Clathrin is a protein that plays a major role in the formation of coated vesicles.
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(above). Cytoplasmic face of the plasma membrane, prepared using a rapid freeze, deep etch, rotary shadowing method. Note the flat clathrin lattices, consisting mainly of hexagons, and the rounder, deeply invaginated clathrin-coated pits, consisting of pentagons as well as hexagons. Electron micrograph courtesy of Dr. John Heuser. (link)