Bijvoet Center and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands.
Cold Spring Harb Perspect Biol. 2011 May 1;3(5):a004671. doi: 10.1101/cshperspect.a004671.
Cells have thousands of different lipids. In the plasma membrane, and in membranes of the late secretory and endocytotic pathways, these lipids are not evenly distributed over the two leaflets of the lipid bilayer. The basis for this transmembrane lipid asymmetry lies in the fact that glycerolipids are primarily synthesized on the cytosolic and sphingolipids on the noncytosolic surface of cellular membranes, that cholesterol has a higher affinity for sphingolipids than for glycerolipids. In addition, P4-ATPases, "flippases," actively translocate the aminophospholipids phosphatidylserine and phosphatidylethanolamine to the cytosolic surface. ABC transporters translocate lipids in the opposite direction but they generally act as exporters rather than "floppases." The steady state asymmetry of the lipids can be disrupted within seconds by the activation of phospholipases and scramblases. The asymmetric lipid distribution has multiple implications for physiological events at the membrane surface. Moreover, the active translocation also contributes to the generation of curvature in the budding of transport vesicles.
细胞内有数千种不同的脂质。在质膜和晚期分泌及内吞途径的膜中，这些脂质在脂双层的两个叶面上并不是均匀分布的。这种跨膜脂质不对称性的基础在于甘油磷脂主要在细胞质面合成，而鞘脂则在细胞膜的非细胞质面合成，胆固醇与鞘脂的亲和力高于与甘油磷脂的亲和力。此外，P4-ATP 酶（“翻转酶”）可将氨基磷脂磷脂酰丝氨酸和磷脂酰乙醇胺主动转运到细胞质面。ABC 转运蛋白可向相反方向转运脂质，但通常它们作为外排体而不是“翻转酶”发挥作用。在几秒钟内，通过磷脂酶和 scramblase 的激活，可以破坏脂质的稳态不对称性。脂质的不对称分布对膜表面的生理事件有多种影响。此外，主动转运也有助于运输小泡出芽时的曲率产生。
