Why are SNARE complexes rod-shaped?

SNARE proteins constitute the core of a cellular machinery that fuses membranes for vital processes such as exocytotic release of neurotransmitters or hormones and intracellular trafficking. Despite years of study, the mechanism remains highly controversial. Here we developed molecularly explicit simulations that fused synaptic-sized vesicle membranes on physiological millisecond timescales for the first time, and demonstrated that membrane fusion is driven by entropic forces among the rodlike SNARE complexes (SNAREpins) and membranes.