Devaux Philippe F, Morris Roger
Institut de Biologie Physico-Chimique, 13 rue Pierre et Marie 75005 Paris, France.
Traffic. 2004 Apr;5(4):241-6. doi: 10.1111/j.1600-0854.2004.0170.x.
It is generally assumed that rafts exist in both the external and internal leaflets of the membrane, and that they overlap so that they are coupled functionally and structurally. However, the two monolayers of the plasma membrane of eukaryotic cells have different chemical compositions. This out-of-equilibrium situation is maintained by the activity of lipid translocases, which compensate for the slow spontaneous transverse diffusion of lipids. Thus rafts in the outer leaflet, corresponding to domains enriched in sphingomyelin and cholesterol, cannot be mirrored in the inner cytoplasmic leaflet. The extent to which lipids contribute to raft properties can be conveniently studied in giant unilamellar vesicles. In these, cholesterol can be seen to condense with saturated sphingolipids or phosphatidylcholine to form microm scale domains. However, such rafts fail to model biological rafts because they are symmetric, and because their membranes lack the mechanism that establishes this asymmetry, namely proteins. Biological rafts are in general of nm scale, and almost certainly differ in size and stability in inner and outer monolayers. Any coupling between rafts in the two leaflets, should it occur, is probably transient and dependent not upon the properties of lipids, but on transmembrane proteins within the rafts.
一般认为，筏在膜的外层和内层小叶中均存在，且它们相互重叠，从而在功能和结构上相互耦合。然而，真核细胞膜的两个单层具有不同的化学组成。这种失衡状态由脂质转位酶的活性维持，脂质转位酶可补偿脂质缓慢的自发横向扩散。因此，外层小叶中的筏，对应于富含鞘磷脂和胆固醇的结构域，在细胞质内层小叶中无法镜像存在。脂质对筏特性的贡献程度可在巨型单层囊泡中方便地进行研究。在这些囊泡中，可以看到胆固醇与饱和鞘脂或磷脂酰胆碱凝聚形成微米级别的结构域。然而，这种筏无法模拟生物筏，因为它们是对称的，且其膜缺乏建立这种不对称性的机制，即蛋白质。生物筏通常为纳米级，几乎可以肯定的是，其在内层和外层单层中的大小和稳定性不同。两个小叶中的筏之间的任何耦合，如果发生的话，可能是短暂的，且不取决于脂质的特性，而是取决于筏内的跨膜蛋白。
