page_title: Vesicle fusion
section_title: Lipid-lined fusion pore theory
hierarchical_section_title: Vesicle fusion / Mechanisms / SNARE protein and pore formation / Lipid-lined fusion pore theory
caption_attribution_description: English: fusion image for cell cell fusion
context_page_description: Vesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis. Vesicles can also fuse with other target cell compartments, such as a lysosome. Exocytosis occurs when secretory vesicles transiently dock and fuse at the base of cup-shaped structures at the cell plasma membrane called porosome, the universal secretory machinery in cells. Vesicle fusion may depend on SNARE proteins in the presence of increased intracellular calcium concentration.
context_section_description: One possible model for fusion pore formation is the lipid-line pore theory.  In this model, once the membranes have been brought into sufficiently close proximity via the "zipper" mechanism of the SNARE complex, membrane fusion occurs spontaneously.  It has been shown that when the two membranes are brought within a critical distance, it is possible for hydrophilic lipid headgroups of one membrane to merge with the opposing membrane. In the lipid-lined fusion pore model, the SNARE complex acts as a scaffold, pulling on the membrane, causing both membranes to pucker so they may reach the critical fusion distance.  As the two membranes begin to fuse, a lipid-lined stalk is produced, expanding radially outward as fusion proceeds.
  While a lipid-lined pore is possible and can achieve all the same properties observed in early pore formation, sufficient data does not exist to prove it is the sole method of formation.  There is not currently a proposed mechanism on inter-cellular regulation for fluctuation of lipid-lined pores, and they would have a substantially more difficult time producing effects such as the "kiss-and-run" when compared with their protein-lined counterparts.  Lipid-lined pores effectiveness would also be highly dependent on the composition of both membranes, and its success or failure could vary wildly with changes in elasticity and rigidity.
caption: In the lipid-lined pore theory both membranes curve toward each other to form the early fusion pore.  When the two membranes are brought to a "critical" distance, the lipid head-groups from one membrane insert into the other, creating the basis for the fusion pore.
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