Endocytosis is the process by which cells are able to internalize membrane and extracellular materials through the formation of a vesicle. The process of membrane budding to form a vesicle is generally mediated by a protein coat, which acts both as a means to deform the membrane and to concentrate specific types of proteins inside the nascent vesicle. Clathrin is a coat protein that has been shown to function in receptor-mediated endocytosis events at the plasma membrane.
The animation below shows the process of clathrin-mediated endocytosis. Iron-bound transferrin is bound to its receptor on the exterior cell membrane. The transferrin receptor in turn binds to adaptor proteins in the interior of the cell, triggering the formation of a clathrin cage around the bound transferrin receptors. Soon after the vesicle has budded off the membrane, clathrin cage disassembly begins. Disassembly is mediated by HSC70, and its cofactor auxilin.
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Clathrin: Molecule of the Month at the Protein Data Bank
Many thanks to Tom Kirchhausen (HMS) for collaborating on this project.