Lipid composition and properties play an important role in many cellular properties such as fusion of vesicles to cell membranes, an essential process for exocytosis. Using a model system composed of artificial vesicles (liposomes) and artificial membranes (planar lipid bilayers), we observed that fusion is significantly affected by the lipid phase of the planar membrane. To determine the effect of lipid phases on fusion rates, we utilized the nystatin/ergosterol fusion assay and stimulated fusion with an osmotic gradient. Phase of the planar membrane was altered by changing cholesterol or temperature while the vesicular lipids were held constant. Liquid disordered (L(d) or L(α)) planar membranes were formed from phosphatidylethanolamine and phosphatidylcholine with unsaturated acyl chains. Addition of cholesterol shifts these membranes to the liquid ordered (L(o)) phase and increases liposome fusion. Planar membranes in the L(α) phase were also made from dipalmitoylphoshatidylcholine (DPPC) above the transition temperature (T(m)) of 41.5 ° C. Decreasing the temperature below T(m) shifts these membranes into the ripple phase (P(β')) and also increases liposome fusion. The cholesterol and temperature data are consistent with the hypothesis that fusion is promoted in membranes that have greater exposure of their lipid tails or in membranes which can form leaflet domains with negative curvature. The data are not consistent with the hypothesis that lipid mismatch drives fusion.

译文

脂质的组成和性质在许多细胞性质中起着重要作用,例如囊泡与细胞膜的融合是胞吐作用的重要过程。使用由人工囊泡(脂质体)和人工膜(平面脂质双层)组成的模型系统,我们观察到融合受到平面膜脂质相的显着影响。为了确定脂质相对融合速率的影响,我们利用制霉菌素/麦角固醇融合测定法并通过渗透梯度刺激融合。通过改变胆固醇或温度来改变平面膜的相,而囊泡脂质保持恒定。液体无序(L(d)或L(α))平面膜由具有不饱和酰基链的磷脂酰乙醇胺和磷脂酰胆碱形成。胆固醇的添加将这些膜转移到液体有序(L(o))相,并增加脂质体融合。 L(α)相中的平面膜也是由高于过渡温度(T(m))41.5°C的二棕榈酰磷脂酰胆碱(DPPC)制成的。降低温度至T(m)以下会使这些膜转变为波纹相(P( β')),还可以增加脂质体融合。胆固醇和温度数据与以下假设相一致:在脂类尾巴暴露量更大的膜中或在可形成具有负曲率的小叶区域的膜中促进融合。数据与脂质错配驱动融合的假设不一致。

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