【多选题】How are proteins further modified and sorted in the Golgi apparatus?
A.
The proteins travel through the cisternae in sequence in two ways: (1) by means of transport vesicles that bud from one cisterna and fuse with the next; and (2) by a maturation process in which the Golgi cisternae themselves migrate through the Golgi stack. Proteins finally exit from the trans Golgi network in transport vesicles destined for either the cell surface or another organelle of the endomembrane system.
B.
Both the ci s and trans Golgi networks are thought to be important for protein sorting: proteins entering the ci s Golgi network can either move onward through the Golgi stack or, if they contain an ER retention signal, be returned to the ER; proteins exiting from the trans Golgi network are sorted according to whether they are destined for lysosomes (via endosomes) or for the cell surface.
C.
Many of the oligosaccharide chains that are added to proteins in the ER undergo further modifications in the Golgi apparatus. On some proteins, for example, more complex oligosaccharide chains are created by a highly ordered process in which sugars are added and removed by a series of enzymes that act in a rigidly determined sequence as the protein passes through the Golgi stack.
D.
As would be expected, the enzymes that act early in the chain of processing events are located in cisternae close to the ci s face, while enzymes that act late are located in cisternae near the trans face.