細胞膜融合在生物系統中扮演了非常重要的角色，例如細胞間的物質傳遞；不過其基本的融合機制卻尚未完全瞭解。在本篇論文中，我們使用全原子分子動態模擬來探討由鈣離子誘導POPE組成的微胞(Micelle)及囊泡(Vesicle)自發性融合的反應機制。研究結果顯示鈣離子具有較好的催化能力來加速POPE微胞的融合，相對的，在含鈉離子或鎂離子的系統中，我們並未觀察到微胞發生融合或是彼此互相靠近的現象。鈣離子在催化膜融合的過程中扮演以下幾種角色：(1). 鈣離子會使微胞的表面變得更具疏水性，引發兩個微胞互相靠近；(2). 鈣離子可以同時吸附在兩顆微胞之間，將兩顆微胞限制在一定的距離；(3). 鈣離子會使微胞變的較不穩定，增加生成pre-stalk state的機率。細胞膜融合的過程依序生成的步驟分別為pre-stalk、stalk、hemi-fused以及fused state。Pre-stalk 是整個融合過程的速率決定步驟，其特徵是將疏水性的磷脂質長碳鏈暴露至微胞表面或水層上。而stalk state是由疏水性的長碳鏈在兩顆微胞之間聚集形成一個具有疏水性核心的結構，其生成的過程中會伴隨著排水現象的發生。而在大尺寸的囊泡模擬中，我們也觀察到stalk state的結構中不只含有外層磷脂質，同時也含有些微的內層磷脂質；而hemi-fused state的橫膈膜只由內層磷脂質所組成。除此之外，模擬結果中也發現多個穩定的hemi-fused state。在本篇研究中，我們從能量、結構以及動力學等不同的角度來觀察細胞膜融合的分子機制。 Although membrane fusion plays key roles in many biological functions, its underlying molecular mechanism remains poorly understood. We employed all-atom molecular dynamics simulations to investigate the fusion mechanism, catalyzed by Ca2+ ions, of two highly hydrated 1-palmitoyl-2-oleoyl-sn-3-phosphoethanolamine (POPE) micelles and vesicels. Our simulations revealed that Ca2+ ions are capable of catalyzing the fusion of POPE micelles in contrast, we did not observe close contact of the two micelles in the presence of only Na+ or Mg2+ ions. The Ca2+ ions play a key role in catalyzing the micelle fusion in three aspects: creating a more-hydrophobic surface on the micelles, binding two micelles together, and enhancing the formation of the pre-stalk state. Effective fusion proceeds through sequential formation of pre-stalk, stalk, hemifused-like, and fused states. The pre-stalk state is a state featuring solvent-exposed lipid tails its formation is the rate-limiting step. The stalk state is a state where a localized hydrophobic core is formed connecting two micelles its formation occurs in conjunction with a dehydration process between two micelles. Our large-scale simulations of vesicle fusion show the stalk formation is mainly dominated by the outer lipids, but it also involves with the inner lipids. On the other hand, the formation of hemi-fused state is dominated by the inner lipids to a diaphragm. Moreover, more than one stable hemi-fused state can be formed. This study provides insight into the molecular mechanism of membrane fusion from the points of view of energetics, structure, and dynamics.
