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| '''Membrane fusion''' is a key biophysical process that is essential for the functioning of life itself. It is defined as the event where two [[lipid bilayers]] approach each other and then merge to form a single continuous structure.<ref>Yang et al.,Science,2002,297,1877</ref> In living beings, cells are made of an outer coat made of lipid bilayers; which then cause fusion to take place in events such as [[fertilization]], [[embryogenesis]] and even infections by various types of [[bacteria]] and [[viruses]].<ref>Jahn et al.,Current Opinion in Cell Biology 2002,14,488</ref> It is therefore an extremely important event to study. From an evolutionary angle, fusion is an extremely controlled phenomenon. Random fusion can result in severe problems to the normal functioning of the human body. Fusion of [[biological membranes]] is mediated by [[membrane fusion protein|protein]]s. Regardless of the complexity of the system, fusion essentially occurs due to the interplay of various interfacial forces, namely hydration repulsion, hydrophobic attraction and [[van der Waals force]]s.<ref>Israelachvili et al.,Biochemistry,1992,31,1794</ref><!-- "van", see [[Talk:Van der Waals#Van should be capitalized unless preceded by first name]] rebuttal -->
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| __TOC__
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| [[Image:Membrane fusion via stalk formation.jpg|900px|center]]
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| ==Inter-bilayer forces==
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| [[Lipid bilayers]] are structures of [[lipid]] molecules consisting of a [[hydrophobic]] tail and a [[hydrophilic]] head group. Therefore, these structures experience all the characteristic Interbilayer forces involved in that regime.
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| ===Hydration repulsion===
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| Two hydrated bilayers experience strong repulsion as they approach each other. These forces have been measured using the [[Surface forces apparatus]] (S.F.A), an instrument used for measuring forces between surfaces. This repulsion was first proposed by [[Irving Langmuir|Langmuir]] and was thought to arise due to water molecules that [[hydrate]] the bilayers. Hydration repulsion can thus be defined as the work required in removing the water molecules around [[hydrophilic]] molecules (like [[lipid]] head groups) in the bilayer system.<ref>R.P Rand,Annual Reviews of Biophysics and Bioengineering,1981,10,277</ref> As water molecules have an affinity towards [[hydrophilic]] head groups, they try to arrange themselves around the head groups of the [[lipid]] molecules and it becomes very hard to separate this favorable combination.
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| Experiments performed through SFA have confirmed that the nature of this force is an exponential decline.<ref>McIntosh et al.,Biochemistry,1987,26,7325</ref> The [[potential]] ''V<sub>R</sub>'' is given by<ref>Ruckenstein et al.,2001,17,2455</ref>
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| :<math>V_R = C_R \cdot \exp\!\left[{-z\over\lambda_R}\right]</math>
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| where ''C<sub>R</sub>'' (>0) is a measure of the hydration interaction energy for [[hydrophilic]] molecules of the given system, ''λ<sub>R</sub>'' is a characteristic length scale of hydration repulsion and ''z'' is the distance of separation. In other words, it is on distances up to this length that molecules/surfaces fully experience this repulsion.
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| ===Hydrophobic attraction===
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| [[Hydrophobic]] forces are the attractive forces between any two hydrophobic groups in aqueous media, e.g. the forces between two long hydrocarbon chains in aqueous solutions. The magnitude of these forces depends on the [[hydrophobicity]] of the interacting groups as well as the distance separating them (they are found to decrease roughly exponentially with the distance). The physical origin of these forces is a debated issue but they have been found to be long-ranged and are the strongest among all the physical interaction forces operating between biological surfaces and molecules.<ref name="Israel">Israelachvili et al.,Quarterly Reviews of Biophysics,2001,34,2,105</ref> Due to their long range nature, they are responsible for rapid [[coagulation]] of [[hydrophobic]] particles in water and play important roles in various biological phenomena including folding and stabilization of macromolecules such as [[proteins]] and fusion of cell membranes.
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| The potential ''V<sub>A</sub>'' is given by<ref name="Israel"/> | |
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| :<math>V_A = C_A \cdot \exp\!\left[{-z\over\lambda_A}\right]</math>
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| where ''C<sub>A</sub>'' (<0) is a measure of the [[hydrophobic]] interaction energy for the given system, ''λ<sub>A</sub>'' is a characteristic length scale of [[hydrophobic]] attraction and ''z'' is the distance of separation.
