Clathrin-coated pits and vesicles are included in both the endocytosis of material at the plasma membrane and the exocytosis of proteins from the Golgi apparatus. Electron micrographs of clathrin-coated pits reveal which these pits are invaginations of the plasma membrane which are coated on their cytosolic surface with a densely packed material made up predominantly of the protein clathrin. This protein that has been highly conserved by evolution having of three huge and three little polypeptide chains which together form a three-legged structure called as a triskelion. These clathrin triskelions assemble into a basket-like convex framework of pentagons and hexagons to develop the coated pits.