Golgi Apparatus (Part 2 of Functions of the Endomembrane System)

Transport vesicles journey from the endoplasmic reticulum (ER) to the Golgi apparatus through a series of coordinated steps:

  1. Budding from ER: Vesicles carrying newly synthesized proteins and lipids bud off from the ER exit sites. These vesicles are often coated with COPII proteins, which help in shaping the vesicle and selecting cargo.

  2. Movement to Golgi: Once formed, these COPII-coated vesicles uncoat and are then transported along microtubules using motor proteins towards the cis-Golgi network (CGN), which is the entry face of the Golgi apparatus.

  3. Fusion with Golgi: The vesicles fuse with the cis-Golgi network membrane, releasing their contents into the Golgi lumen. This fusion is mediated by SNARE proteins: v-SNAREs on the vesicle interact with t-SNAREs on the Golgi membrane, facilitating the docking and fusion of the membranes.

The Golgi apparatus itself is composed of stacked, flattened sacs called cisternae, and it has distinct functional regions:

  • cis-Golgi Network (CGN): This is the entry face, located closest to the ER. It receives vesicles containing proteins and lipids from the ER. Here, initial processing and phosphorylation of some cargo may occur. (Receiving side of the Golgi).

  • trans-Golgi Network (TGN): This is the exit face of the Golgi, positioned furthest from the ER. Its primary function is the sorting and packaging of modified proteins and lipids into new transport vesicles, which then bud off to deliver their contents to various destinations such as lysosomes, the plasma membrane, or secretion outside the cell. (Shipping side of the Golgi).

Transport vesicles journey from the endoplasmic reticulum (ER) to the Golgi apparatus through a series of coordinated steps:

1. Budding from ER: Vesicles carrying newly synthesized proteins and lipids bud off from the ER exit sites. These vesicles are often coated with COPII proteins, which help in shaping the vesicle and selecting cargo.

2. Movement to Golgi: Once formed, these COPII-coated vesicles uncoat and are then transported along microtubules using motor proteins towards the cis-Golgi network (CGN), which is the entry face of the Golgi apparatus.

3. Fusion with Golgi: The vesicles fuse with the cis-Golgi network membrane, releasing their contents into the Golgi lumen. This fusion is mediated by SNARE proteins: v-SNAREs on the vesicle interact with t-SNAREs on the Golgi membrane, facilitating the docking and fusion of the membranes. The Golgi apparatus itself is composed of stacked, flattened sacs called cisternae, and it has distinct functional regions: - cis-Golgi Network (CGN): This is the entry face, located closest to the ER. It receives vesicles containing proteins and lipids from the ER. Here, initial processing and phosphorylation of some cargo may occur. (Receiving side of the Golgi). - Intra-Golgi and Retrograde Transport: While COPII vesicles mediate anterograde transport from the ER to the Golgi, some proteins need to be transported back. COPI-coated vesicles mediate retrograde transport, moving proteins from the Golgi back to the ER or from later Golgi cisternae to earlier (less mature) ones. This ensures that ER-resident proteins that accidentally escape to the Golgi are returned and facilitates the recycling of SNARE proteins and other transport machinery. - trans-Golgi Network (TGN): This is the exit face of the Golgi, positioned furthest from the ER. Its primary function is the sorting and packaging of modified proteins and lipids into new transport vesicles, which then bud off to deliver their contents to various destinations such as lysosomes, the plasma membrane, or secretion outside the cell.