page_title: Lipid bilayer fusion
section_title: Fusion proteins
hierarchical_section_title: Lipid bilayer fusion / Fusion proteins
caption_attribution_description: Molecular machinery engaged in synaptic transmitter release. Also used in "Nano and Molecular Electronics Handbook". Lyshevski SE (editor), Nano and Microengineering Series, Boca Raton: CRC Press, 2007, pp. 17-11--17-41.
context_page_description: In membrane biology, fusion is the process by which two initially distinct lipid bilayers merge their hydrophobic cores, resulting in one interconnected structure. If this fusion proceeds completely through both leaflets of both bilayers, an aqueous bridge is formed and the internal contents of the two structures can mix. Alternatively, if only one leaflet from each bilayer is involved in the fusion process, the bilayers are said to be hemifused. In hemifusion, the lipid constituents of the outer leaflet of the two bilayers can mix, but the inner leaflets remain distinct. The aqueous contents enclosed by each bilayer also remain separated.
  Fusion is involved in many cellular processes, particularly in eukaryotes since the eukaryotic cell is extensively sub-divided by lipid bilayer membranes. Exocytosis, fertilization of an egg by sperm and transport of waste products to the lysosome are a few of the many eukaryotic processes that rely on some form of fusion. Fusion is also an important mechanism for transport of lipids from their site of synthesis to the membrane where they are needed.
context_section_description: The situation is further complicated when considering fusion in vivo since biological fusion is almost always regulated by the action of membrane-associated proteins. The first of these proteins to be studied were the viral fusion proteins, which allow an enveloped virus to insert its genetic material into the host cell (enveloped viruses are those surrounded by a lipid bilayer; some others have only a protein coat). Broadly, there are two classes of viral fusion proteins: acidic and pH-independent. pH independent fusion proteins can function under neutral conditions and fuse with the plasma membrane, allowing viral entry into the cell. Viruses utilizing this scheme included HIV, measles and herpes. Acidic fusion proteins such as those found on influenza are only activated when in the low pH of acidic endosomes and must first be endocytosed to gain entry into the cell.
  Eukaryotic cells use entirely different classes of fusion proteins, the best studied of which are the SNAREs. SNARE proteins are used to direct all vesicular intracellular trafficking. Despite years of study, much is still unknown about the function of this protein class. In fact, there is still an active debate regarding whether SNAREs are linked to early docking or participate later in the fusion process by facilitating hemifusion. Even once the role of SNAREs or other specific proteins is illuminated, a unified understanding of fusion proteins is unlikely as there is an enormous diversity of structure and function within these classes, and very few themes are conserved.
caption: Diagram of the action of SNARE proteins docking a vesicle for exocytosis. Complementary versions of the protein on the vesicle and the target membrane bind and wrap around each other, drawing the two bilayers close together in the process.[14]
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