Explain the steps involved in anterograde transport

Anterograde transport refers to the forward movement of proteins and lipids from the endoplasmic reticulum (ER) towards their final destinations, such as the Golgi apparatus, plasma membrane, or extracellular space. This process is a highly regulated and directional pathway, which ensures that proteins follow a proper route, allowing cells to maintain organisation, compartmentalisation and functional efficiency. The transport is carried out using vesicles, and involves several organelles and protein complexes working in coordination.

This process can be described in five major steps, beginning in the rough ER and progressing toward the cell membrane or beyond:

Step 1: Protein Synthesis and Folding in the Rough ER

Anterograde transport begins with the synthesis of proteins on membrane-bound ribosomes of the rough ER. As the proteins are translated, they are inserted into the ER lumen or membrane. Within the ER, they undergo initial folding, assembly, and modifications, including N-linked glycosylation. Only properly folded proteins are allowed to move forward, misfolded proteins are retained or degraded.

Step 2: Packaging into COPII-Coated Vesicles

Once folded correctly, these proteins are selected and packaged into vesicles at specialised regions of the ER known as ER exit sites (ERES). The formation of these transport vesicles is driven by the COPII coat complex, which includes proteins like Sec23/24 and Sec13/31. The Sar1 GTPase initiates coat assembly. These COPII vesicles bud off from the ER, carrying cargo proteins toward the Golgi apparatus.

Step 3: Transport to the Golgi Apparatus

After budding, the COPII vesicles move through the cytoplasm, often guided along microtubules with the help of motor proteins. Vesicles first fuse with the ER-Golgi Intermediate Compartment (ERGIC), a sorting station between the ER and Golgi. From there, cargo is passed on to the cis-Golgi.

Step 4: Processing in the Golgi Complex

As proteins move from the cis- to the trans-Golgi cisternae, they undergo further modifications, such as complex glycosylation, phosphorylation, or sulfation. The Golgi apparatus also plays a key role in sorting proteins based on their final destination, such as lysosomes, the plasma membrane, or for secretion.

Step 5: Exit from Trans-Golgi and Delivery to Final Destination

In the trans-Golgi network (TGN), proteins are sorted and packed into different types of vesicles depending on their target. For example:
  • Clathrin-coated vesicles transport enzymes to lysosomes.
  • Secretory vesicles move proteins to the plasma membrane for constitutive or regulated secretion.
  • Membrane proteins are sent to specific membrane domains.
These vesicles then fuse with their target membranes, releasing their contents or integrating proteins into the membrane.






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