Figure 1. Cartoon of the intracellular compartments involved in receptor-mediated endocytosis. Membrane proteins found at the cell surface or associated with endocytic organelles are synthesized in the endoplasmic reticulum and transported to the endocytic pathway via the Golgi apparatus. At least two types of transport vesicle are formed on the trans side of the Golgi one of which contains a clathrin coat (blue). Endocytic clathrin-coated vesicles are formed at the cell surface and transport receptors (green) and ligands (red) to endosomes. In coated pits, adaptor complexes (pink) couple receptors to the clathrin scaffold (blue). Clathrin-coated vesicles deliver receptors and ligands to early endosomes. In early endosomes receptors and ligands can be sorted for return to the cell surface or transport to late endocytic compartments where degradation or other processing events occur.

Figure 2. Thin section electron micrograph illustrating the bristle coat associated with a clathrin-coated pits. Bar, 60 nm. Provided by Annegret Pelchen-Matthews (MRC-LMCB, UCL, London).

Figure 3. Left-hand panel, diagram of arrangement of clathrin triskelions in a hexagonal barrel (see Smith et al., 1998). Four triskelions are indicated in different colours. In vivo coated vesicles are larger. The minimum composition required to accommodate a vesicle is believed to be 12 pentagons and 20 hexagons containing 60 triskelions, a mixture of hexagons and pentagons being necessary to achieve curvature. Each side of each polygon in the cage contains four segments of leg derived from four different triskelions. The proximal leg domains from adjacent hubs form an antiparallel pair on the top of each edge and the distal domains make an antiparallel pair beneath. The right-hand panel illustrates the domains of a single triskelion.