fusion pore – have been proposed. However, the exact nature of these processes remains debated. In accordance with the "zipper" hypothesis, as the SNARE complex forms, the tightening helix bundle puts torsional force on the transmembrane (TM) domains of synaptobrevin and syntaxin. This causes the TM domains to tilt within the separate membranes as the proteins coil more tightly. The unstable configuration of the TM domains eventually causes the two membranes to fuse and the SNARE proteins come together within the same membrane, which is referred to as a "cis"-SNARE complex. As a result of the lipid rearrangement, a fusion pore opens and allows the chemical contents of the vesicle to leak into the outside environment. The continuum explanation of stalk formation suggests that membrane fusion begins with an infinitesimal radius until it radially expands into a stalk-like structure. However, such a description fails to take into account the molecular dynamics of membrane lipids. Recent molecular simulations show that the close proximity of the membranes allows the lipids to splay, where a population of lipids insert their hydrophobic tails into the neighboring membrane – effectively keeping a "foot" in each membrane. The resolution of the splayed lipid state proceeds spontaneously to form the stalk structure. In this molecular view, the splayed-lipid intermediate state is the rate determining barrier rather than the formation of the stalk, which now becomes the free energy minimum. The energetic barrier for establishment of the splayed-lipid conformation is directly proportional to the intermembrane distance. The SNARE complexes and their pressing of the two membranes together, therefore, could provide the free energy required to overcome the barrier. Disassembly The energy input that is required for SNARE-mediated fusion to take place comes from SNARE-complex disassembly. The suspected energy source is N-ethylmaleimide-sensitive factor (NSF), an ATPase that is involved with membrane fusion. NSF homohexamers, along with the NSF cofactor α-SNAP, bind and dissociate the SNARE complex by coupling the process with ATP hydrolysis. This process allows for reuptake of synaptobrevin for further use in vesicles, whereas the other SNARE proteins remain associated with the cell membrane. The dissociated SNARE proteins have a higher energy state than the more stable cis-SNARE complex. It is believed that the energy that drives fusion is derived from the transition to a lower energy cis-SNARE complex. The ATP hydrolysis-coupled dissociation of SNARE complexes is an energy investment that can be compared to "cocking the