Order of lipid phases in model and plasma membranes
Order of lipid phases in model and plasma membranes
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DOI：
10.1073/pnas.0908987106
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发表时间：
2009-09-29
影响因子：
11.1
通讯作者：
Simons, Kai
中科院分区：
文献类型：
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作者：
Kaiser, Hermann-Josef;Lingwood, Daniel;Simons, Kai
脂筏是鞘脂、胆固醇和特定膜蛋白的纳米级组装体，有助于真核细胞膜的横向异质性。将人造膜分离成液体有序相（Lo）和液体无序相被认为是这种划分的常见模型。然而，Lo 相中紧密的脂质堆积似乎与筏相关跨膜 (TM) 蛋白的有效分配相冲突。为了评估膜秩序作为筏组织的组成部分，我们使用膜探针 Laurdan 和 C-laurdan 进行了荧光光谱和显微镜检查。首先，我们评估了各种成分的模型膜中的脂质堆积，并发现了膜顺序的胆固醇和酰基链依赖性。然后，我们通过使用两种表现出诱导相分离的新颖系统来探测细胞膜：巨型质膜囊泡[Baumgart 等人。 (2007)Proc Natl Acad Sci USA 104:3165-3170]和质膜球。值得注意的是，只有后者支持将 raft TM 蛋白与神经节苷脂 GM1 选择性包含到一个相中。我们测量了两种生物膜的分离相之间顺序上相当小的差异。巨型质膜囊泡有序相中的横向堆积类似于模型膜的 Lo 结构域，而质膜球中的 GM1 相表现出相当低的有序性，这与脂质和 TM 蛋白标记物的不同分配一致。因此，脂质介导的 GM1 筏结构域的聚结似乎与 Lo 相的形成不同，这表明蛋白质和脂质之间的额外相互作用是有效的。
Lipid rafts are nanoscopic assemblies of sphingolipids, cholesterol, and specific membrane proteins that contribute to lateral heterogeneity in eukaryotic membranes. Separation of artificial membranes into liquid-ordered (Lo) and liquid-disordered phases is regarded as a common model for this compartmentalization. However, tight lipid packing in Lo phases seems to conflict with efficient partitioning of raft-associated transmembrane (TM) proteins. To assess membrane order as a component of raft organization, we performed fluorescence spectroscopy and microscopy with the membrane probes Laurdan and C-laurdan. First, we assessed lipid packing in model membranes of various compositions and found cholesterol and acyl chain dependence of membrane order. Then we probed cell membranes by using two novel systems that exhibit inducible phase separation: giant plasma membrane vesicles [Baumgart et al. (2007) Proc Natl Acad Sci USA 104: 3165-3170] and plasma membrane spheres. Notably, only the latter support selective inclusion of raft TM proteins with the ganglioside GM1 into one phase. We measured comparable small differences in order between the separated phases of both biomembranes. Lateral packing in the ordered phase of giant plasma membrane vesicles resembled the Lo domain of model membranes, whereas the GM1 phase in plasma membrane spheres exhibited considerably lower order, consistent with different partitioning of lipid and TM protein markers. Thus, lipid-mediated coalescence of the GM1 raft domain seems to be distinct from the formation of a Lo phase, suggesting additional interactions between proteins and lipids to be effective.