Clathrin-coated pits and vesicles

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 consists of three huge and three small polypeptide chains which together form a three-legged structure known as a triskelion.  These clathrin triskelions   assemble   into a basket-like   convex framework   of pentagons and hexagons   to form the coated pits. The assembly of the clathrin coat is by to drive the membrane to invaginate at which point. As further clathrin triskelions are added to the structure a whole cage forms pinching off a region of the membrane and establishing a clathrin-coated vesicle.  In the endocytosis these vesicles then migrate into the cell shed their clathrin coat and become early endosomes. The before endosomes then migrate towards the Golgi apparatus and the nucleus becoming late endosomes before fusing with the lysosomes.