Cell biology: Bulky tether proteins aid membrane fusion.

Abstract

The energy source that drives vesicle fusion with a target organelle in vivo has been unclear. It emerges that proteins that tether fusing structures together also decrease the energy needed for the final fusion step. See Letter p.634 Membrane fusion affects many cellular events and is essential for cell survival. It is thought that the various factors that mediate this process have distinct functions. For example, tethering proteins are responsible for the initial recognition and attachment of the fusing membranes, whereas SNARE protein complexes do the rest, providing mechanical energy to distort membranes through a hemifusion intermediate to form a fusion pore. But Andreas Mayer and colleagues now provide evidence that the division of labour is not so clear cut and that the tethering factors are not the lesser partner. They find that, in yeast cells, tether proteins are also essential for the transition from the hemifused state to the fusion pore. Specifically, these proteins play a mechanical part by increasing the volume of SNARE complexes and deforming the site of hemifusion. This lowers the energy barrier for pore opening.