Editing Endosome (section)

== Types ==
There are three different types of endosomes: ''early endosomes'', ''late endosomes'', and ''recycling endosomes''.<ref name="endo1" /> They are distinguished by the time it takes for endocytosed material to reach them, and by markers such as [[Rab (G-protein)|Rabs]].<ref name="Stenmark-2009">{{cite journal | vauthors = Stenmark H | title = Rab GTPases as coordinators of vesicle traffic | journal = Nature Reviews. Molecular Cell Biology | volume = 10 | issue = 8 | pages = 513–25 | date = August 2009 | pmid = 19603039 | doi = 10.1038/nrm2728 | s2cid = 33236823 }}</ref> They also have different morphology. Once [[Endocytosis|endocytic]] vesicles have uncoated, they fuse with early endosomes. Early endosomes then ''mature'' into late endosomes before fusing with lysosomes.<ref name="Futter-1996">{{cite journal | vauthors = Futter CE, Pearse A, Hewlett LJ, Hopkins CR | title = Multivesicular endosomes containing internalized EGF-EGF receptor complexes mature and then fuse directly with lysosomes | journal = The Journal of Cell Biology | volume = 132 | issue = 6 | pages = 1011–23 | date = March 1996 | pmid = 8601581 | pmc = 2120766 | doi = 10.1083/jcb.132.6.1011 }}</ref><ref name="eele2">{{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 ( Pt 9) | pages = 1515–24 | date = May 2000 | issue = 9 | doi = 10.1242/jcs.113.9.1515 | pmid = 10751143 | doi-access = free }}</ref>

Early endosomes mature in several ways to form late endosomes. They become increasingly acidic mainly through the activity of the V-ATPase.<ref name="Lafourcade-2008">{{cite journal | vauthors = Lafourcade C, Sobo K, Kieffer-Jaquinod S, Garin J, van der Goot FG | title = Regulation of the V-ATPase along the endocytic pathway occurs through reversible subunit association and membrane localization | journal = PLOS ONE | volume = 3 | issue = 7 | pages = e2758 | date = July 2008 | pmid = 18648502 | pmc = 2447177 | doi = 10.1371/journal.pone.0002758 | editor1-last = Joly | editor1-first = Etienne | bibcode = 2008PLoSO...3.2758L | doi-access = free }}</ref> Many molecules that are recycled are removed by concentration in the tubular regions of early endosomes. Loss of these tubules to recycling pathways means that late endosomes mostly lack tubules. They also increase in size due to the homotypic fusion of early endosomes into larger vesicles.<ref name="Rink-2005">{{cite journal | vauthors = Rink J, Ghigo E, Kalaidzidis Y, Zerial M | title = Rab conversion as a mechanism of progression from early to late endosomes | journal = Cell | volume = 122 | issue = 5 | pages = 735–49 | date = September 2005 | pmid = 16143105 | doi = 10.1016/j.cell.2005.06.043 | doi-access = free }}</ref> Molecules are also sorted into smaller vesicles that bud from the perimeter membrane into the endosome lumen, forming '''intraluminal vesicles''' (ILVs); this leads to the multivesicular appearance of late endosomes and so they are also known as '''multivesicular endosomes''' or '''multivesicular bodies''' (MVBs). Removal of recycling molecules such as [[transferrin receptor]]s and mannose 6-phosphate receptors continues during this period, probably via budding of vesicles out of endosomes.<ref name="Futter-1996" /> Finally, the endosomes lose [[RAB5A]] and acquire [[RAB7A]], making them competent for fusion with lysosomes.<ref name="Rink-2005" />

Fusion of late endosomes with lysosomes has been shown to result in the formation of a 'hybrid' compartment, with characteristics intermediate of the two source compartments.<ref name="endo-lysfus">{{cite journal | vauthors = Mullock BM, Bright NA, Fearon CW, Gray SR, Luzio JP | title = Fusion of lysosomes with late endosomes produces a hybrid organelle of intermediate density and is NSF dependent | journal = The Journal of Cell Biology | volume = 140 | issue = 3 | pages = 591–601 | date = February 1998 | pmid = 9456319 | pmc = 2140175 | doi = 10.1083/jcb.140.3.591 }}</ref> For example, lysosomes are more dense than late endosomes, and the hybrids have an intermediate density. Lysosomes reform by recondensation to their normal, higher density. However, before this happens, more late endosomes may fuse with the hybrid.

Some material recycles to the plasma membrane directly from early endosomes,<ref name="fastrecyc1">{{cite journal | vauthors = Hopkins CR, Trowbridge IS | title = Internalization and processing of transferrin and the transferrin receptor in human carcinoma A431 cells | journal = The Journal of Cell Biology | volume = 97 | issue = 2 | pages = 508–21 | date = August 1983 | pmid = 6309862 | pmc = 2112524 | doi = 10.1083/jcb.97.2.508 }}</ref> but most traffics via recycling endosomes.

