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.