A lipid bound actin meshwork organizes liquid phase separation in model membranes.
A lipid bound actin meshwork organizes liquid phase separation in model membranes.
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作者：
Honigmann A;Sadeghi S;Keller J;Hell SW;Eggeling C;Vink R
真核细胞膜与致密的富含肌动蛋白的皮质相连。我们提出的 FCS 和 STED 实验表明，致密的膜结合肌动蛋白网络对脂质相分离具有严重影响。最小的肌动蛋白皮层通过生物素化的脂质链霉亲和素复合物（钉扎位点）与支持的脂质双层结合。一般来说，即使在低温下，肌动蛋白与三元膜的结合也能阻止宏观液体有序和液体无序结构域的形成。相反，根据钉扎脂质的类型，观察到肌动蛋白相关的多结构域模式。 FCS 测量显示，在肌动蛋白网络存在的情况下，脂质扩散受到阻碍。为了解释我们的实验结果，提出了一种新的模拟模型，其中膜组成、膜曲率和肌动蛋白钉扎位点都是耦合的。我们的结果揭示了细胞如何防止其膜成分宏观分层，同时调节局部膜组成的机制。 DOI：http://dx.doi.org/10.7554/eLife.01671.001 所有细胞都被脂质膜包围，该膜保护细胞、控制分子进出细胞的运动，并将有关环境条件的信息传递给细胞。这种膜由两层称为脂质的分子组成，其中嵌入了各种蛋白质。有许多不同类型的脂质分子共同帮助保持膜的柔韧性。此外，特定类型的脂质分子也可以聚集在一起形成“筏”，帮助膜发挥其各种作用。鉴于细胞膜的复杂性，细胞生物学家经常使用更简单的模型膜和计算机模拟来探索不同类型的脂质分子在膜内的组织方式。根据“栅栏”模型，细胞膜由于与充当细胞内骨架的致密肌动蛋白纤维网络相互作用而被分成小隔室。最近的计算机模拟预测这些相互作用可以影响膜内脂质和蛋白质的分布。特别是，它们可以防止脂质在低温下剧烈重新排列成高粘度和低粘度区域。这种依赖于温度的膜重新排列被称为脂质相分离。霍尼格曼等人。现在，他们使用计算机模拟和两种先进技术（超分辨率光学 STED 显微镜和荧光相关光谱）来详细探索存在致密肌动蛋白纤维网络的模型膜的特性。结果表明，与“栅栏”模型的模拟预测一致，与膜结合的肌动蛋白纤维可防止低温下发生脂质相分离。此外，肌动蛋白纤维还有助于在生理温度下组织膜内脂质和蛋白质的分布。霍尼格曼等人。还表明肌动蛋白纤维导致膜以某种方式弯曲，从而增强“栅栏”的影响。结果表明，“筏”和“栅栏”模型是相互联系的，细胞可以通过控制膜和构成细胞骨架的肌动蛋白纤维之间的相互作用来控制其膜的特性。 DOI：http://dx.doi.org/10.7554/eLife.01671.002
The eukaryotic cell membrane is connected to a dense actin rich cortex. We present FCS and STED experiments showing that dense membrane bound actin networks have severe influence on lipid phase separation. A minimal actin cortex was bound to a supported lipid bilayer via biotinylated lipid streptavidin complexes (pinning sites). In general, actin binding to ternary membranes prevented macroscopic liquid-ordered and liquid-disordered domain formation, even at low temperature. Instead, depending on the type of pinning lipid, an actin correlated multi-domain pattern was observed. FCS measurements revealed hindered diffusion of lipids in the presence of an actin network. To explain our experimental findings, a new simulation model is proposed, in which the membrane composition, the membrane curvature, and the actin pinning sites are all coupled. Our results reveal a mechanism how cells may prevent macroscopic demixing of their membrane components, while at the same time regulate the local membrane composition. DOI: http://dx.doi.org/10.7554/eLife.01671.001 All cells are surrounded by a lipid membrane that protects the cell, controls the movement of molecules into and out of the cell, and passes messages about environmental conditions to the cell. This membrane is made of two layers of molecules called lipids, with various proteins embedded in it. There are many different types of lipid molecules that together help to keep the membrane flexible. Moreover, lipid molecules of particular types can also come together to form ‘rafts’ that help the membrane to carry out its various roles. Given the complexity of the cell membrane, cell biologists often use simpler model membranes and computer simulations to explore how the different types of lipid molecules are organized within the membrane. According to the ‘picket fence’ model the cell membrane is divided into small compartments as a result of its interaction with the dense network of actin fibers that acts as a skeleton inside the cell. Recent computer simulations have predicted that these interactions can influence the distribution of lipids and proteins within the membrane. In particular, they can prevent the drastic re-arrangement of lipids into regions of high and low viscosity at low temperature. This temperature dependent re-arrangement of the membrane is known as lipid phase separation. Honigmann et al. have now used computer simulations and two advanced techniques—super-resolution optical STED microscopy and fluorescence correlation spectroscopy—to explore the properties of a model membrane in the presence of a dense network of actin fibers in fine detail. The results show that, in agreement with the simulation predictions of the ‘picket fence’ model, the actin fibers bound to the membrane prevent lipid phase separation happening at low temperatures. Moreover, the actin fibers also help to organize the distribution of lipids and proteins within the membrane at physiological temperatures. Honigmann et al. also suggest that the actin fibers cause the membrane to curve in a way that can reinforce the influence of the ‘picket fence’. The results show that the ‘raft’ and ‘picket fence’ models are connected, and that a cell can control the properties of its membrane by controlling the interactions between the membrane and the actin fibers that make up the skeleton of the cell. DOI: http://dx.doi.org/10.7554/eLife.01671.002