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| ===van der Waals forces in bilayers===
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| [[Image:Lipid bilayer fluid.JPG|right|300px]]
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| These forces arise due to [[dipole-dipole interaction]]s (induced/permanent) between molecules of bilayers. As molecules come closer, this attractive force arises due to the ordering of these dipoles; like in the case of magnets that align and attract each other as they approach.<ref name="Israel"/> This also implies that any surface would experience a van der waals attraction. In bilayers, the form taken by van der Waals interaction potential ''V<sub>VDW</sub>'' is given by<ref>Petrarche et al.,Physical Review E,1998,57,6,7014</ref>
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| :<math>V_{VDW} = -{H\over 12\pi} * \left( {1\over z^2} - {2\over(z+D)^2} + {1\over(z+2D)^2} \right)</math>
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| where ''H'' is the [[Hamaker constant]] and ''D'' and ''z'' are the bilayers thickness and the distance of separation respectively.
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| ==Background==
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| For fusion to take place, it has to overcome huge repulsive forces due to the strong hydration repulsion between [[hydrophilic]] [[lipid]] head groups.<ref name="Israel"/> However, it has been hard to exactly determine the connection between [[adhesion]], fusion and interbilayer forces. The forces that promote [[cell adhesion]] are not the same as the ones that promote membrane fusion. Studies show that by creating a [[stress (mechanics)|stress]] on the interacting bilayers, fusion can be achieved without disrupting the interbilayer interactions. It has also been suggested that membrane fusion takes place through a sequence of structural rearrangements that help to overcome the barrier that prevents fusion.<ref name="Israel"/> Thus, interbilayer fusion takes place through
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| * local approach of membrane
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| * structural rearrangements causing [[Hydrate|hydration]] repulsion forces to be overcome
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| * complete merging to form a single entity
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| ==Interbilayer interactions during membrane fusion==
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| When two [[lipid bilayers]] approach each other, they experience weak van der Waals attractive forces and much stronger repulsive forces due to hydration repulsion.<ref>Leikin et al., Journal of Theoretical Biology,1987,129,411</ref> These forces are normally dominant over the [[hydrophobic]] attractive forces between the membranes. Studies done on membrane bilayers using [[Surface forces apparatus]] (SFA) indicate that membrane fusion can instantaneously occur when two bilayers are still at a finite distance from each other without them having to overcome the short-range repulsive force barrier.<ref name="Israel"/> This is attributed to the molecular rearrangements that occur resulting in the bypassing of these forces by the membranes. During fusion, the [[hydrophobic]] tails of a small patch of [[lipids]] on the [[cell membrane]] are exposed to the aqueous phase surrounding them. This results in very strong [[hydrophobic]] attractions (which dominate the repulsive force) between the exposed groups leading to membrane fusion.<ref>Israelachvili et al.,Science,1989,246,4932</ref> The attractive van der Waals forces play a negligible role in membrane fusion. Thus, fusion is a result of the hydrophobic attractions between internal hydrocarbon chain groups that are exposed to the normally inaccessible aqueous environment. Fusion is observed to start at points on the membranes where the membrane stresses are either the weakest or the strongest.<ref name="Israel"/>
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| ==Applications==
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| Interbilayer forces play a key role in mediating membrane fusion, which has extremely important biomedical applications.<ref>Chen at al,Science,2005,308,369</ref>
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| * The most important application of membrane fusion is in the production of [[hybridomas]] which are cells that arise as a result of the fusion of [[antibody]]-secreting and immortal [[B-cells]]. [[Hybridomas]] are used in the industry for the production of [[monoclonal antibodies]].
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| * Membrane fusion also has a major role in [[cancer immunotherapy]]. Currently, one of the approaches in cancer immunotherapy involves [[vaccination]] of [[dendritic cells]] which express a specific [[tumor]] [[antigen]] on their membranes. Instead, the hybrid cells obtained from the fusion of dendritic cells with tumor cells can be used. These hybrids would help in the expression of a range of tumor-associated antigens on their membranes.
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| * Understanding membrane fusion better can also lead to improvements in [[gene therapy]].
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| ==See also==
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| * [[Lipid bilayers]]
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| * [[Hydrophobic effect]]
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| * [[Surface forces apparatus]]
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| * [[Cell membrane]]
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| * [[Hydrate]]
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| ==References==
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| {{reflist}}
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| [[Category:Intermolecular forces]]
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| [[Category:Membrane biology]]
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| [[Category:Surface chemistry]]
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| [[Category:Biophysics]]
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