Figure 1.

Exosome biogenesis within endosomal system. Early endosomes are formed by the fusion of endocytic vesicles, further transformed into late endosomes/MVBs through inward budding of the membrane (packing the cargo into ILVs). ILVs protein sorting has two methods, ESCRT-dependent and ESCRT-independent. ESCRT-0, ESCRT-I, ESCRT-II and ESCRT-III are the four components of the ESCRT. They generalize the substrate on the inwardly budding endosomal membrane. Subsequently, a part of MVBs is fused with lysosomes to be degraded, while another part of MVBs can be fused with the plasma membrane to release ILVs into the extracellular space, now called exosomes. In addition, Rab27A and Rab27B are important mediators leading MVBs to the periphery of cells, and the SNARE complex helps MVBs fuse with the plasma membrane.