The assembly of three synaptic SNARE proteins into a four-helix bundle drives membrane fusion by bringing vesicular and plasma membranes into close proximity. The role of SNARE transmembrane domains (TMDs) in this process remains controversial. Using high-resolution optical tweezers, we characterized the reversible folding and assembly of a single full-length SNARE complex anchored in a model membrane. Contrary to the zippered TMDs observed in crystal structures, our findings reveal that SNARE TMDs exhibit no detectable affinity. PIP2 slows down SNARE zippering, whereas complexin-1, a key SNARE-binding protein, significantly accelerates it, thereby enhancing neurotransmitter release. In contrast to SNARE-mediated quantal lipid trafficking, bridge-like lipid transfer proteins (BLTPs) facilitate continuous bulk lipid transport through lipid conduits, supporting lipid homeostasis and organelle dynamics. We propose lipid osmosis as a novel biophysical mechanism that generates membrane tension to drive bulk lipid flow, potentially regulating membrane expansion during vesicle and autophagosome formation. Theoretical analyses confirm that lipid osmosis provides sufficient force to overcome membrane bending and tension to expand membranes during organelle biogenesis.
Yongli Zhang (Sun,) studied this question.