FIGURE 10:

On the basis of membrane topological data, measurements of endocytosis, and imaging of lumenal/membrane protein release, we propose that Syt isoforms favor different fusion modes in adrenal chromaffin cells. The isoforms themselves are preferentially activated by mild (Syt-7) and strong (Syt-1) depolarization. Postfusion topologies of Syt-1 granules are consistent with a rapid transition between an initial and wide pore (i → iii), thereby promoting the release of granule lumen and membrane constituents. On the other hand, postfusion topologies of Syt-7 granules indicate a slow transition between an initial and wide pore (i → ii), thereby restricting the release of granule lumen and membrane constituents. In addition, Syt-7 fusion events more frequently result in endocytosis at the site of fusion (i.e., cavicapture) than do Syt-1 events.