Editing Endocytosis (section)

== Clathrin-mediated ==

The major route for endocytosis in most cells, and the best-understood, is that mediated by the molecule [[clathrin]].<ref>{{cite journal | vauthors = Kirchhausen T, Owen D, Harrison SC | title = Molecular structure, function, and dynamics of clathrin-mediated membrane traffic | journal = Cold Spring Harbor Perspectives in Biology | volume = 6 | issue = 5 | pages = a016725 | date = May 2014 | pmid = 24789820 | pmc = 3996469 | doi = 10.1101/cshperspect.a016725 }}</ref><ref>{{cite journal | vauthors = Bitsikas V, Corrêa IR, Nichols BJ | title = Clathrin-independent pathways do not contribute significantly to endocytic flux | journal = eLife | volume = 3 | pages = e03970 | date = September 2014 | pmid = 25232658 | pmc = 4185422 | doi = 10.7554/eLife.03970 | doi-access = free }}</ref> This large protein assists in the formation of a coated pit on the inner surface of the [[plasma membrane]] of the cell. This pit then buds into the cell to form a coated vesicle in the cytoplasm of the cell. In so doing, it brings into the cell not only a small area of the surface of the cell but also a small volume of fluid from outside the cell.<ref name="endo15">{{cite journal | vauthors = Benmerah A, Lamaze C | title = Clathrin-coated pits: vive la différence? | journal = Traffic | volume = 8 | issue = 8 | pages = 970–982 | date = August 2007 | pmid = 17547704 | doi = 10.1111/j.1600-0854.2007.00585.x | s2cid = 12685926 | doi-access =  }}</ref><ref name="endo16">{{cite journal | vauthors = Rappoport JZ | title = Focusing on clathrin-mediated endocytosis | journal = The Biochemical Journal | volume = 412 | issue = 3 | pages = 415–423 | date = June 2008 | pmid = 18498251 | doi = 10.1042/BJ20080474 | s2cid = 24174632 }}</ref><ref name="endo17">{{cite journal | vauthors = Granseth B, Odermatt B, Royle SJ, Lagnado L | title = Clathrin-mediated endocytosis: the physiological mechanism of vesicle retrieval at hippocampal synapses | journal = The Journal of Physiology | volume = 585 | issue = Pt 3 | pages = 681–686 | date = December 2007 | pmid = 17599959 | pmc = 2375507 | doi = 10.1113/jphysiol.2007.139022 }}</ref>

Coats function to deform the donor membrane to produce a vesicle, and they also function in the selection of the vesicle cargo. Coat complexes that have been well characterized so far include coat protein-I (COP-I), COP-II, and clathrin.<ref name="endo12">{{cite journal | vauthors = Robinson MS | title = Coats and vesicle budding | journal = Trends in Cell Biology | volume = 7 | issue = 3 | pages = 99–102 | date = March 1997 | pmid = 17708916 | doi = 10.1016/S0962-8924(96)10048-9 }}</ref><ref name="Glick">{{cite journal | vauthors = Glick BS, Malhotra V | title = The curious status of the Golgi apparatus | journal = Cell | volume = 95 | issue = 7 | pages = 883–889 | date = December 1998 | pmid = 9875843 | doi = 10.1016/S0092-8674(00)81713-4 | doi-access = free }}</ref> Clathrin coats are involved in two crucial transport steps: (i) receptor-mediated and fluid-phase endocytosis from the plasma membrane to early endosome and (ii) transport from the TGN to endosomes. In endocytosis, the clathrin coat is assembled on the cytoplasmic face of the plasma membrane, forming pits that invaginate to pinch off (scission) and become free CCVs. In cultured cells, the assembly of a CCV takes ~ 1min, and several hundred to a thousand or more can form every minute.<ref name="Gaid">{{cite journal | vauthors = Gaidarov I, Santini F, Warren RA, Keen JH | title = Spatial control of coated-pit dynamics in living cells | journal = Nature Cell Biology | volume = 1 | issue = 1 | pages = 1–7 | date = May 1999 | pmid = 10559856 | doi = 10.1038/8971 | s2cid = 12553151 }}</ref> The main scaffold component of clathrin coat is the 190-kD protein called clathrin heavy chain (CHC), which is associated with a 25- kD protein called clathrin light chain (CLC), forming three-legged trimers called triskelions.

Vesicles selectively concentrate and exclude certain proteins during formation and are not representative of the membrane as a whole. [[AP2 adaptors]] are multisubunit complexes that perform this function at the plasma membrane. The best-understood receptors that are found concentrated in coated vesicles of mammalian cells are the [[LDL receptor]] (which removes [[Low-density lipoprotein|LDL]] from circulating blood), the transferrin receptor (which brings ferric ions bound by [[transferrin]] into the cell) and certain hormone receptors (such as that for [[Epidermal growth factor|EGF]]).

At any one moment, about 25% of the plasma membrane of a fibroblast is made up of coated pits. As a coated pit has a life of about a minute before it buds into the cell, a fibroblast takes up its surface by this route about once every 50 minutes. Coated vesicles formed from the plasma membrane have a diameter of about 100&nbsp;nm and a lifetime measured in a few seconds. Once the coat has been shed, the remaining vesicle fuses with [[endosomes]] and proceeds down the endocytic pathway. The actual budding-in process, whereby a pit is converted to a vesicle, is carried out by clathrin; Assisted by a set of cytoplasmic proteins, which includes [[dynamin]] and adaptors such as [[adaptin]].

