Molecular Mechanisms of Vesicular Traffic

Small membrane-bounded vesicles that transport proteins from one organelle to another are common elements in the secretory and endocytic pathways (see Figure 17-1). These vesicles bud from the membrane of a particular "parent" (donor) organelle and fuse with the membrane of a particular "target" (destination) organelle. Although each step in the secretory and endocytic pathways employs a different type of vesicle, studies employing genetic and biochemical techniques described in the previous section have revealed that each of the different vesicular transport steps is simply a variation on a common theme. In this section we explore that common theme, the basic mechanisms underlying vesicle budding and fusion.

(a) Coated vesicle budding

Soluble cargo protein

(a) Coated vesicle budding

Soluble cargo protein

v-SNARE protein

Membrane cargo protein

Membrane cargo-receptor protein

Cytosol

GTP-binding protein v-SNARE protein

Membrane cargo protein

Membrane cargo-receptor protein

Donor membrane

Coat proteins

Cytosol

(b) Uncoated vesicle fusion

Cytosol Target membrane

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