Brown Deborah
Department of Biochemistry and Cell Biology, SUNY at Stony Brook, NY 11794-5215, USA.
Int J Med Microbiol. 2002 Feb;291(6-7):433-7. doi: 10.1078/1438-4221-00150.
Lipids do not always mix uniformly in membranes, but can cluster to form microdomains. We will consider one type of microdomain that can form in cell membranes. These are enriched in cholesterol and sphingolipids, and are referred to as rafts. Rafts probably exist in membranes in the liquid-ordered phase or a phase with similar properties. We will briefly review membrane lipid phase behavior, and the differences between liquid-crystalline, liquid-ordered, and gel-phase membrane bilayer domains. We will present evidence suggesting that phospholipid-rich, liquid-crystalline phase domains and sphingolipid-rich, liquid-ordered phase domains (rafts) can exist in equilibrium in biological membranes, especially the plasma membrane. Preferential partitioning of membrane proteins into rafts can affect function. Among the proteins that are targeted to rafts are those anchored in the outer leaflet of the membrane through covalent attachment to a special glycolipid, glycosyl phosphatidylinositol (GPI). Other proteins that are linked to saturated acyl chains, such as those that are directly acylated with two or more palmitate chains, or a palmitate and a myristate chain, are also targeted to rafts. Targeting of GPI-anchored proteins and other proteins to rafts plays a role in signal transduction in hematopoietic cells, and possibly also in sorting in intracellular membranes and regulation of cell-surface proteolysis in other mammalian cells.
脂质在膜中并不总是均匀混合，而是可以聚集形成微区。我们将考虑一种可在细胞膜中形成的微区类型。这些微区富含胆固醇和鞘脂，被称为脂筏。脂筏可能以液晶态或具有类似性质的相存在于膜中。我们将简要回顾膜脂的相行为，以及液晶相、液晶有序相和凝胶相膜双层结构域之间的差异。我们将提供证据表明，富含磷脂的液晶相结构域和富含鞘脂的液晶有序相结构域（脂筏）可以在生物膜，尤其是质膜中处于平衡状态。膜蛋白优先分配到脂筏中会影响其功能。定位于脂筏的蛋白质包括那些通过与一种特殊糖脂——糖基磷脂酰肌醇（GPI）共价连接而锚定在膜外小叶的蛋白质。其他与饱和酰基链相连的蛋白质，例如那些直接被两条或更多条棕榈酸链或一条棕榈酸链和一条肉豆蔻酸链酰化的蛋白质，也定位于脂筏。GPI锚定蛋白和其他蛋白质定位于脂筏在造血细胞的信号转导中起作用，在其他哺乳动物细胞的内膜分选和细胞表面蛋白水解调控中可能也起作用。
