Monck J R, Alvarez de Toledo G, Fernandez J M
Department of Physiology and Biophysics, Mayo Clinic, Rochester, MN 55905.
Proc Natl Acad Sci U S A. 1990 Oct;87(20):7804-8. doi: 10.1073/pnas.87.20.7804.
For fusion to occur the repulsive forces between two interacting phospholipid bilayers must be reduced. In model systems, this can be achieved by increasing the surface tension of at least one of the membranes. However, there has so far been no evidence that the secretory granule membrane is under tension. We have been studying exocytosis by using the patch-clamp technique to measure the surface area of the plasma membrane of degranulating mast cells. When a secretory granule fuses with the plasma membrane there is a step increase in the cell surface area. Some fusion events are reversible, in which case we have found that the backstep is larger than the initial step, indicating that there is a net decrease in the area of the plasma membrane. The decrease has the following properties: (i) the magnitude is strongly dependent on the lifetime of the fusion event and can be extensive, representing as much as 40% of the initial granule surface area; (ii) the rate of decrease is independent of granule size; and (iii) the decrease is not dependent on swelling of the secretory granule matrix. We conclude that the granule membrane is under tension and that this tension causes a net transfer of membrane from the plasma membrane to the secretory granule, while they are connected by the fusion pore. The high membrane tension in the secretory granule may be the critical stress necessary for bringing about exocytotic fusion.
为了发生融合，两个相互作用的磷脂双层之间的排斥力必须降低。在模型系统中，这可以通过增加至少一层膜的表面张力来实现。然而，迄今为止，尚无证据表明分泌颗粒膜处于张力状态。我们一直在使用膜片钳技术研究胞吐作用，以测量脱颗粒肥大细胞质膜的表面积。当分泌颗粒与质膜融合时，细胞表面积会有一个阶梯式增加。一些融合事件是可逆的，在这种情况下，我们发现回退步骤大于初始步骤，这表明质膜面积有净减少。这种减少具有以下特性：（i）幅度强烈依赖于融合事件的持续时间，并且可能很大，相当于初始颗粒表面积的40%；（ii）减少速率与颗粒大小无关；（iii）减少不依赖于分泌颗粒基质的肿胀。我们得出结论，颗粒膜处于张力状态，并且这种张力导致膜在通过融合孔连接时从质膜向分泌颗粒的净转移。分泌颗粒中的高膜张力可能是实现胞吐融合所需的关键应力。
