FIGURE 2.14 • Steps in formation, targeting, docking, and fusion of transport vesicles with the target membrane. (1) Lipid raft with cargo receptors ready to interact with cargo protein. Note the presence of the specifc targeting protein v-SNARE. (2) Initial step in vesicle formation: The binding of the adaptin complex and clathrin forms a coated pit. (3) Formation (budding) of fully assembled coated vesicle. (5) Disassembly of clathrin coat. Note the expression of Rab-GTPase activity. (6) Tethering of the vesicle to the target membrane by the interaction between Rab-GTPase and tethering proteins. (7) Beginning of the docking process (recruitment of tethering proteins). (8) Formation of the docking complex between Rab-GTPase and its protein in the target membrane: v-SNAREs on the immobilized vesicle interact with t-SNAREs on the target membrane to form the cis-SNARE complex. (9) Fusion of the vesicle to the target membrane. (10) Discharge of the cargo protein into the early endosomal compartment and disassembly of the cis complex by the interaction of the NSF/ -SNAP protein complex. (11) Recycling of v-SNAREs in the transport vesicles for use in another round of vesicle targeting and fusion.
FIGURE 2.15 • Electron micrograph of an early endosome.
This deep-etch electron micrograph shows the structure of an early endosome in Dictyostelium. Early endosomes are located near the plasma membrane and, as in many other sorting compartments, have a typical tubulovesicle structure. The tubular portions contain the majority of integral membrane proteins destined for membrane recycling, whereas the luminal portions collect secretory cargo proteins. The lumen of the endosome is subdivided into multiple compartments, or cisternae, by the invagination of its membrane and undergoes frequent changes in shape. 15,000. (Courtesy of Dr. John E. Heuser, Washington University School of Medicine.)
Endosomes can be viewed either as stable cytoplasmic organelles or as transient structures formed as the result of endocytosis.
Recent experimental observations of endocytotic pathways conducted in vitro and in vivo suggest two different models that explain the origin and formation of the endosomal compartments in the cell:  The stable compartment model describes early and late endosomes as stable cellular organelles connected by vesicular transport with the external environment of the cell and with the Golgi apparatus. Coated vesicles formed at the plasma membrane fuse only with early endosomes because of their expression of specific surface receptors. The receptor remains a resident component of the early endosomal membrane.
 In the maturation model, early endosomes are formed de novo from endocytotic vesicles originating from the plasma membrane. Therefore, the composition of the early endosomal membrane changes progressively as some components are recycled between the cell surface and the Golgi apparatus. This maturation process leads to formation of late endosomes and then to lysosomes. Specific receptors present on early endosomes (e.g., for coated vesicles) are removed by recycling, degradation, or inactivation as this compartment matures.
Both models actually complement rather than contradict each other in describing, identifying, and studying the pathways of internalized molecules.
Endosomes destined to become lysosomes receive newly synthesized lysosomal enzymes that are targeted via the mannose-6-phosphate receptor.