Department of Biochemistry and Cell Biology, State University of New York, Stony Brook, NY, USA.
Biophys J. 2012 Dec 5;103(11):2311-9. doi: 10.1016/j.bpj.2012.10.033.
A long-standing question about membrane structure and function is the degree to which the physical properties of the inner and outer leaflets of a bilayer are coupled to one another. Using our recently developed methods to prepare asymmetric vesicles, coupling was investigated for vesicles containing phosphatidylcholine (PC) in the inner leaflet and sphingomyelin (SM) in the outer leaflet. The coupling of both lateral diffusion and membrane order was monitored as a function of PC and SM acyl chain structure. The presence in the outer leaflet of brain SM, which decreased outer-leaflet lateral diffusion, had little effect upon lateral diffusion in inner leaflets composed of dioleoyl PC (i.e., diffusion was only weakly coupled in the two leaflets) but did greatly reduce lateral diffusion in inner leaflets composed of PC with one saturated and one oleoyl acyl chain (i.e., diffusion was strongly coupled in these cases). In addition, reduced outer-leaflet diffusion upon introduction of outer-leaflet milk SM or a synthetic C24:0 SM, both of which have long interdigitating acyl chains, also greatly reduce diffusion of inner leaflets composed of dioleoyl PC, indicative of strong coupling. Strikingly, several assays showed that the ordering of the outer leaflet induced by the presence of SM was not reflected in increased lipid order in the inner leaflet, i.e., there was no detectable coupling between inner and outer leaflet membrane order. We propose a model for how lateral diffusion can be coupled in opposite leaflets and discuss how this might impact membrane function.
关于膜结构和功能的一个长期存在的问题是双层膜的内外叶层的物理性质相互耦合的程度。利用我们最近开发的制备不对称囊泡的方法，研究了含有内层磷脂酰胆碱（PC）和外层鞘磷脂（SM）的囊泡的耦合。作为 PC 和 SM 酰基链结构的函数，监测了侧向扩散和膜有序性的耦合。在外层叶中存在脑 SM，其降低了外层叶的侧向扩散，对内层叶中由二油酰基 PC 组成的扩散几乎没有影响（即扩散在两个叶层中仅弱耦合），但大大降低了内层叶中由一个饱和和一个油酰基酰链组成的扩散（即，在这些情况下扩散是强耦合的）。此外，在外层叶中引入具有长互贯酰链的外层叶牛奶 SM 或合成的 C24:0 SM 会降低外层叶扩散，这也大大降低了由二油酰基 PC 组成的内层叶的扩散，表明存在强耦合。引人注目的是，几种测定表明，SM 存在诱导的外层叶的有序性并没有反映在内层叶中脂质有序性的增加，即内层叶和外层叶膜有序性之间没有可检测到的耦合。我们提出了一种模型，说明如何在相反的叶层中实现侧向扩散的耦合，并讨论了这可能如何影响膜功能。
