In eukaryotic cells, the endoplasmic reticulum (ER) and the Golgi apparatus are part of an elaborate network of membrane compartments called the endomembrane system. These organelles work together to make and process many of the proteins, lipids, and carbohydrates that the cell uses or exports. The subset of cellular proteins that begin their journeys in the ER are those that will be exported from the cell, incorporated into membranes, or moved into organelles of the endomembrane system, such as lysosomes. In this animation, we examine the path of some of these proteins, from their production in the ER, to their transport through the Golgi, to their release from the cell.
The endoplasmic reticulum, or ER, and the Golgi apparatus produce and process proteins. These proteins move from their site of production in the ER through the Golgi, after which they proceed to their destinations: either taking residence in the plasma membrane, or being secreted outside of the cell, or fusing with lysosomes.
Proteins from the ER travel in transport vesicles to a region of the Golgi called the cis Golgi network, where protein modification begins. We can follow the maturation of this compartment by marking it with a dark color.
Vesicles fuse with the cis Golgi network until it begins to mature and move into the Golgi stack, and then the vesicles create a new compartment in its place. The compartments, with their proteins, mature to become the medial and then the trans compartments of the Golgi stack, where further protein modifications take place.
The compartments of the Golgi gradually mature and progressively move through the Golgi in the cis-to-trans direction, carrying their contents with them.
The trans Golgi network acts as a sorting and distribution center, directing molecules to their final destinations. The proteins leave in vesicles, which bud off until this final Golgi compartment disappears and is replaced by a maturing compartment immediately behind it in the stack.
The transport vesicles that are associated with the Golgi apparatus function to return Golgi resident proteins back to earlier Golgi compartments for reuse. The mechanism by which proteins move through the Golgi apparatus has been a long-standing area of controversy, but the model described here has considerable support from recent studies.
Proteins that begin life in the endoplasmic reticulum (ER) follow a path from this organelle through each compartment of the Golgi apparatus to their final destinations. En route, the proteins are chemically modified by enzymes within the ER and Golgi. Enzymes in the ER attach sugars to the proteins, and enzymes in the Golgi compartments add or remove sugars, resulting in the formation of mature glycoproteins. Both organelles are required to produce mature proteins, which are then targeted via transport vesicles to their correct location within or outside the cell.