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Endocytosis is a form of active transport in which a cell transports molecules (such as proteins) into the cell ( + cytosis) by engulfing them in an energy-using process. Endocytosis and its counterpart, exocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane by passive means. Endocytosis includes pinocytosis (cell drinking) and phagocytosis (cell eating).


The term was proposed by De Duve in 1963.Rieger, R.; Michaelis, A.; Green, M.M. 1991. Glossary of Genetics.Classical and Molecular (Fifth edition). Springer-Verlag, Berlin, linkhttps://books.google.com/books?id=5azxCAAAQBAJ. Phagocytosis was discovered by Élie Metchnikoff in 1882.

Endocytosis pathways

membranes.]] Endocytosis pathways can be subdivided into four categories: namely, receptor-mediated endocytosis (also known as clathrin-mediated endocytosis), caveolae, macropinocytosis, and phagocytosis.
  • Clathrin-mediated endocytosis is mediated by the production of small (approx. 100 nm in diameter) vesicles that have a morphologically characteristic coat made up of the cytosolic protein clathrin. Clathrin-coated vesicles (CCVs) are found in virtually all cells and form domains of the plasma membrane termed clathrin-coated pits. Coated pits can concentrate large extracellular molecules that have different receptors responsible for the receptor-mediated endocytosis of ligands, e.g. low density lipoprotein, transferrin, growth factors, antibodies and many others.{{cite journal| volume = 285
| issue = 5425 | pages = 215–20 |date=July 1999 | pmid = 10398591 | doi = 10.1126/science.285.5425.215 | journal = Science | url = http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=10398591 | accessdate = 2009-06-19 | title = The structural era of endocytosis | last1 = Marsh | first1 = M. | last2 = McMahon | first2 = HT }} Study by {{cite journal | volume = 13 | pages = 1455–1462 |date=2017 | pmid = 10398591 | doi = 10.1039/C6SM02623K | journal = Soft matter | title = Clathrin polymerization exhibits high mechano-geometric sensitivity | last1 = Irajizad | first1 = E. | last2 = Agrawal | first2 = A. }} in mammalian cells confirm a reduction in clathrin coat size in an increased tension environment. In addition, it suggests that the two apparently distinct clathrin assembly modes, namely coated pits and coated plaques, observed in experimental investigations might be a consequence of varied tensions in the plasma membrane.
  • Caveolae are the most common reported non-clathrin-coated plasma membrane buds, which exist on the surface of many, but not all cell types. They consist of the cholesterol-binding protein caveolin (Vip21) with a bilayer enriched in cholesterol and glycolipids. Caveolae are small (approx. 50 nm in diameter) flask-shape pits in the membrane that resemble the shape of a cave (hence the name caveolae). They can constitute up to a third of the plasma membrane area of the cells of some tissues, being especially abundant in smooth muscle, type I pneumocytes, fibroblasts, adipocytes, and endothelial cells.{{cite journal
|vauthors=Parton RG, Simons K | title = The multiple faces of caveolae | journal = Nature Reviews Molecular Cell Biology | volume = 8 | issue = 3 | pages = 185–94 |date=March 2007 | pmid = 17318224 | doi = 10.1038/nrm2122 }} Uptake of extracellular molecules is also believed to be specifically mediated via receptors in caveolae.
  • * Potocytosis is a form of receptor-mediated endocytosis that uses caveolae vesicles to bring molecules of various sizes into the cell. Unlike most endocytosis that uses caveolae to deliver contents of vesicles to lysosomes or other organelles, material endocytosed via potocytosis is released into the cytosol.
  • Macropinocytosis, which usually occurs from highly ruffled regions of the plasma membrane, is the invagination of the cell membrane to form a pocket, which then pinches off into the cell to form a vesicle (0.5–5 µm in diameter) filled with a large volume of extracellular fluid and molecules within it (equivalent to ~100 CCVs). The filling of the pocket occurs in a non-specific manner. The vesicle then travels into the cytosol and fuses with other vesicles such as endosomes and lysosomes.{{cite journal
|vauthors=Falcone S, Cocucci E, Podini P, Kirchhausen T, Clementi E, Meldolesi J | title = Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events | journal = Journal of Cell Science | volume = 119 | issue = Pt 22 | pages = 4758–69 |date=November 2006 | pmid = 17077125| doi = 10.1242/jcs.03238 }} More recent experiments have suggested that these morphological descriptions of endocytic events may be inadequate, and a more appropriate method of classification may be based upon the clathrin-dependence of particular pathways, with multiple subtypes of clathrin-dependent and clathrin-independent endocytosis. Mechanistic insight into non-phagocytic, clathrin-independent endocytosis has been lacking, but a recent study has shown how Graf1 regulates a highly prevalent clathrin-independent endocytic pathway known as the CLIC/GEEC pathway.{{cite journal |vauthors=Lundmark R, Doherty GJ, Howes MT | title = The GTPase-Activating Protein GRAF1 Regulates the CLIC/GEEC Endocytic Pathway | journal = Current Biology | volume = 18 | issue = 22 | pages = 1802–8 |date=November 2008 | pmid = 19036340 | doi = 10.1016/j.cub.2008.10.044 | pmc = 2726289 |display-authors=etal}}

