Shendrik Petr, Sorkin Raya, Golani Gonen
School of Chemistry, Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel.
Department of Physics, University of Haifa, Haifa, 3498838, Israel.
Faraday Discuss. 2025 May 19. doi: 10.1039/d4fd00189c.
The fusion of lipid membranes progresses through a series of intermediate steps with two significant energy barriers: hemifusion-stalk formation and fusion-pore expansion. The cell's ability to tune these energy barriers is crucial as they determine the rate of many biological processes involving membrane fusion. However, a mechanism that allows the cell to manipulate both barriers in the same direction remains elusive, since membrane properties that the cell could dynamically tune during its life cycle, such as the lipids' spontaneous curvatures and membrane tension, have an opposite effect on the two barriers: tension inhibits stalk formation while promoting fusion-pore expansion. In contrast, increasing the total membrane concentration of lipids with negative intrinsic curvatures, such as cholesterol, promotes hemifusion-stalk formation while inhibiting pore expansion, and for lipids with positive intrinsic curvatures. Therefore, changes in these membrane properties increase one energy barrier at the expense of the other, resulting in a mixed effect on the fusion reaction. A possible mechanism to change both barriers in the same direction is by inducing lipid composition asymmetry, which results in tension and spontaneous curvature differences between the monolayers. To test the feasibility of this mechanism, a continuum elastic model was used to simulate the fusion intermediates and calculate the changes in the energy barriers. The calculations showed that a reasonable lipid composition asymmetry could lead to a 10-20 difference in both energy barriers, depending on the direction from which fusion occurs. We further provide experimental support to the model predictions, demonstrating changes in the time to hemifusion upon asymmetry introduction. These results indicate that biological membranes, which are asymmetric, have a preferred direction for fusion.
脂质膜的融合通过一系列中间步骤进行，存在两个显著的能量屏障：半融合柄的形成和融合孔的扩张。细胞调节这些能量屏障的能力至关重要，因为它们决定了许多涉及膜融合的生物过程的速率。然而，一种能让细胞在同一方向上操纵这两个屏障的机制仍然难以捉摸，因为细胞在其生命周期中可以动态调节的膜特性，如脂质的自发曲率和膜张力，对这两个屏障有相反的影响：张力抑制柄的形成，同时促进融合孔的扩张。相比之下，增加具有负固有曲率的脂质（如胆固醇）的总膜浓度，会促进半融合柄的形成，同时抑制孔的扩张，而对于具有正固有曲率的脂质则相反。因此，这些膜特性的变化会以牺牲另一个为代价增加一个能量屏障，从而对融合反应产生混合效应。一种在同一方向上改变两个屏障的可能机制是诱导脂质组成不对称，这会导致单分子层之间的张力和自发曲率差异。为了测试这种机制的可行性，使用了连续弹性模型来模拟融合中间体并计算能量屏障的变化。计算结果表明，合理的脂质组成不对称可能导致两个能量屏障相差10 - 20倍，这取决于融合发生的方向。我们进一步为模型预测提供了实验支持，证明了引入不对称后半融合时间的变化。这些结果表明，不对称的生物膜具有优先的融合方向。
