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==Formation and transport== {{main|Membrane vesicle trafficking}} {{Organelle diagram}} Some vesicles are made when part of the membrane pinches off the endoplasmic reticulum or the Golgi complex. Others are made when an object outside of the cell is surrounded by the cell membrane.{{citation needed|date=March 2023}} ===Vesicle coat and cargo molecules=== The vesicle "coat" is a collection of proteins that serve to shape the curvature of a donor membrane, forming the rounded vesicle shape. Coat proteins can also function to bind to various transmembrane receptor proteins, called cargo receptors. These receptors help select what material is endocytosed in [[receptor-mediated endocytosis]] or intracellular transport. There are three types of vesicle coats: [[clathrin]], [[COPI]] and [[COPII]]. The various types of coat proteins help with sorting of vesicles to their final destination. Clathrin coats are found on vesicles trafficking between the [[Golgi apparatus|Golgi]] and [[plasma membrane]], the Golgi and [[endosome]]s and the plasma membrane and endosomes. COPI coated vesicles are responsible for retrograde transport from the Golgi to the ER, while COPII coated vesicles are responsible for anterograde transport from the ER to the Golgi. The [[clathrin]] coat is thought to assemble in response to regulatory [[G protein]]. A protein coat assembles and disassembles due to an [[ADP ribosylation factor]] (ARF) protein. ===Vesicle docking=== Surface proteins called [[SNARE]]s identify the vesicle's cargo and complementary SNAREs on the target membrane act to cause fusion of the vesicle and target membrane. Such v-SNARES are hypothesised to exist on the vesicle membrane, while the complementary ones on the target membrane are known as t-SNAREs.{{citation needed|date=March 2023}} Often SNAREs associated with vesicles or target membranes are instead classified as Qa, Qb, Qc, or R SNAREs owing to further variation than simply v- or t-SNAREs. An array of different SNARE complexes can be seen in different tissues and subcellular compartments, with 38 isoforms currently identified in humans.<ref>{{cite journal |last1=Dingjan |first1=Peter |title=Endosomal and Phagosomal SNAREs |url=https://journals.physiology.org/doi/full/10.1152/physrev.00037.2017?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org |journal=Physiological Reviews |date=2018 |volume=98 |issue=3 |pages=1465β1492 |doi=10.1152/physrev.00037.2017 |pmid=29790818 |access-date=17 June 2024}}</ref> Regulatory [[Rab (G-protein)|Rab]] proteins are thought to inspect the joining of the SNAREs. Rab protein is a regulatory GTP-binding protein and controls the binding of these complementary SNAREs for a long enough time for the Rab protein to hydrolyse its bound GTP and lock the vesicle onto the membrane. [[SNAREs in plants|SNAREs proteins in plants]] are understudied compared to fungi and animals. The cell botanist [[Natasha Raikhel]] has done some of the basic research in this area, including Zheng et al 1999 in which she and her team found [[AtVTI1a]] to be essential to [[Golgi apparatus|Golgi]]β[[vacuole]] transport.<ref name="Raikhel-2017">{{cite journal |last1=Raikhel |first1=Natasha V. |author1-link=Natasha Raikhel |title=Firmly Planted, Always Moving |journal=[[Annual Review of Plant Biology]] |publisher=[[Annual Reviews (publisher)|Annual Reviews]] |volume=68 |issue=1 |date=2017-04-28 |issn=1543-5008 |doi=10.1146/annurev-arplant-042916-040829 |pages=1β27|pmid=27860488 |doi-access=free |bibcode=2017AnRPB..68....1R }}</ref> ===Vesicle fusion=== {{Further|Vesicle fusion}} Vesicle fusion can occur in one of two ways: full fusion or [[kiss-and-run fusion]]. Fusion requires the two membranes to be brought within 1.5 nm of each other. For this to occur water must be displaced from the surface of the vesicle membrane. This is energetically unfavorable and evidence suggests that the process requires [[adenosine triphosphate|ATP]], [[guanosine triphosphate|GTP]] and [[acetyl-coA]]. Fusion is also linked to budding, which is why the term budding and fusing arises. ===In receptor downregulation=== Membrane proteins serving as [[receptor (biochemistry)|receptor]]s are sometimes tagged for [[downregulation]] by the attachment of [[ubiquitin]]. After arriving an [[endosome]] via the pathway described above, vesicles begin to form inside the endosome, taking with them the membrane proteins meant for degradation; When the endosome either matures to become a [[lysosome]] or is united with one, the vesicles are completely degraded. Without this mechanism, only the extracellular part of the membrane proteins would reach the lumen of the [[lysosome]] and only this part would be degraded.