Fission mechanisms of cylindrical membrane tubes under tension

Membrane remodeling, such as fusion and fission, is involved in a variety of basic, cellular processes. This work investigates the mechanisms and pathways for the fission of phospholipid membranes, in particular double-membrane fission as it occurs in mitochondrial division. We employ self-consistent field theory and utilize the string method to find the Minimum Free Energy Path (MFEP) in order to determine the most likely pathway for the transition. Our results suggest that the free energy barrier to membrane fission, as well as the dominant pathway, can be controlled by the tension experienced by the membrane. At high tension, the inner tube partially collapses into a worm-like micelle, which then ruptures, resulting in two capped tubes. The outer membrane then follows similarly. This pathway is non-leaky, i.e. the solvent inside the inner membrane, between the membranes and outside the outer membrane never mix. At lower tension, the barrier to forming a worm-like micelle becomes prohibitive, and instead, the inner and outer membranes fuse. This pathway is leaky as pores form close to the fusion sites. We also investigate the role of a constricting protein, such as dynamin, on the fission mechanism and barrier.