which membrane oligomers form are distinct from those surrounding proteins inside cells. Membrane proteins are also more difficult to study and characterize than water-soluble proteins inside the cell, and yet many therapeutic drugs such as antibiotics specifically target membrane proteins. Overall, our understanding of how the unique properties of lipid membranes affect the formation of protein structures embedded within, is lacking and warrants further investigation. Now, Chadda, Bernhardt et al. focused on one membrane protein, known as CLC, which tends to exist in pairs – or dimers. To understand why these proteins form dimers (a process called dimerization) Chadda, Bernhardt et al. first used computer simulations, and then validated the findings in experimental tests. These complementary approaches demonstrated that the main reason CLC proteins ‘dimerize’ lies in their interaction with the lipid membrane, and not the attraction of one protein to its partner. When CLC proteins are on their own, they deform the surrounding membrane and create structural defects that put the membrane under strain. But when two CLC proteins join as a dimer, this membrane strain disappears – making dimerization the more stable and energetically favorable option. Chadda, Bernhardt et al. also showed that with the addition of a few certain lipids, specifically smaller lipids, cell membranes become more tolerant of protein-induced structural changes. This might explain how cells could use various lipids to fine-tune the activity of membrane proteins by controlling how oligomers form. However, the theory needs to be examined further. Altogether, this work has provided fundamental insights into the physical forces shaping membrane-bound proteins, relevant to researchers studying cell biology and pharmacology alike.