*''Early endosomes'' consist of a dynamic tubular-vesicular network (vesicles up to 1&nbsp;μm in diameter with connected tubules of approx. 50&nbsp;nm diameter). Markers include [[RAB5A]] and RAB4, [[Transferrin]] and its [[Transferrin receptor|receptor]] and [[EEA1]].
*''Late endosomes'', also known as MVBs, are mainly spherical, lack tubules, and contain many close-packed intraluminal vesicles. Markers include RAB7, RAB9, and mannose 6-phosphate receptors.<ref name="Russell-2006">{{cite journal | vauthors = Russell MR, Nickerson DP, Odorizzi G | title = Molecular mechanisms of late endosome morphology, identity and sorting | journal = Current Opinion in Cell Biology | volume = 18 | issue = 4 | pages = 422–8 | date = August 2006 | pmid = 16781134 | doi = 10.1016/j.ceb.2006.06.002 }}</ref> In addition to this, the late endosomal membrane (and consequently the lysosome) contains a peculiar and unique [[lipid]] named BMP or LBPA, which is not found in any other organelle membrane.<ref>{{Cite journal |last1=Kobayashi |first1=Toshihide |last2=Beuchat |first2=Marie-Hélène |last3=Chevallier |first3=Julien |last4=Makino |first4=Asami |last5=Mayran |first5=Nathalie |last6=Escola |first6=Jean-Michel |last7=Lebrand |first7=Cecile |last8=Cosson |first8=Pierre |last9=Kobayashi |first9=Tetsuyuki |last10=Gruenberg |first10=Jean |date=2002-08-30 |title=Separation and characterization of late endosomal membrane domains |journal=The Journal of Biological Chemistry |volume=277 |issue=35 |pages=32157–32164 |doi=10.1074/jbc.M202838200 |issn=0021-9258 |pmid=12065580|doi-access=free }}</ref><ref name="Hullin-Matsuda 313–324">{{Cite journal |last1=Hullin-Matsuda |first1=F. |last2=Luquain-Costaz |first2=C. |last3=Bouvier |first3=J. |last4=Delton-Vandenbroucke |first4=I. |date=November 2009 |title=Bis(monoacylglycero)phosphate, a peculiar phospholipid to control the fate of cholesterol: Implications in pathology |url=https://pubmed.ncbi.nlm.nih.gov/19857945/ |journal=Prostaglandins, Leukotrienes, and Essential Fatty Acids |volume=81 |issue=5–6 |pages=313–324 |doi=10.1016/j.plefa.2009.09.006 |issn=1532-2823 |pmid=19857945}}</ref>
*''Recycling endosomes'' are concentrated at the microtubule organizing center and consist of a mainly tubular network. Marker; RAB11.<ref name="Ullrich-1996">{{cite journal | vauthors = Ullrich O, Reinsch S, Urbé S, Zerial M, Parton RG | title = Rab11 regulates recycling through the pericentriolar recycling endosome | journal = The Journal of Cell Biology | volume = 135 | issue = 4 | pages = 913–24 | date = November 1996 | pmid = 8922376 | pmc = 2133374 | doi = 10.1083/jcb.135.4.913 }}</ref>

More subtypes exist in specialized cells such as polarized cells and [[macrophage]]s.

[[Phagosomes]], [[macropinosome]]s and [[autophagosome]]s<ref name="Fader-2009">{{cite journal | vauthors = Fader CM, Colombo MI | title = Autophagy and multivesicular bodies: two closely related partners | journal = Cell Death and Differentiation | volume = 16 | issue = 1 | pages = 70–8 | date = January 2009 | pmid = 19008921 | doi = 10.1038/cdd.2008.168 | doi-access = free }}</ref> mature in a manner similar to endosomes, and may require fusion with normal endosomes for their maturation. Some intracellular pathogens subvert this process, for example, by preventing RAB7 acquisition.<ref name="Körner-2006">{{cite journal | vauthors = Körner U, Fuss V, Steigerwald J, Moll H | title = Biogenesis of Leishmania major-harboring vacuoles in murine dendritic cells | journal = Infection and Immunity | volume = 74 | issue = 2 | pages = 1305–12 | date = February 2006 | pmid = 16428780 | pmc = 1360340 | doi = 10.1128/IAI.74.2.1305-1312.2006 }}</ref>

Late endosomes/MVBs are sometimes called ''endocytic carrier vesicles'', but this term was used to describe vesicles that bud from early endosomes and fuse with late endosomes. However, several observations (described above) have now demonstrated that it is more likely that transport between these two compartments occurs by a maturation process, rather than vesicle transport.

Another unique identifying feature that differs between the various classes of endosomes is the lipid composition in their membranes. Phosphatidyl inositol phosphates (PIPs), one of the most important [[lipid]] signaling molecules, is found to differ as the endosomes mature from early to late. [[Phosphatidylinositol 4,5-bisphosphate|PI(4,5)P<sub>2</sub>]] is present on [[Cell membrane|plasma membranes]], [[Phosphatidylinositol 3-phosphate|PI(3)P]] on early endosomes, PI(3,5)P<sub>2</sub> on late endosomes and [[Phosphatidylinositol 4-phosphate|PI(4)P]] on the [[trans Golgi network]].<ref>{{cite journal | vauthors = van Meer G, Voelker DR, Feigenson GW | title = Membrane lipids: where they are and how they behave | journal = Nature Reviews. Molecular Cell Biology | volume = 9 | issue = 2 | pages = 112–24 | date = February 2008 | pmid = 18216768 | pmc = 2642958 | doi = 10.1038/nrm2330 }}</ref> These lipids on the surface of the endosomes help in the specific recruitment of proteins from the cytosol, thus providing them an identity. The inter-conversion of these lipids is a result of the concerted action of phosphoinositide [[kinase]]s and [[phosphatases]] that are strategically localized<ref>{{cite journal | vauthors = Di Paolo G, De Camilli P | title = Phosphoinositides in cell regulation and membrane dynamics | journal = Nature | volume = 443 | issue = 7112 | pages = 651–7 | date = October 2006 | pmid = 17035995 | doi = 10.1038/nature05185 | bibcode = 2006Natur.443..651D | s2cid = 4421550 }}</ref>