<ref>{{cite journal | vauthors = Katzmann DJ, Odorizzi G, Emr SD | title = Receptor downregulation and multivesicular-body sorting | journal = Nature Reviews. Molecular Cell Biology | volume = 3 | issue = 12 | pages = 893β905 | date = December 2002 | pmid = 12461556 | doi = 10.1038/nrm973 | s2cid = 1344520 }}</ref> It is because of these vesicles that the endosome is sometimes known as a ''multivesicular body''. The pathway to their formation is not completely understood; unlike the other vesicles described above, the outer surface of the vesicles is not in contact with the [[cytosol]]. ===Preparation=== ====Isolated vesicles==== Producing membrane vesicles is one of the methods to investigate various membranes of the cell. After the living tissue is crushed into [[suspension (chemistry)|suspension]], various membranes form tiny closed bubbles. Big fragments of the crushed cells can be discarded by low-speed centrifugation and later the fraction of the known origin ([[plasmalemma]], [[tonoplast]], etc.) can be isolated by precise high-speed centrifugation in the density gradient. Using [[osmotic shock]], it is possible temporarily open vesicles (filling them with the required solution) and then centrifugate down again and resuspend in a different solution. Applying ionophores like [[valinomycin]] can create electrochemical gradients comparable to the gradients inside living cells. Vesicles are mainly used in two types of research: * To find and later isolate membrane receptors that specifically bind hormones and various other important substances.<ref>{{cite journal | vauthors = Sidhu VK, VorhΓΆlter FJ, Niehaus K, Watt SA | title = Analysis of outer membrane vesicle associated proteins isolated from the plant pathogenic bacterium Xanthomonas campestris pv. campestris | journal = BMC Microbiology | volume = 8 | pages = 87 | date = June 2008 | pmid = 18518965 | pmc = 2438364 | doi = 10.1186/1471-2180-8-87 | doi-access = free }}</ref> * To investigate transport of various ions or other substances across the membrane of the given type.<ref>{{cite journal |vauthors=Scherer GG, Martiny-Baron G |title=K+/H+ exchange transport in plantmembranevesicles is evidence for K+ transport |journal=Plant Science |volume=41 |issue=3 |pages=161β8 |year=1985 |doi=10.1016/0168-9452(85)90083-4 }}</ref> While transport can be more easily investigated with [[patch clamp]] techniques, vesicles can also be isolated from objects for which a patch clamp is not applicable. ===Artificial vesicles=== {{See also|Unilamellar liposome}} Artificial vesicles are classified into three groups based on their size: small unilamellar liposomes/vesicles (SUVs) with a size range of 20β100 nm, large unilamellar liposomes/vesicles (LUVs) with a size range of 100β1000 nm and giant unilamellar liposomes/vesicles (GUVs) with a size range of 1β200 ΞΌm.<ref>{{cite journal | vauthors = Walde P, Cosentino K, Engel H, Stano P | title = Giant vesicles: preparations and applications | journal = ChemBioChem | volume = 11 | issue = 7 | pages = 848β65 | date = May 2010 | pmid = 20336703 | doi = 10.1002/cbic.201000010 | s2cid = 30723166 }}</ref> Smaller vesicles in the same size range as trafficking vesicles found in living cells are frequently used in [[biochemistry]] and related fields. For such studies, a homogeneous phospholipid vesicle suspension can be prepared by extrusion or [[sonication]],<ref>{{cite journal | vauthors = Barenholz Y, Gibbes D, Litman BJ, Goll J, Thompson TE, Carlson RD | title = A simple method for the preparation of homogeneous phospholipid vesicles | journal = Biochemistry | volume = 16 | issue = 12 | pages = 2806β10 | date = June 1977 | pmid = 889789 | doi = 10.1021/bi00631a035 }}</ref> or by rapid injection of a phospholipid solution into an aqueous buffer solution.<ref>{{cite journal | vauthors = Batzri S, Korn ED | title = Single bilayer liposomes prepared without sonication | journal = Biochimica et Biophysica Acta (BBA) - Biomembranes | volume = 298 | issue = 4 | pages = 1015β9 | date = April 1973 | pmid = 4738145 | doi = 10.1016/0005-2736(73)90408-2 }}</ref> In this way, aqueous vesicle solutions can be prepared of different phospholipid composition, as well as different sizes of vesicles. Larger synthetically made vesicles such as GUVs are used for in vitro studies in [[cell biology]] in order to mimic cell membranes. These vesicles are large enough to be studied using traditional fluorescence light microscopy. A variety of methods exist to encapsulate biological reactants like protein solutions within such vesicles, making GUVs an ideal system for the in vitro recreation (and investigation) of cell functions in cell-like model membrane environments.<ref>{{cite journal | vauthors = Litschel T, Schwille P | title = Protein Reconstitution Inside Giant Unilamellar Vesicles | journal = Annual Review of Biophysics | date = March 2021 | volume = 50 | pages = 525β548 | pmid = 33667121 | doi = 10.1146/annurev-biophys-100620-114132 | s2cid = 232131463 }}</ref> These methods include microfluidic methods, which allow for a high-yield production of vesicles with consistent sizes.<ref>{{cite journal | vauthors = Sato Y, Takinoue M | title = Creation of Artificial Cell-Like Structures Promoted by Microfluidics Technologies | journal = Micromachines | volume = 10 | issue = 4 | pages = 216 | date = March 2019 | pmid = 30934758 | pmc = 6523379 | doi = 10.3390/mi10040216 | doi-access = free }}</ref>
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