Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON, Canada.
Nanoscale. 2020 Jan 23;12(3):1438-1447. doi: 10.1039/c9nr07977g.
Membrane undulations play a vital role in many biological processes, including the regulation of membrane protein activity. The asymmetric lipid composition of most biological membranes complicates theoretical description of these bending fluctuations, yet experimental data that would inform any such a theory is scarce. Here, we used neutron spin-echo (NSE) spectroscopy to measure the bending fluctuations of large unilamellar vesicles (LUV) having an asymmetric transbilayer distribution of high- and low-melting lipids. The asymmetric vesicles were prepared using cyclodextrin-mediated lipid exchange, and were composed of an outer leaflet enriched in egg sphingomyelin (ESM) and an inner leaflet enriched in 1-palmitoyl-2-oleoyl-phosphoethanolamine (POPE), which have main transition temperatures of 37 °C and 25 °C, respectively. The overall membrane bending rigidity was measured at three temperatures: 15 °C, where both lipids are in a gel state; 45 °C, where both lipids are in a fluid state; and 30 °C, where there is gel-fluid co-existence. Remarkably, the dynamics for the fluid asymmetric LUVs (aLUVs) at 30 °C and 45 °C do not follow trends predicted by their symmetric counterparts. At 30 °C, compositional asymmetry suppressed the bending fluctuations, with the asymmetric bilayer exhibiting a larger bending modulus than that of symmetric bilayers corresponding to either the outer or inner leaflet. We conclude that the compositional asymmetry and leaflet coupling influence the internal dissipation within the bilayer and result in membrane properties that cannot be directly predicted from corresponding symmetric bilayers.
膜波动在许多生物过程中起着至关重要的作用，包括膜蛋白活性的调节。大多数生物膜的不对称脂质组成使这些弯曲波动的理论描述变得复杂，然而，能够为任何这样的理论提供信息的实验数据却很少。在这里，我们使用中子自旋回波（NSE）光谱测量了具有高熔点和低熔点脂质不对称跨双层分布的大单室囊泡（LUV）的弯曲波动。使用环糊精介导的脂质交换制备了不对称囊泡，其外层富含蛋黄鞘磷脂（ESM），内层富含 1-棕榈酰基-2-油酰基-磷酸乙醇胺（POPE），其主相变温度分别为 37°C 和 25°C。在三个温度下测量了整体膜弯曲刚度：15°C，两种脂质均处于凝胶状态；45°C，两种脂质均处于流动状态；30°C，存在凝胶-流动共存。值得注意的是，在 30°C 和 45°C 下，流体不对称 LUV（aLUV）的动力学不遵循与其对称对应物预测的趋势。在 30°C 时，组成不对称抑制了弯曲波动，不对称双层的弯曲模量大于对应于外层或内层的对称双层的弯曲模量。我们得出结论，组成不对称和双层耦合影响双层内的内部耗散，并导致不能直接从相应的对称双层预测的膜性质。
