Domains in biologically relevant lipid membranes

细胞被膜包围着——脂质和蛋白质的复杂组合，在水环境中，脂质自然倾向于形成两层，只有两个分子厚。几乎所有的生物膜都以液晶的物理状态存在，膜成分可以在自己的层内扩散，但通常不能自发地跳到另一层。膜脂具有很大的结构可变性，最近的证据表明，许多生物膜含有结构域，可通过组成和物理特性加以区分。这些结构域通常富含鞘脂，鞘脂往往具有相对较高的融化温度（想想猪油与橄榄油的对比），以及胆固醇等固醇。基于对“模型”（即无蛋白膜）的研究，假设这些结构域具有不同的物理状态（“相”），其中脂质具有比周围膜更“有序”的液晶结构。膜内长时间的相共存强烈影响膜特性：局部膜厚度、膜内脂质和蛋白质的扩散、脂质和蛋白质之间的相互作用以及膜蛋白的功能/柔韧性都可能在细胞膜中受到影响。我们感兴趣的是绘制出模型膜中磷脂、甾醇和鞘脂之间的基本相互作用，因为尽管它们构成了其他细胞膜成分之间相互作用的背景，但其中许多相互作用仍知之甚少。氘核磁共振（NMR）是研究膜中相共存的一种强有力的实验方法：不同相导致清晰可区分的光谱，并且可以明确地测量特定相中标记脂质的数量。重要的是，氘核磁共振不依赖于大型的非天然探针来监测膜结构域，因为即使少量的这种探针也可以改变结构域的性质。从拟议的研究中获得的知识将有助于细胞生物学家研究与膜结构域相关的现象，例如程序性细胞死亡、蛋白质定位/信号传导、神经元成熟和细菌/病毒感染。

Cells are surrounded by membranes - complex assemblies of lipids and proteins held together by lipids' natural tendency to form bilayers, only two molecules thick, in watery environments. Nearly all biological membranes exist in a liquid crystalline physical state, where membrane components can diffuse within their own layer but typically cannot spontaneously hop to the opposite layer. Membrane lipids have large structural variability and recent evidence suggests that many biological membranes contain domains, distinguishable by both composition and physical characteristics. These domains are often enriched in sphingolipids, which tend in isolation to have relatively high melting temperatures (think of lard compared with olive oil), and sterols such as cholesterol. Based on work with 'model', i.e. protein-free, membranes, such domains are postulated to have a different physical state ("phase"), where the lipids have a more 'orderly' liquid crystalline structure than the surrounding membrane. Long-lived phase coexistence within a membrane strongly influences membrane character: local membrane thickness, diffusion of lipids and proteins within the membrane, interactions between lipids and proteins, and membrane protein function/flexibility could all be affected in cell membranes. We are interested in mapping out the fundamental interactions between phospholipids, sterols and sphingolipids in model membranes, as many of these are poorly understood even though they form the backdrop for interactions among other cell membrane components. Deuterium nuclear magnetic resonance (NMR) is a powerful experimental approach to the study of phase coexistence in membranes: different phases result in clearly distinguishable spectra, and the quantity of labeled lipid in a particular phase can be measured unambiguously. Importantly, deuterium NMR does not rely on large unnatural probes to monitor membrane domains, as even small amounts of such probes can modify domain properties. Knowledge gained from the proposed research will aid cell biologists investigating phenomena related to membrane domains, e.g. programmed cell death, protein localization/signaling, neuronal maturation and bacterial/viral infection.