Why are so many fusogens rod-shaped?

Molecular fusogens catalyze membrane fusion for many basic biological processes. In eukaryotic cells, SNARE proteins drive membrane fusion for trafficking and for exocytic release in many contexts, from neurotransmission to enzymatic digestion, while other fusogens mediate cell-cell fusion for organ formation, placental development and gamete fusion. Enveloped viruses use glycoprotein fusogens for host cell entry and delivery of the viral genome. Despite this breadth of roles, a structural feature shared by many of these fusogens is their rod shape, conserved across the SNARE superfamily, the class I and II fusogen superfamilies and the class III fusogen family. Here we used highly coarse-grained molecular dynamics (MD) simulations to examine the collective behavior of rod-like fusogens on the microscopically long timescales of physiological membrane fusion. Rod-generated entropic forces maintained a cleared fusion site, squeezed and hemifused the membranes, and then expanded and ruptured the hemifused connection to yield fusion. More fusogens generated higher entropic forces and faster fusion, consistent with electrophysiological measurements at neuronal synapses. The required fusogenic feature was the rod shape, since simulated SNARE complexes, class II EFF-1 fusogens, and model rod-shaped complexes entropically drove fusion along similar pathways, whereas globular complexes failed. Thus, rod-like fusogens are optimally shaped generators of entropic forces that drive membrane fusion. These results suggest a universal rod-based fusion mechanism may have been the evolutionary driver of structural convergence among major classes of eukaryotic and viral fusogens.