Principal components of endocytic pathway

The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degradation (as in late endosomes and lysosomes). The principal components of the endocytic pathway are:
  • Early endosomes are the first compartment of the endocytic pathway. Early endosomes are often located in the periphery of the cell, and receive most types of vesicles coming from the cell surface. They have a characteristic tubulo-vesicular structure (vesicles up to 1 µm in diameter with connected tubules of approx. 50 nm diameter) and a mildly acid pH. They are principally sorting organelles where many endocytosed ligands dissociate from their receptors in the acid pH of the compartment, and from which many of the receptors recycle to the cell surface (via tubules).{{cite journal
| author = Mellman I | title = Endocytosis and molecular sorting | journal = Annual Review of Cell and Developmental Biology | volume = 12 | issue = | pages = 575–625 | year = 1996 | pmid = 8970738 | doi = 10.1146/annurev.cellbio.12.1.575 }}{{cite journal |vauthors=Mukherjee S, Ghosh RN, Maxfield FR | title = Endocytosis | journal = Physiological Reviews | volume = 77 | issue = 3 | pages = 759–803 |date=July 1997 | pmid = 9234965 | url = http://physrev.physiology.org/cgi/pmidlookup?view=long&pmid=9234965 | accessdate = 2009-06-19 }} It is also the site of sorting into transcytotic pathway to later compartments (like late endosomes or lysosomes) via transvesicular compartments (like multivesicular bodies (MVB) or endosomal carrier vesicles (ECVs)).
  • Late endosomes receive endocytosed material en route to lysosomes, usually from early endosomes in the endocytic pathway, from trans-Golgi network (TGN) in the biosynthetic pathway, and from phagosomes in the phagocytic pathway.{{cite journal
|vauthors=Stoorvogel W, Strous GJ, Geuze HJ, Oorschot V, Schwartz AL | title = Late endosomes derive from early endosomes by maturation | journal = Cell | volume = 65 | issue = 3 | pages = 417–27 |date=May 1991 | pmid = 1850321 | doi = 10.1016/0092-8674(91)90459-C }} Late endosomes often contain proteins characteristic of lysosomes, including lysosomal membrane glycoproteins and acid hydrolases. They are acidic (approx. pH 5.5), and are part of the trafficking pathway of mannose-6-phosphate receptors. Late endosomes are thought to mediate a final set of sorting events prior to delivery of material to lysosomes.
  • Lysosomes are the last compartment of the endocytic pathway. Their chief function is to break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds. These are then returned to the cytoplasm as new cell-building materials. To accomplish this, lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum, modified in the Golgi apparatus and function in an acidic environment.Weissmann, G. - Studies on Lysosomes,1965) The approximate pH of a lysosome is 4.8 and by electron microscopy (EM) usually appear as large vacuoles (1-2 µm in diameter) containing electron dense material. They have a high content of lysosomal membrane proteins and active lysosomal hydrolases, but no mannose-6-phosphate receptor. They are generally regarded as the principal hydrolytic compartment of the cell.{{cite journal
|vauthors=Gruenberg J, Maxfield FR | title = Membrane transport in the endocytic pathway | journal = Current Opinion in Cell Biology | volume = 7 | issue = 4 | pages = 552–63 |date=August 1995 | pmid = 7495576 | doi = 10.1016/0955-0674(95)80013-1 }}{{cite journal |vauthors=Luzio JP, Rous BA, Bright NA, Pryor PR, Mullock BM, Piper RC | title = Lysosome-endosome fusion and lysosome biogenesis | journal = Journal of Cell Science | volume = 113 | issue = 9 | pages = 1515–24 | pmid = 10751143 | url = http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=10751143 | accessdate = 2009-06-19 | date = May 1, 2000 }} It was recently found that an eisosome serves as a portal of endocytosis in yeast.

Clathrin-mediated endocytosis

The major route for endocytosis in most cells, and the best-understood, is that mediated by the molecule clathrin. 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.{{cite journal |vauthors=Benmerah A, Lamaze C | title = Clathrin-coated pits: vive la différence? | journal = Traffic (Copenhagen, Denmark) | volume = 8 | issue = 8 | pages = 970–82 |date=August 2007 | pmid = 17547704 | doi = 10.1111/j.1600-0854.2007.00585.x }}{{cite journal | author = Rappoport JZ | title = Focusing on clathrin-mediated endocytosis | journal = The Biochemical Journal | volume = 412 | issue = 3 | pages = 415–23 |date=June 2008 | pmid = 18498251 | doi = 10.1042/BJ20080474 }}{{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 = 681–6 |date=December 2007 | pmid = 17599959 | pmc = 2375507 | doi = 10.1113/jphysiol.2007.139022 }} 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.{{cite journal | author = 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 }}{{cite journal |vauthors=Glick BS, Malhotra V | title = The curious status of the Golgi apparatus | journal = Cell | volume = 95 | issue = 7 | pages = 883–9 |date=December 1998 | pmid = 9875843 | doi = 10.1016/S0092-8674(00)81713-4 }} 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.{{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 }} 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 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 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 16 minutes. Coated vesicles formed from the plasma membrane have a diameter of about 36 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 Matt Lions and Parker George. 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. Coated vesicles were first purified by Barbara Pearse, who discovered the clathrin coat molecule in 1976.

See also


External links

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