Metastable hemifusion diaphragms regulate rim-pore expansion dynamics.

HYPOTHESIS: The interaction of large, membrane-bound vesicles leads to the development of extended diaphragms that act as intermediates in the fusion process. Such hemifusion diaphragms confined inside a stiffer container may disassemble through energetically distinct rim-pore expansion modes (radial or lateral), and the preferred mode depends on diaphragm size and lipid reabsorption costs. We propose a theoretical model based on constrained-wetting physics to explain why large hemifusion diaphragms favor lateral rim-pore propagation while small hemifusion diaphragms prefer radial growth.

EXPERIMENTS: We combine coarse-grained unbiased molecular dynamics of confined vesicle-in-vesicle systems in the μs-scale, with an analytical geometrical model derived from interface wetting. Energies include line tensions for membrane building blocks (edges, arcs and Y-junctions), diaphragm lateral tension, and a lipid reabsorption term accounting for area transfer and flip-flop controlled stress-asymmetry. Simulation observables (rim-pore area, arc/edge/Y-junction lengths and contact angles) are directly compared to model predictions.

FINDINGS: The wetting-inspired energy model predicts two regimes. For large hemifusion diaphragms lateral propagation minimizes both edge length and reabsorption costs and therefore is energetically preferred, producing rapid lateral expansion and micelle formation via a final fission. For small hemifusion diaphragms radial propagation is favored, producing slow bleb reabsorption without fragment excision. Independent unbiased trajectory replicas (n=10 per regime) reproduce these predictions quantitatively (rim-pore area, contact angles, expansion timescales). Results unify geometric and wetting analogies with numerical simulation evidence, and reveal a mechanistic pathway for selective fast or slow hemifusion diaphragm disassembly with implications for controlled membrane self-reorganization under spatial confinement.