Institute for Theoretical Physics, Georg-August University, 37077 Göttingen, Germany;
Institute for Theoretical Physics, Georg-August University, 37077 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2019 Feb 12;116(7):2571-2576. doi: 10.1073/pnas.1818200116. Epub 2019 Jan 30.
Biological membrane fusion proceeds via an essential topological transition of the two membranes involved. Known players such as certain lipid species and fusion proteins are generally believed to alter the free energy and thus the rate of the fusion reaction. Quantifying these effects by theory poses a major challenge since the essential reaction intermediates are collective, diffusive and of a molecular length scale. We conducted molecular dynamics simulations in conjunction with a state-of-the-art string method to resolve the minimum free-energy path of the first fusion intermediate state, the so-called stalk. We demonstrate that the isolated transmembrane domains (TMDs) of fusion proteins such as SNARE molecules drastically lower the free energy of both the stalk barrier and metastable stalk, which is not trivially explained by molecular shape arguments. We relate this effect to the local thinning of the membrane (negative hydrophobic mismatch) imposed by the TMDs which favors the nearby presence of the highly bent stalk structure or prestalk dimple. The distance between the membranes is the most crucial determinant of the free energy of the stalk, whereas the free-energy barrier changes only slightly. Surprisingly, fusion enhancing lipids, i.e., lipids with a negative spontaneous curvature, such as PE lipids have little effect on the free energy of the stalk barrier, likely because of its single molecular nature. In contrast, the lipid shape plays a crucial role in overcoming the hydration repulsion between two membranes and thus rather lowers the total work required to form a stalk.
生物膜融合通过涉及的两个膜的基本拓扑转变进行。已知的参与者，如某些脂质种类和融合蛋白，通常被认为会改变自由能，从而改变融合反应的速率。通过理论来量化这些影响是一个重大挑战，因为基本的反应中间体是集体的、扩散的和分子长度尺度的。我们进行了分子动力学模拟，并结合了最先进的弦方法，以确定第一个融合中间状态（所谓的茎）的最小自由能路径。我们证明，融合蛋白（如 SNARE 分子）的跨膜结构域（TMD）会极大地降低茎的自由能障碍和亚稳态茎的自由能，这不能简单地用分子形状来解释。我们将这种效应与 TMD 施加的膜局部变薄（负疏水失配）联系起来，这有利于高度弯曲的茎结构或前茎凹陷的存在。膜之间的距离是茎自由能的最关键决定因素，而自由能障碍变化很小。令人惊讶的是，融合增强脂质，即具有负自发曲率的脂质，如 PE 脂质，对茎的自由能障碍几乎没有影响，可能是因为其单一分子性质。相比之下，脂质形状在克服两个膜之间的水合排斥方面起着至关重要的作用，因此可以降低形成茎所需的总工作量。
