Lin Qingqing, London Erwin
Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, New York.
Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, New York.
Biophys J. 2015 May 5;108(9):2212-22. doi: 10.1016/j.bpj.2015.03.056.
Sphingolipid- and cholesterol-rich liquid-ordered (Lo) lipid domains (rafts) are thought to be important organizing elements in eukaryotic plasma membranes. How they form in the sphingolipid-poor cytosolic (inner) membrane leaflet is unclear. Here, we characterize how outer-leaflet Lo domains induce inner-leaflet-ordered domains, i.e., interleaflet coupling. Asymmetric vesicles studied contained physiologically relevant cholesterol levels (∼ 37 mol %), a mixture of SM (sphingomyelin) and DOPC (dioleoylphosphatidylcholine) in their outer leaflets, and DOPC in their inner leaflets. Lo domains were observed in both leaflets, and were in register, indicative of coupling between SM-rich outer-leaflet-ordered domains and inner-leaflet-ordered domains. For asymmetric vesicles with outer-leaflet egg SM or milk SM, a fluorescent lipid with unsaturated acyl chains (NBD-DOPE) was depleted in both the outer- and inner-leaflet-ordered domains. This suggests the inner-leaflet-ordered domains were depleted in unsaturated lipid (i.e., DOPC) and thus rich in cholesterol. For asymmetric vesicles containing egg SM, outer-leaflet Lo domains were also depleted in a saturated fluorescent lipid (NBD-DPPE), while inner-leaflet Lo domains were not. This indicates that inner- and outer-leaflet Lo domains can have significantly different physical properties. In contrast, in asymmetric vesicles containing outer-leaflet milk SM, which has long acyl chains capable of interdigitating into the inner leaflet, both outer- and inner-leaflet Lo domains were depleted, to a similar extent, in NBD-DPPE. This is indicative of interdigitation-enhanced coupling resulting in inner- and outer-leaflet Lo domains with similar physical properties.
富含鞘脂和胆固醇的液态有序（Lo）脂结构域（脂筏）被认为是真核细胞质膜中的重要组织成分。它们如何在鞘脂含量低的胞质（内）膜小叶中形成尚不清楚。在这里，我们描述了外层小叶Lo结构域如何诱导内层小叶有序结构域，即叶间偶联。所研究的不对称囊泡在外层小叶中含有生理相关水平的胆固醇（约37摩尔%）、鞘磷脂（SM）和二油酰磷脂酰胆碱（DOPC）的混合物，在内层小叶中含有DOPC。在两个小叶中均观察到Lo结构域，且它们是对齐的，这表明富含SM的外层小叶有序结构域与内层小叶有序结构域之间存在偶联。对于外层小叶含有鸡蛋SM或牛奶SM的不对称囊泡，具有不饱和酰基链的荧光脂质（NBD-DOPE）在外层和内层小叶有序结构域中均减少。这表明内层小叶有序结构域中不饱和脂质（即DOPC）减少，因此富含胆固醇。对于含有鸡蛋SM的不对称囊泡，外层小叶Lo结构域在饱和荧光脂质（NBD-DPPE）中也减少，而内层小叶Lo结构域则没有。这表明内层和外层小叶Lo结构域可能具有显著不同的物理性质。相比之下，在含有外层小叶牛奶SM的不对称囊泡中，其长酰基链能够插入内层小叶，外层和内层小叶Lo结构域在NBD-DPPE中的减少程度相似。这表明相互穿插增强的偶联导致内层和外层小叶Lo结构域具有相似的物理性质。
