Clathrin-coated vesicles mediate selective transport. They are, for example, the means by which protein in the trans Golgi that bears mannose-6-phosphate is collected into a vesicle bound for the lysosome. Clathrin-coated vesicles carry proteins and lipids from the plasma membrane to the endosome, and operate in other places where selective transport is required.
Figure 10.10ft illustrates how clathrin generates a vesicle. The process starts when the cargo of interest binds to integral proteins of the donor membrane that are selective receptors for that cargo. Clathrin adaptor proteins then bind to cargo-loaded receptors and begin to associate, forming a complex. Lastly, clathrin molecules bind to this complex, forming the coat and bending the membrane into the bud shape.
Even though clathrin can force the membrane into a bud shape, it cannot force the bud to leave as an independent vesicle. One of the best-studied membrane fission events is endocytosis. Here a GTPase called dynamin forms a ring around the neck of a budding vesicle. GTP hydrolysis then causes a change in dynamin's shape that mechanically pinches the vesicle off from its membrane of origin. Unlike coatamer, clathrin coats dissociate as soon as the vesicle is formed, leaving the vesicle ready to fuse with the target membrane.
If phospholipids are shaken up with aqueous medium, they spontaneously form artificial vesicles called liposomes. If these are then incubated with purified coatomer proteins, ARF, and GTP, bud formation and coated vesicles can be observed by electron microscopy. In contrast neither buds nor vesicles form if liposomes are incubated with clathrin and clathrin adaptor proteins. Clathrin-mediated budding requires ligand to be present in the vesicles, and receptors for that ligand to be present in the membrane of the vesicle. Furthermore even if buds did form, clathrin on its own, unlike coatamer, cannot cause fission. Clathrin-coated buds require another protein such as dynamin to trigger fission.
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