GLTP Mediated Non-Vesicular GM1 Transport between Native Membranes

Lipid transfer proteins (LTPs) are emerging as key players in lipid homeostasis by mediating non-vesicular transport steps between two membrane surfaces. Little is known about the driving force that governs the direction of transport in cells. Using the soluble LTP glycolipid transfer protein (GLTP), we examined GM1 (monosialotetrahexosyl-ganglioside) transfer to native membrane surfaces. With artificial GM1 donor liposomes, GLTP can be used to increase glycolipid levels over natural levels in either side of the membrane leaflet, i.e., external or cytosolic. In a system with native donor- and acceptor-membranes, we find that GLTP balances highly variable GM1 concentrations in a population of membranes from one cell type, and in addition, transfers lipids between membranes from different cell types. Glycolipid transport is highly efficient, independent of cofactors, solely driven by the chemical potential of GM1 and not discriminating between the extra- and intracellular membrane leaflet. We conclude that GLTP mediated non-vesicular lipid trafficking between native membranes is driven by simple thermodynamic principles and that for intracellular transport less than 1 µM GLTP would be required in the cytosol. Furthermore, the data demonstrates the suitability of GLTP as a tool for artificially increasing glycolipid levels in cellular membranes.

脂质转运蛋白（Lipid transfer proteins, LTPs）正逐渐成为脂质稳态调控的关键因子，可介导两个膜表面之间的非囊泡转运过程。目前人们对调控细胞内脂质转运方向的驱动力仍知之甚少。本研究以可溶性糖脂转运蛋白（Glycolipid transfer protein, GLTP）为研究模型，探究了单唾液酸四己糖神经节苷脂（monosialotetrahexosyl-ganglioside, GM1）向天然膜表面的转运过程。通过人工构建的GM1供体脂质体，GLTP可使膜小叶的任意一侧（即胞外侧或胞质侧）的糖脂水平高于天然状态下的水平。在包含天然供体膜与受体膜的实验体系中，研究团队发现GLTP可平衡同一细胞类型的膜群体中高度不均一的GM1浓度，此外还能介导不同细胞类型的膜之间的脂质转运。糖脂转运效率极高，且不依赖任何辅因子，仅由GM1的化学势驱动，不会对胞外侧与胞质侧膜小叶产生选择性偏好。本研究结论表明，GLTP介导的天然膜之间非囊泡脂质运输由基础热力学原理驱动，且胞质内仅需低于1 μM的GLTP即可完成胞内转运。此外，本研究数据证实，GLTP可作为人工提升细胞膜糖脂水平的实用工具。