Cohen F S, Niles W D, Akabas M H
Rush Medical College, Department of Physiology, Chicago, Illinois 60612.
J Gen Physiol. 1989 Feb;93(2):201-10. doi: 10.1085/jgp.93.2.201.
Phospholipid vesicles fuse with a planar membrane when they are osmotically swollen. Channels in the vesicle membrane are required for swelling to occur when the vesicle-containing compartment is made hyperosmotic by adding a solute (termed an osmoticant). We have studied fusion using two different channels, porin, a highly permeable channel, and nystatin, a much less permeable channel. We report that an osmoticant's ability to support fusion (defined as the magnitude of osmotic gradient necessary to obtain sustained fusion) depends on both its permeability through lipid bilayer as well as its permeability through the channel by which it enters the vesicle interior. With porin as the channel, formamide requires an osmotic gradient about ten times that required with urea, which is approximately 1/40th as permeant as formamide through bare lipid membrane. When nystatin is the channel, however, fusion rates sustained by osmotic gradients of formamide are within a factor of two of those obtained with urea. Vesicles containing a porin-impermeant solute can be induced to swell and fuse with a planar membrane when the impermeant bathing the vesicles is replaced by an isosmotic quantity of a porin-permeant solute. With this method of swelling, formamide is as effective as urea in obtaining fusion. In addition, we report that binding of vesicles to the planar membrane does not make the contact region more permeable to the osmoticant than is bare lipid bilayer. In the companion paper, we quantitatively account for the observation that the ability of a solute to promote fusion depends on its permeability properties and the method of swelling. We show that the intravesicular pressure developed drives fusion.
当磷脂囊泡发生渗透膨胀时，它们会与平面膜融合。当通过添加一种溶质（称为渗透剂）使含有囊泡的隔室变为高渗时，囊泡膜中的通道是膨胀发生所必需的。我们使用两种不同的通道——孔蛋白（一种高度通透的通道）和制霉菌素（一种通透率低得多的通道）来研究融合。我们报告称，一种渗透剂支持融合的能力（定义为获得持续融合所需的渗透梯度大小）既取决于其通过脂质双层的通透性，也取决于其通过进入囊泡内部的通道的通透性。以孔蛋白作为通道时，甲酰胺所需的渗透梯度约为尿素所需渗透梯度的十倍，而尿素通过裸露脂质膜的通透率约为甲酰胺的1/40。然而，当制霉菌素作为通道时，由甲酰胺的渗透梯度维持的融合速率与由尿素获得的融合速率相差不超过两倍。当用等渗量的孔蛋白可通透溶质取代浸泡囊泡的不通透溶质时，含有孔蛋白不通透溶质的囊泡能够被诱导膨胀并与平面膜融合。用这种膨胀方法，甲酰胺在获得融合方面与尿素一样有效。此外，我们报告称，囊泡与平面膜的结合不会使接触区域比裸露脂质双层对渗透剂更通透。在配套论文中，我们定量解释了溶质促进融合的能力取决于其通透性特性和膨胀方法这一观察结果。我们表明，所产生的囊泡内压力驱动融合。
