The different types of endocytosis

Endocytosis is a form of active transport in which a cell transports molecules (such as proteins) into the cell (endo- + 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.[1] Phagocytosis was discovered by Élie Metchnikoff. [2]

Endocytosis pathways

Schematic drawing illustrating clathrin-mediated (left) and clathrin-independent endocytosis (right) of synaptic vesicle membranes.

Endocytosis pathways can be subdivided into four categories: namely, receptor-mediated endocytosis, caveolae, macropinocytosis, and phagocytosis.[3]

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.[7]

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:[3]

It was recently found that an eisosome serves as a portal of endocytosis in yeast.[14]

Clathrin-mediated endocytosis

The major route for endocytosis in most cells, and the best-understood, is that mediated by the molecule clathrin.[15][16] 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.[17][18][19]

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.[20][21] 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.[22] 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.[23] Coated vesicles were first purified by Barbara Pearse, who discovered the clathrin coat molecule in 1976.[24]

See also


  1. Rieger, R.; Michaelis, A.; Green, M.M. 1991. Glossary of Genetics. Classical and Molecular (Fifth edition). Springer-Verlag, Berlin, .
  2. "Ilya Mechnikov - Biographical". www.nobelprize.org. Retrieved 2016-10-10.
  3. 1 2 Marsh, Mark (2001). Endocytosis. Oxford University Press. p. vii. ISBN 978-0-19-963851-2.
  4. Marsh, M.; McMahon, HT (July 1999). "The structural era of endocytosis". Science. 285 (5425): 215–20. doi:10.1126/science.285.5425.215. PMID 10398591. Retrieved 2009-06-19.
  5. Parton RG, Simons K (March 2007). "The multiple faces of caveolae". Nature Reviews Molecular Cell Biology. 8 (3): 185–94. doi:10.1038/nrm2122. PMID 17318224.
  6. Falcone S, Cocucci E, Podini P, Kirchhausen T, Clementi E, Meldolesi J (November 2006). "Macropinocytosis: regulated coordination of endocytic and exocytic membrane traffic events". Journal of Cell Science. 119 (Pt 22): 4758–69. doi:10.1242/jcs.03238. PMID 17077125.
  7. Lundmark R, Doherty GJ, Howes MT, et al. (November 2008). "The GTPase-Activating Protein GRAF1 Regulates the CLIC/GEEC Endocytic Pathway". Current Biology. 18 (22): 1802–8. doi:10.1016/j.cub.2008.10.044. PMC 2726289Freely accessible. PMID 19036340.
  8. Mellman I (1996). "Endocytosis and molecular sorting". Annual Review of Cell and Developmental Biology. 12: 575–625. doi:10.1146/annurev.cellbio.12.1.575. PMID 8970738.
  9. Mukherjee S, Ghosh RN, Maxfield FR (July 1997). "Endocytosis". Physiological Reviews. 77 (3): 759–803. PMID 9234965. Retrieved 2009-06-19.
  10. Stoorvogel W, Strous GJ, Geuze HJ, Oorschot V, Schwartz AL (May 1991). "Late endosomes derive from early endosomes by maturation". Cell. 65 (3): 417–27. doi:10.1016/0092-8674(91)90459-C. PMID 1850321.
  11. Weissmann, G. - Studies on Lysosomes,1965)
  12. Gruenberg J, Maxfield FR (August 1995). "Membrane transport in the endocytic pathway". Current Opinion in Cell Biology. 7 (4): 552–63. doi:10.1016/0955-0674(95)80013-1. PMID 7495576.
  13. Luzio JP, Rous BA, Bright NA, Pryor PR, Mullock BM, Piper RC (May 1, 2000). "Lysosome-endosome fusion and lysosome biogenesis". Journal of Cell Science. 113 (9): 1515–24. PMID 10751143. Retrieved 2009-06-19.
  14. Walther TC, Brickner JH, Aguilar PS, Bernales S, Pantoja C, Walter P (February 2006). "Eisosomes mark static sites of endocytosis". Nature. 439 (7079): 998–1003. doi:10.1038/nature04472. PMID 16496001.
  15. Kirchhausen, T.; Owen, D.; Harrison, S. C. (1 May 2014). "Molecular Structure, Function, and Dynamics of Clathrin-Mediated Membrane Traffic". Cold Spring Harbor Perspectives in Biology. 6 (5): a016725–a016725. doi:10.1101/cshperspect.a016725.
  16. Bitsikas, V; Corrêa IR, Jr; Nichols, BJ (17 September 2014). "Clathrin-independent pathways do not contribute significantly to endocytic flux.". eLife. 3: e03970. doi:10.7554/eLife.03970. PMID 25232658.
  17. Benmerah A, Lamaze C (August 2007). "Clathrin-coated pits: vive la différence?". Traffic (Copenhagen, Denmark). 8 (8): 970–82. doi:10.1111/j.1600-0854.2007.00585.x. PMID 17547704.
  18. Rappoport JZ (June 2008). "Focusing on clathrin-mediated endocytosis". The Biochemical Journal. 412 (3): 415–23. doi:10.1042/BJ20080474. PMID 18498251.
  19. Granseth B, Odermatt B, Royle SJ, Lagnado L (December 2007). "Clathrin-mediated endocytosis: the physiological mechanism of vesicle retrieval at hippocampal synapses". The Journal of Physiology. 585 (Pt 3): 681–6. doi:10.1113/jphysiol.2007.139022. PMC 2375507Freely accessible. PMID 17599959.
  20. Robinson MS (March 1997). "Coats and vesicle budding". Trends in Cell Biology. 7 (3): 99–102. doi:10.1016/S0962-8924(96)10048-9. PMID 17708916.
  21. Glick BS, Malhotra V (December 1998). "The curious status of the Golgi apparatus". Cell. 95 (7): 883–9. doi:10.1016/S0092-8674(00)81713-4. PMID 9875843.
  22. Gaidarov I, Santini F, Warren RA, Keen JH (May 1999). "Spatial control of coated-pit dynamics in living cells". Nature Cell Biology. 1 (1): 1–7. doi:10.1038/8971. PMID 10559856.
  23. Anderson, RG; Brown, MS; Goldstein, JL (March 1977). "Role of the coated endocytic vesicle in the uptake of receptor-bound low density lipoprotein in human fibroblasts.". Cell. 10 (3): 351–64. doi:10.1016/0092-8674(77)90022-8. PMID 191195.
  24. Pearse, BM (April 1976). "Clathrin: a unique protein associated with intracellular transfer of membrane by coated vesicles.". Proceedings of the National Academy of Sciences of the United States of America. 73 (4): 1255–9. doi:10.1073/pnas.73.4.1255. PMC 430241Freely accessible. PMID 1063406.

External links

This article is issued from Wikipedia - version of the 11/10/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.