Figure 8.

A, Exocytosis event in a process observed using TIRFM. As expected, the punctum at first brightens without spreading and then spreads in the images (left to right) as the DCG approaches the membrane and fuses. Scale bar, 2 μm. Three-dimensional trajectories of fusing DCGs showing the existence of prefusion movement that is substantially lateral (B), axial (C), and random (D) are shown. The numbers along the axes indicate distance in nanometers; the numbers in the top left corners indicate elapsed time. The starting DCG positions are labeled with a filled circle, exocytosis points are labeled with an asterisk, and the plasma membrane is labeled with the letters PM. We attributed brightening that occurs just before the apparent onset of fusion to axial movement and not to increases in DCG pH associated with the opening of a fusion pore, if the DCG was very dim before brightening and also moved laterally.