Why are so many fusogens rod-shaped?

Abstract

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. SignificanceLarge classes of eukaryotic and viral membrane fusion complexes are rod-shaped, a common structural feature despite a multiplicity of functions, from neurotransmission to organogenesis to cell entry. Here we used coarse-grained molecular dynamics to access the millisecond timescales of physiological fusion. Simulations revealed a universal mechanism whereby, regardless of structural details, rod-shaped fusogens generate entropic forces that drive fusion. This suggests the mechanism was a major evolutionary driver of structural convergence and multiple transmissions between eukaryotic hosts and viruses. Rod-mediated fusion is faster with more fusogens, explaining why release at presynaptic nerve terminals is enhanced or suppressed depending on the number of active SNARE fusogens. Thus, modulation of SNARE numbers and fusogenicities may represent a significant mechanism of synaptic plasticity.