Mechanism of endocytic downregulation of RTKs. The figure illustrates RTK downregulation, with EGFR as an example. Ligand binding triggers receptor autophosphorylation via receptor dimerization. c-Cbl associates with the tyrosine-phosphorylated RTK and mediates its multi-ubiquitination. This facilitates endocytosis of the receptor via clathrin-coated pits. Proteins such as Epsin, Eps15 and Hip1 may be involved in cargo recruitment into clathrin-coated pits. Clathrin polymerization is catalysed by AP-180/CALM, and vesicle formation is facilitated by Dynamin and Endophilin. The endocytic vesicle uncoats (facilitated by Synaptojanin) and fuses with an early endosome, by a mechanism catalysed by the small GTPase Rab5 and its effectors. At the early endosome, the ubiquitinated RTK is sorted into a ‘bilayered' clathrin coat, probably by associating with ubiquitin-binding proteins such as Hrs and STAM. From here, the RTK is sorted into intraluminal vesicles of the endosome, in a process mediated by ESCRT-I (whose subunit Tsg101 binds Hrs as well as ubiquitin), ESCRT-II and ESCRT-III. The RTK is probably dephosphorylated and deubiquitinated during this sorting step. From the early endosome, a multivesicular body (MVB)/endosomal carrier vesicle is formed in an Annexin-II-dependent fashion (), which ultimately fuses with a late endosome or a lysosome. Intraluminal vesicles with their content are then degraded by lysosomal hydrolases. Ubiquitin-binding/monoubiquitinated proteins are in red, proteins found as oncogenic fusion proteins (see ) are in italics, and proteins experimentally implicated in cancer (see ) are in bold. A recycling pathway from the early endosome to the plasma membrane, which is followed by non-ubiquitinated membrane proteins, is indicated by dashed arrows. For simplicity, a number of known components of the endocytosis machinery, including specific lipids (), have been omitted from the figure. Note that RTKs can also be endocytosed from other invaginations than clathrin-coated pits, but the significance of this alternative internalization in RTK downregulation is not known ().