Cubic phases in phosphatidylcholine-cholesterol mixtures: cholesterol as membrane "fusogen".
Cubic phases in phosphatidylcholine-cholesterol mixtures: cholesterol as membrane "fusogen".
复制标题
DOI：
10.1529/biophysj.106.083766
复制
发表时间：
2006-10
影响因子：
3.4
通讯作者：
B. Tenchov;R. Macdonald;D. Siegel
中科院分区：
文献类型：
--
作者：
B. Tenchov;R. Macdonald;D. Siegel
X射线衍射表明，一些不饱和磷脂酰胆碱(PC)和胆固醇(CHOL)的混合物很容易形成在生理条件下稳定的倒置双连续立方相。对于摩尔比为1：1和3：7的二油酰基PC/Chol混合物的这一影响进行了详细的研究。在磷酸盐缓冲溶液中、蔗糖溶液中、在混合物Lpha-HII转变温度附近的水中以及在HII相冷却期间，分别容易形成晶格常数为30-50 nm和25-30 nm的Im3m和Pn3m相。一旦形成，立方相就显示出在广泛的生理温度范围内过冷和替换初始Lpha相的能力。Chol与二甘油酰基PC的混合物也观察到了稳定的立方相，而与棕榈酰亚油酰基PC或棕榈酰油酰基PC的混合物则没有观察到稳定的立方相，它们在高温下只记录到了短暂的立方相。一种饱和的支链PC，二植酸基PC，在与Chol的混合物中也显示出立方相。与PE不同，膜PC本质上是非融合性脂类：在过量的水中，它们只形成片层相，而不是任何本身的倒相。因此，Chol在与不饱和PC的混合物中诱导立方相的发现可能对其在熔融中的作用具有重要的意义。在三元混合物中，饱和的PC和鞘磷脂与Chol一起被分离成液体有序结构域。因此，我们的结果表明，被排除在这些结构域之外的不饱和PC可以与Chol形成融合结构域。Chol的这种双重作用可能解释了细胞膜看似矛盾的能力，即同时形成坚硬的、低曲率的木筏状斑块，同时仍然能够进行容易的膜融合。
X-ray diffraction reveals that mixtures of some unsaturated phosphatidylcholines (PCs) with cholesterol (Chol) readily form inverted bicontinuous cubic phases that are stable under physiological conditions. This effect was studied in most detail for dioleoyl PC/Chol mixtures with molar ratios of 1:1 and 3:7. Facile formation of Im3m and Pn3m phases with lattice constants of 30-50 nm and 25-30 nm, respectively, took place in phosphate-buffered saline, in sucrose solution, and in water near the temperature of the Lalpha-HII transition of the mixtures, as well as during cooling of the HII phase. Once formed, the cubic phases displayed an ability to supercool and replace the initial Lalpha phase over a broad range of physiological temperatures. Conversion into stable cubic phases was also observed for mixtures of Chol with dilinoleoyl PC but not for mixtures with palmitoyl-linoleoyl PC or palmitoyl-oleoyl PC, for which only transient cubic traces were recorded at elevated temperatures. A saturated, branched-chain PC, diphytanoyl PC, also displayed a cubic phase in mixture with Chol. Unlike the PEs, the membrane PCs are intrinsically nonfusogenic lipids: in excess water they only form lamellar phases and not any of the inverted phases on their own. Thus, the finding that Chol induces cubic phases in mixtures with unsaturated PCs may have important implications for its role in fusion. In ternary mixtures, saturated PCs and sphingomyelin are known to separate into liquid-ordered domains along with Chol. Our results thus suggest that unsaturated PCs, which are excluded from these domains, could form fusogenic domains with Chol. Such a dual role of Chol may explain the seemingly paradoxical ability of cell membranes to simultaneously form rigid, low-curvature raft-like patches while still being able to undergo facile membrane fusion.