Clathrin is a major protein component of the polyhedral coat of coated pits and vesicles. Two different adapter protein complexes link the clathrin lattice to either the plasma membrane or the trans-Golgi network. These specialised organelles are involved in the intracellular trafficking of receptors and endocytosis of a variety of macromolecules. Clathrin triskelions, composed of 3 heavy chains and 3 light chains, are the basic subunits of the clathrin coat. In the presence of light chains, hub assembly is influenced by both the pH and the concentration of calcium. Clathrin localises to the cytoplasmic face of coated pit and vesicle membranes.