(i) forward (expanding pore, black line) and (ii) backward (shrinking pore, red line). For membranes-only system we report here a decrease of ∼100 kJ/mol (∼40k B T ) to drive a just nucleated fusion pore of radius ∼0.2nm to a metastable state of radius ∼0.5nm. Also, we have measured that ∼60 kJ/mol (∼24k B T ) are needed to effectively increase the fusion pore radius from 0.5nm to 0.8nm (see figure ). In the following sections, we show that the introduction of one or two Syt1-C2B domains in the cytosolic space significantly equalize these energetic barriers, ultimately conditioning the kinetics of the fusion pore.