Rawicz W, Smith B A, McIntosh T J, Simon S A, Evans E
Department of Physics and Department of Pathology, University of British Columbia, Vancouver, Canada.
Biophys J. 2008 Jun;94(12):4725-36. doi: 10.1529/biophysj.107.121731. Epub 2008 Mar 13.
Bilayers composed of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (CHOL) are commonly used as systems to model the raft-lipid domain structure believed to compartmentalize particular cell membrane proteins. In this work, micropipette aspiration of giant unilamellar vesicles was used to test the elasticities, water permeabilities, and rupture tensions of single-component PC, binary 1:1 PC/CHOL, and 1:1 SM/CHOL, and ternary 1:1:1 PC/SM/CHOL bilayers, one set of measurements with dioleoyl PC (DOPC; C18:1/C18:1 PC) and the other with stearoyloleoyl PC (SOPC; C18:0/C18:1 PC). Defining the elastic moduli (K(A)), the initial slopes of the increase in tension (sigma) versus stretch in lipid surface area (alpha(e)) were determined for all systems at low (15 degrees C) and high (32-33 degrees C) temperatures. The moduli for the single-component PC and binary phospholipid/CHOL bilayers followed a descending hierarchy of stretch resistance with SM/CHOL > SOPC/CHOL > DOPC/CHOL > PC. Although much more resistant to stretch than the single-component PC bilayers, the elastic response of vesicle bilayers made from the ternary phospholipid/CHOL mixtures showed an abrupt softening (discontinuity in slope), when immediately subjected to a steady ramp of tension at the low temperature (15 degrees C). However, the discontinuities in elastic stretch resistance at low temperature vanished when the bilayers were held at approximately 1 mN/m prestress for long times before a tension ramp and when tested at the higher temperature 32-33 degrees C. The elastic moduli of single-component PC and DOPC/CHOL bilayers changed very little with temperature, whereas the moduli of the binary SOPC/CHOL and SM/CHOL bilayers diminished markedly with increase in temperature, as did the ternary SOPC/SM/CHOL system. For all systems, increasing temperature increased the water permeability but decreased rupture tension. Concomitantly, the measurements of permeability exhibited a prominent correlation with the rupture tension across all the systems. Together, these micromechanical tests of binary and ternary phospholipid/CHOL bilayers demonstrate that PC hydrocarbon chain unsaturation and temperature are major determinants of the mechanical and permeation properties of membranes composed of raft microdomain-forming lipids.
由磷脂酰胆碱（PC）、鞘磷脂（SM）和胆固醇（CHOL）组成的双层膜通常被用作模型系统，以模拟被认为可分隔特定细胞膜蛋白的筏脂域结构。在这项工作中，使用微量移液器抽吸巨型单层囊泡来测试单组分PC、二元1:1 PC/CHOL和1:1 SM/CHOL以及三元1:1:1 PC/SM/CHOL双层膜的弹性、水渗透性和破裂张力，一组测量使用二油酰PC（DOPC；C18:1/C18:1 PC），另一组使用硬脂酰油酰PC（SOPC；C18:0/C18:1 PC）。通过定义弹性模量（K(A)），在低温（15℃）和高温（32 - 33℃）下确定了所有系统中张力（σ）随脂质表面积（α(e)）增加的初始斜率。单组分PC和二元磷脂/CHOL双层膜的模量呈现出拉伸抗性的降序排列，即SM/CHOL > SOPC/CHOL > DOPC/CHOL > PC。尽管三元磷脂/CHOL混合物制成的囊泡双层膜比单组分PC双层膜更耐拉伸，但在低温（15℃）下立即受到稳定的张力斜坡作用时，其弹性响应显示出突然软化（斜率不连续）。然而，当双层膜在张力斜坡之前长时间保持在约1 mN/m的预应力下并在较高温度32 - 33℃下测试时，低温下弹性拉伸抗性的不连续性消失。单组分PC和DOPC/CHOL双层膜的弹性模量随温度变化很小，而二元SOPC/CHOL和SM/CHOL双层膜的模量以及三元SOPC/SM/CHOL系统的模量随温度升高显著降低。对于所有系统，温度升高会增加水渗透性但降低破裂张力。同时，渗透率的测量在所有系统中都与破裂张力呈现出显著的相关性。总之，这些二元和三元磷脂/CHOL双层膜的微观力学测试表明，PC烃链不饱和度和温度是由形成筏微区的脂质组成的膜的力学和渗透性质的主要决定因素。
