FIGURE 14-33 Model of the mechanism for formation of multivesicular endosomes. In endosomal budding, ubiquitinylated Hrs on the endosomal membrane directs the loading of specific membrane cargo proteins (blue) into vesicle buds and then recruits cytosolic ESCRT protein complexes to the membrane (step 1). Note that both Hrs and the recruited cargo proteins are tagged with ubiquitin. After the set of bound ESCRT complexes mediates the completion and pinching off of the inwardly budding vesicles (step 2), these complexes are disassembled by the ATPase Vps4 and returned to the cytosol (step 3). See text for discussion. See O. Pornillos et al., 2002, Trends Cell Biol. 12:569.