Structure of a clathrin-coated pit

a. Clathrin, the major protein in coated pits, forms triskelions (named after the symbol of three legs radiating from the center), which assemble into polyhedral lattices composed of hexagons and pentagons, such as the barrel shown in the next panel.

b. Image reconstruction from electron cryomicroscopy of a clathrin barrel formed from 36 triskelions. A single clathrin triskelion is highlighted in light blue.

c. A coated pit on the inner surface of the plasma membrane of a cultured mammalian cell is visualized by freeze-fracture electron microscopy.

The cage-like structure of the pit is due to the clathrin lattice.