Fig. 1

Endocytosis and endosome maturation. Protein internationalization is often dependent on a ubiquitous clathrin-mediated mechanism. Subsequent to internationalization, cargo proteins are transported to early endosomes via endocytic vesicles under the control of Rab5. Early endosomes serve as the major sorting stations where proteins can be sorted into recycling endosomes for recycling back to the cell surface, into a retrogradee pathway mediated by retromer to be sent back to the trans-golgi network (TGN), or into a degradation pathway for eventual targeting to the lysosome. As the number of intraluminal vesicles increases, early endosomes mature into late endosomes, and then late endosomes fuse with lysosomes.Due to their different capacities of acidification, a stable pH gradient is established in different compartments during the maturation process: early endosomes maintain pH at about 6.5, late endosomes at about 5.5 and lysosomes at about 4.5