Protein excretion from a cell generally involves a complex process of synthesis, modification, and transport, ultimately leading to the protein's release outside the cellular membrane. This process often utilizes the endomembrane system.
Protein Excretion Pathways
Several pathways exist for protein excretion, but a common and well-understood mechanism involves the endoplasmic reticulum (ER) and the Golgi apparatus.
1. Synthesis and Entry into the ER
- Protein Synthesis: Protein synthesis begins on ribosomes. If the protein is destined for secretion, it contains a signal peptide.
- Signal Recognition Particle (SRP): The signal peptide is recognized by the SRP, which pauses translation.
- ER Translocation: The SRP escorts the ribosome-mRNA complex to the ER membrane, where it binds to an SRP receptor.
- Translocon: The ribosome then binds to a protein channel called the translocon. The signal peptide guides the nascent polypeptide chain through the translocon and into the ER lumen.
- Signal Peptide Cleavage: Once inside the ER lumen, the signal peptide is usually cleaved off by a signal peptidase.
2. Folding and Modification in the ER
- Protein Folding: Within the ER lumen, proteins fold into their correct three-dimensional structure, often assisted by chaperone proteins like BiP (Binding Immunoglobulin Protein).
- Post-Translational Modifications: Proteins can undergo post-translational modifications, such as glycosylation (addition of sugar molecules), which are important for protein stability, folding, and function.
3. Transport to the Golgi Apparatus
- Vesicle Formation: Properly folded and modified proteins are packaged into transport vesicles that bud off from the ER.
- ER-to-Golgi Intermediate Compartment (ERGIC): These vesicles fuse to form the ERGIC and then move to the Golgi apparatus.
4. Further Modification and Sorting in the Golgi
- Golgi Processing: As proteins move through the Golgi apparatus (cis, medial, and trans compartments), they undergo further modifications, such as glycosylation and phosphorylation.
- Sorting and Packaging: In the trans-Golgi network (TGN), proteins are sorted according to their final destination.
5. Excretion via Vesicles
- Vesicle Budding: Proteins destined for secretion are packaged into secretory vesicles that bud off from the TGN.
- Constitutive Secretion: Some proteins are secreted continuously (constitutive secretion) via vesicles that fuse directly with the plasma membrane, releasing their contents into the extracellular space.
- Regulated Secretion: Other proteins are stored in secretory vesicles until a specific signal (e.g., hormone or neurotransmitter) triggers their release. These vesicles then fuse with the plasma membrane in response to the signal.
Alternative Excretion Pathways
While the ER-Golgi pathway is the most common, other less conventional pathways exist, including:
- Direct translocation across the plasma membrane: Some proteins may be directly transported across the plasma membrane without involving the ER or Golgi.
- Exosomes: Proteins can be packaged into exosomes (small vesicles) that are released from the cell.
In summary, protein excretion typically involves synthesis, translocation into the ER, folding and modification, transport through the Golgi, and packaging into vesicles for release via constitutive or regulated secretion.
