The membrane protein, clathrin is involved in:

Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. Clathrin molecules have the shape of triskelions, with three "legs" radiating from a central hub. As endocytosis progresses, the clathrin molecules form a geometric array that surrounds the endocytotic vesicle. At the neck of the vesicle, the GTP binding protein dynamin is involved, either directly or indirectly, in pinching off the vesicle. Once the complete vesicle is formed, the clathrin falls off and the three-legged proteins recycle to form another vesicle. The vesicle fuses with and dumps its contents into an early endosome. From the early endosome, a new vesicle can bud off and return to the cell membrane. Alternatively, the early endosome can become a late endosome and fuse with a lysosome in which the contents are digested by the lysosomal proteases. Clathrin-mediated endocytosis is responsible for the internalization of many receptors and the ligands bound to them--including, for example, nerve growth factor (NGF) and low-density lipoproteins. It also plays a major role in synaptic function. Also Know: In exocytosis the cytoplasmic sides of two membranes fuse, whereas in endocytosis two non cytoplasmic sides fuse. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 2. Overview of Cellular Physiology in Medical Physiology. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.