The proteins and glycoproteins leaving the RER are transported to the Golgi apparatus in small spherical membrane-bounded compartments, known as transport vesicles, which pinch off from the RER. The membrane of the transport vesicle fuses with the Golgi membrane, so proteins inside the vesicle are released into the Golgi lumen while proteins embedded in the vesicle membranes are incorporated into the Golgi membrane. After further processing, sorting and packaging in the Golgi apparatus, the modified proteins are again packaged into vesicles (Figure 19b) and delivered to their final destination in a similar manner. Secretory cells usually have large numbers of secretory vesicles which fuse with the cell membrane to release their contents outside the cell. There are many different types of vesicle, even in cells that are not specialised for secretion. Vesicles are also used to excrete waste materials and to transport many different molecules within cells, for example proteins and glycoproteins. Newly synthesised lipids from the SER are incorporated in vesicle membranes and are therefore also delivered to the cell membrane in these small structures.
How do vesicles move within cells, and what mechanism ensures their correct delivery? The surfaces of vesicles have different coatings and surface signals that identify their contents and ensure that they fuse with the correct target membrane. Vesicles can move short distances within the cytosol by diffusion, but movement to more distant sites in the cell is mediated by the action of special 'motor' proteins that are able to 'walk' vesicles along the cytoskeleton network.