The role of the lateral tension in calcium-dependent phospholipid membrane fusion
Fusion of the cortical granules of the sea urchin Strongylocentrotus purpuratus eggs was studied in an experimental system that allows a slow increase in the concentration of calcium in micro-molar range. It was found that in addition to the threshold concentration that trigger fusion of granules with the plasma membrane, there is a threshold for the rate of concentration growth, below which fusion does not occur. It is also shown that incubation of cortex preparations in solutions with an increased calcium content leads to inactivation of granules, which is expressed in the loss of their ability to fuse at higher concentrations of calcium. These data indicate that protein-mediated membrane fusion can be driven by the membrane lateral stress, similarly to the earlier proposed mechanism for fusion of purely phospholipid membranes.