Molecular model of clathrin. Clathrin plays an essential role in the formation of coated vesicles at cell membranes. Clathrin-mediated endocytosis regulates multiple cellular processes, such as internalisation of growth factors and receptors, entry of pathogens, and synaptic transmission. Clathrin assembles under the membrane by interaction with a receptor into a cage-like structure, effectively pulling the bud of a vesicle out of the surface. The cage is a polyhedral shape and clathrin is flexible enough to form hexagons or pentagons. The cage is assembled from clathrin subunits with a complex triple spiral and rotational symmetry, also referred to as triskelion, which contains three light chains and three heavy chains.