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. Author manuscript; available in PMC: 2010 Jul 22.
Published in final edited form as: Nat Rev Mol Cell Biol. 2008 Oct;9(10):781–795. doi: 10.1038/nrm2515

Figure 1. Endocytosis as a paradigm for phagocytosis.

Figure 1

Phagocytosis and receptor mediated endocytosis are similar processes, but also have distinct features.

(a) During receptor-mediated endocytosis, activated receptor sinks into the cell, in this case into a clathrin-coated pit21. Following scission of the vesicle from the membrane mediated by dynamin, clathrin is removed20; receptors are then trafficked to the `recycling endosome,' where they are eventually sorted back to the cell surface22. Activated receptors are also removed from phagosomes and presumably are also targeted to recycling endosomes. Endocytosed vesicles acquire the GTPase Rab5, which is activated by GEFs (such as RabEx-5 and GapEx-5)150 which allows homotypic fusion with other Rab5-positive structures23, 57, as well as heterotypic fusion with Rab4-positive recycling endosomes70. Rab5 is then exchanged for Rab7 via the HOPS complex23, which contains the Rab7 GEF Vps39, resulting in recruitment of the Rab7 effector RILP and ultimately fusion with acidic lysosomes151 and acquisition of the lysosomal markers LAMP-1 and LAMP-2.

(b) In contrast, during phagocytosis the plasma membrane is extended around the apoptotic cell, forming the phagosome8, 14. Phagosomes also `mature' through Rab5(+) and Rab7(+) stages, ultimately resulting in fusion with lysosome structures28, 33, 41, 52, 57, 58, however phagosomes appear to utilize a distinct mechanism for GTPase regulation. In the nematode, known RAB-5 GEFs are not required for phagosome maturation), and the HOPS complex is not required for the recruitment of Rab7 to the phagosome. This may reflect the observations that phagosomes rarely fuse together and tend to mature individually. Other events, such as trafficking of proteins from the Golgi to the phagosome have been described43, although whether this occurs during endocytosis remains to be determined. A number of GTPases have also been found associated with the phagosome during phagosome maturation (see Box 3), many of which have not been described to play a role on the endosome.