CLATHRIN AND ADAPTERS

Clathrin and adaptors are components of clathrin-coated pits and vesic les. The AP-1 adaptor complex is associated with clathrin-coated vesic les budding from the TGN, while the AP-2 adaptor complex is associated with clathrin-coated vesicles budding from the plasma membrane. The c lathrin forms a polyhedral lattice and is believed to be the driving f orce behind membrane invagination leading to vesicle budding. The adap tors attach the clathrin to the membrane and also interact with the cy toplasmic domains of selected transmembrane proteins, causing these pr oteins to become concentrated in clathrin-coated vesicles. Clathrin-co ated vesicles budding from the TGN have been implicated in the sorting of newly synthesised lysosomal enzymes, while clathrin-coated vesicle s budding from the plasma membrane facilitate the receptor-mediated en docytosis of ligands, such as low density lipoproteins and transferrin . A novel adaptor-related complex, AP-3, has recently been identified, which is recruited onto membranes of the TGN and a more peripheral co mpartment but does not appear to be associated with clathrin. Genetic studies indicate that AP-3 plays a role in the sorting of proteins to lysosomes and lysosome-related organelles. (C) 1998 Elsevier Science B .V. All rights reserved.