Kinetic Pathway and Micromechanics of Vesicle Fusion/Fission

Despite the wide existence of vesicles in living cells as well as their important applications like drug-delivery, the underlying mechanism of vesicle fusion/fission remains under debate. Here, we develop a constrained self-consistent field theory (SCFT) which allows tracking the shape evolution and free energy as a function of center-of-mass separation distance. Fusion and fission are described in a unified framework. Both the kinetic pathway and the mechanical response can be simultaneously captured. By taking vesicles formed by polyelectrolytes as a model system, we predict discontinuous transitions between the three morphologies: parent vesicle with a single cavity, hemifission/hemifusion and two separated child vesicles, as a result of breaking topological isomorphism. With the increase of inter-vesicle repulsion, we observe a great reduction of the cleavage energy, indicating that vesicle fission can be achieved without hemifission, in good agreement with simulation. The force-extension relationship elucidates typical plasticity for separating two vesicles. The super extensibility in the mechanical response of vesicle is in stark contrast to soft particles with other morphologies such as cylinder and sphere.