Three ways to make a vesicle

Abstract

Some steps of membrane transport require the formation of vesicles coated with COPI, COPII or clathrin. The coats probably generate the forces necessary to bend a relatively flat membrane into a vesicle. COPI coats function in transport through the Golgi and in retrograde transport from the Golgi to the endoplasmic reticulum. COPII coats function in transport from the ER to the Golgi. Clathrin functions in receptor-mediated endocytosis at the plasma membrane when associated with AP2 adaptors, and in transport from the trans-Golgi network (TGN) to endosomes when associated with AP1 adaptors. The assembly of COPII coats starts with the activation of the small GTPase Sar1p, followed by the sequential recruitment of Sec24p-Sec23p and then Sec13p-Sec31p. The GTPase activity of Sar1p acts as a timer for coat release. Similarly, the assembly of COPI coats starts with the activation of the small GTPase Arf1, followed by the recruitment of the coatomer, which binds to the cytoplasmic domain of cargo. The GTPase activity of Arf1 acts as a timer for coat release. The formation of clathrin coats is more complicated, as coat assembly, vesicle budding and uncoating are distinguishable steps regulated by different sets of proteins. Adaptor proteins (AP1, AP2 and arrestins) bind to the cytoplasmic signal sequences of cargo molecules. The recruitment of adaptors is only energy dependent at the TGN, and not at the plasma membrane. Many accessory proteins take part in a network of interactions that regulates clathrin-coat formation. In most cases, the precise function of these proteins is not known. Vesicle scission requires GTP hydrolysis by the GTPase dynamin. Vesicle uncoating requires ATP hydrolysis by auxilin-hsc70. The structure of many of the components of the clathrin machinery has been solved, and many of the protein-protein interactions have been analysed at atomic resolution. However, no structural information is available for COPI and COPII coats.