1. Arrival at the Golgi
* Transport Vesicles: Proteins synthesized by ribosomes on the rough endoplasmic reticulum (ER) are packaged into transport vesicles. These vesicles bud off from the ER and move towards the Golgi apparatus.
2. Entry into the Golgi
* Cis-Golgi Network (CGN): The transport vesicles fuse with the cis-Golgi network (CGN), the entry point of the Golgi. Here, the proteins are sorted and modified slightly.
3. Movement through the Golgi Stacks
* Golgi Cisternae: The Golgi apparatus is composed of flattened, membrane-bound sacs called cisternae stacked upon each other. Proteins move through these cisternae in a specific sequence.
* Cisternal Maturation Model: The prevailing model suggests that the cisternae themselves mature, moving from the cis to the trans face of the Golgi. As they mature, their enzymatic content changes, allowing for different modifications to occur.
4. Modifications and Processing
* Glycosylation: The most common modification is glycosylation, where sugars are added to proteins. This can affect protein folding, stability, and function.
* Other Modifications: The Golgi also performs other modifications, including:
* Phosphorylation: Adding phosphate groups.
* Sulfation: Adding sulfate groups.
* Proteolysis: Cleaving proteins into smaller peptides.
* Sorting: During processing, proteins are sorted based on their destination. Some are destined for secretion, while others will go to other organelles within the cell.
5. Exit from the Golgi
* Trans-Golgi Network (TGN): The proteins reach the trans-Golgi network (TGN), the exit point. Here, they undergo final modifications and sorting.
* Secretory Vesicles: Proteins destined for secretion are packaged into secretory vesicles. These vesicles bud off from the TGN and move to the cell surface for release.
* Other Destinations: Proteins targeted for other organelles or the plasma membrane are also packaged into specific vesicles and transported accordingly.
Summary:
The Golgi apparatus plays a crucial role in protein modification, sorting, and packaging. Its structure and the sequential movement of proteins through its cisternae enable a complex and highly regulated process, ensuring that proteins reach their correct destinations and function optimally.