Coated pits and vesicles were first seen in thin sections of tissue in the electron microscope by Thomas F Roth and [[Keith R. Porter]].<ref name="pmid14126875">{{cite journal | vauthors = ROTH TF, PORTER KR | title = Yolk Protein Uptake In The Oocyte Of The Mosquito Aedes Aegypti. L | journal = J Cell Biol | volume = 20 | issue = 2 | pages = 313–32 | date = February 1964 | pmid = 14126875 | pmc = 2106398 | doi = 10.1083/jcb.20.2.313 }}</ref> The importance of them for the clearance of LDL from blood was discovered by Richard G. Anderson, [[Michael S. Brown]] and [[Joseph L. Goldstein]] in 1977.<ref>{{cite journal | vauthors = Anderson RG, Brown MS, Goldstein JL | title = Role of the coated endocytic vesicle in the uptake of receptor-bound low density lipoprotein in human fibroblasts | journal = Cell | volume = 10 | issue = 3 | pages = 351–364 | date = March 1977 | pmid = 191195 | doi = 10.1016/0092-8674(77)90022-8 | s2cid = 25657719 }}</ref> Coated vesicles were first purified by [[Barbara Pearse]], who discovered the clathrin coat molecule in 1976.<ref>{{cite journal | vauthors = Pearse BM | title = Clathrin: a unique protein associated with intracellular transfer of membrane by coated vesicles | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 73 | issue = 4 | pages = 1255–1259 | date = April 1976 | pmid = 1063406 | pmc = 430241 | doi = 10.1073/pnas.73.4.1255 | doi-access = free | bibcode = 1976PNAS...73.1255P }}</ref>

=== Processes and components ===

Caveolin proteins like caveolin-1 ([[Caveolin 1|CAV1]]), caveolin-2 ([[Caveolin 2|CAV2]]), and caveolin-3 ([[Caveolin 3|CAV3]]), play significant roles in the caveolar formation process. More specifically, CAV1 and CAV2 are responsible for caveolae formation in non-muscle cells while CAV3 functions in muscle cells. The process starts with CAV1 being synthesized in the [[Endoplasmic reticulum|ER]] where it forms detergent-resistant [[oligomer]]s. Then, these oligomers travel through the [[Golgi Complex|Golgi complex]] before arriving at the cell surface to aid in caveolar formation. Caveolae formation is also reversible through disassembly under certain conditions such as increased plasma membrane tension. These certain conditions then depend on the type of tissues that are expressing the caveolar function. For example, not all tissues that have caveolar proteins have a caveolar structure i.e. the [[Blood-brain-barrier|blood-brain barrier]].<ref name="pmid">{{cite journal | vauthors = Parton RG, Tillu VA, Collins BM | title = Caveolae | journal = Current Biology | volume = 28 | issue = 8 | pages = R402–R405 | date = April 2018 | pmid = 29689223| doi = 10.1016/j.cub.2017.11.075 | s2cid = 235331463 | doi-access = free }}</ref>
Though there are many morphological features conserved among caveolae, the functions of each CAV protein are diverse. One common feature among caveolins is their hydrophobic stretches of potential hairpin structures that are made of [[α-helices]]. The insertion of these hairpin-like α-helices forms a caveolae coat which leads to membrane curvature. In addition to insertion, caveolins are also capable of oligomerization which further plays a role in membrane curvature. Recent studies have also discovered that polymerase I, transcript release factor, and serum deprivation protein response also play a role in the assembly of caveolae. Besides caveolae assembly, researchers have also discovered that CAV1 proteins can also influence other endocytic pathways. When CAV1 binds to [[CDC42|Cdc42]], CAV1 inactivates it and regulates Cdc42 activity during membrane trafficking events.<ref name="pmid20125123">{{cite journal | vauthors = Kumari S, Mg S, Mayor S | title = Endocytosis unplugged: multiple ways to enter the cell | journal = Cell Research | volume = 20 | issue = 3 | pages = 256–75 | date = March 2010 | pmid = 20125123 | doi = 10.1038/cr.2010.19 | pmc = 7091825 }}</ref>

=== Mechanisms ===
The process of cell uptake depends on the tilt and chirality of constituent molecules to induce membrane budding. Since such chiral and tilted lipid molecules are likely to be in a "raft" form, researchers suggest that caveolae formation also follows this mechanism since caveolae are also enriched in raft constituents. When caveolin proteins bind to the inner leaflet via [[cholesterol]], the membrane starts to bend, leading to spontaneous curvature. This effect is due to the force distribution generated when the caveolin oligomer binds to the membrane. The force distribution then alters the tension of the membrane which leads to budding and eventually vesicle formation.<ref name="pmid17237196">{{cite journal | vauthors = Sarasij RC, Mayor S, Rao M | title = Chirality-induced budding: a raft-mediated mechanism for endocytosis and morphology of caveolae? | journal = Biophysical Journal | volume = 92 | issue = 9 | pages = 3140–58 | date = May 2007 | pmid = 17237196 | doi = 10.1529/biophysj.106.085662 | pmc = 1852369 | bibcode = 2007BpJ....92.3140S }}</ref>