Physics Department, University of Illinois at Chicago , Chicago, Illinois 60607, United States.
Materials Science Division, Argonne National Laboratory , Lemont, Illinois 60439, United States.
Langmuir. 2017 Apr 11;33(14):3384-3394. doi: 10.1021/acs.langmuir.6b04013. Epub 2017 Mar 29.
Maintaining compositional lipid gradients across membranes in animal cells is essential to biological function, but what is the energetic cost to maintain these differences? It has long been recognized that studying the passive movement of lipids in membranes can provide insight into this toll. Confusingly the reported values of inter- and, particularly, intra-lipid transport rates of lipids in membranes show significant differences. To overcome this difficulty, biases introduced by experimental approaches have to be identified. The present study addresses the difference in the reported intramembrane transport rates of dimyristoylphosphatidylcholine (DMPC) on flat solid supports (fast flipping) and in curved free-standing membranes (slow flipping). Two possible scenarios are potentially at play: one is the difference in curvature of the membranes studied and the other the presence (or not) of the support. Using DMPC vesicles and DMPC supported membranes on silica nanoparticles of different radii, we found that an increase in curvature (from a diameter of 30 nm to a diameter of 100 nm) does not change the rates significantly, differing only by factors of order ∼1. Additionally, we found that the exchange rates of DMPC in supported membranes are similar to the ones in vesicles. And as previously reported, we found that the activation energies for exchange on free-standing and supported membranes are similar (84 and 78 kJ/mol, respectively). However, DMPC's flip-flop rates increase significantly when in a supported membrane, surpassing the exchange rates and no longer limiting the exchange process. Although the presence of holes or cracks in supported membranes explains the occurrence of fast lipid flip-flop in many studies, in defect-free supported membranes we find that fast flip-flop is driven by the surface's induced disorder of the bilayer's acyl chain packing as evidenced from their broad melting temperature behavior.
在动物细胞中维持膜的组成脂质梯度对于生物功能至关重要，但维持这些差异的能量成本是多少？长期以来，人们一直认为研究脂质在膜中的被动运动可以深入了解这种代价。令人困惑的是，报道的膜中脂质的内-和，特别是，内脂质转运速率值显示出显著的差异。为了克服这一困难，必须确定实验方法引入的偏差。本研究解决了在平坦的固体支撑物（快速翻转）和弯曲的自由-standing 膜（缓慢翻转）中报道的二肉豆蔻酰磷脂酰胆碱（DMPC）的膜内转运速率的差异。两种可能的情况可能起作用：一种是所研究的膜的曲率差异，另一种是支撑物的存在（或不存在）。使用 DMPC 囊泡和在不同半径的硅纳米粒子上的 DMPC 支撑膜，我们发现曲率的增加（从直径 30nm 到直径 100nm）不会显著改变速率，仅相差约 1 的因子。此外，我们发现 DMPC 在支撑膜中的交换速率与囊泡中的相似。并且如前所述，我们发现自由-standing 和支撑膜上的交换的活化能相似（分别为 84 和 78kJ/mol）。然而，当 DMPC 在支撑膜中时，其翻转速率显著增加，超过了交换速率，不再限制交换过程。尽管支撑膜中的孔或裂缝的存在解释了许多研究中快速脂质翻转的发生，但在无缺陷的支撑膜中，我们发现快速翻转是由双层酰链排列的表面诱导无序驱动的，这从其广泛的熔融温度行为中可以得到证明。
