Proteins are secreted out of the cell primarily through the conventional protein secretion (CPS) pathway, but other pathways also exist.
Here's a breakdown of how proteins are secreted:
1. Conventional Protein Secretion (CPS) Pathway
This is the most well-understood mechanism for protein secretion. Here's how it works:
- Entry into the Endoplasmic Reticulum (ER): Proteins destined for secretion are synthesized with a signal peptide. This signal peptide directs the ribosome and nascent protein to the ER membrane. The protein then translocates into the ER lumen.
- Folding and Modification in the ER: Inside the ER, the protein folds into its correct three-dimensional structure. It may also undergo post-translational modifications like glycosylation. Chaperone proteins in the ER assist with proper folding and prevent aggregation.
- Transport to the Golgi Apparatus (GA): Properly folded and modified proteins are then transported from the ER to the Golgi apparatus via transport vesicles.
- Further Processing in the Golgi: As the protein moves through the Golgi, it undergoes further processing and sorting. Different Golgi compartments contain specific enzymes that modify the protein.
- Sorting and Packaging: In the trans-Golgi network (TGN), proteins are sorted based on their final destination. They are then packaged into transport vesicles.
- Delivery to the Plasma Membrane: Vesicles containing the secreted protein bud off from the TGN and travel to the plasma membrane. The vesicle membrane fuses with the plasma membrane, releasing the protein into the extracellular space.
2. Other Secretion Pathways (Non-Classical)
Besides the CPS pathway, proteins can also be secreted through unconventional mechanisms, often referred to as non-classical secretion. These pathways bypass the ER and Golgi. Examples include:
- Direct Translocation Across the Plasma Membrane: Some proteins are secreted directly across the plasma membrane, bypassing the ER and Golgi entirely. The mechanisms involved in this process are not fully understood but may involve specialized transporter proteins.
- Secretion via Exosomes/Microvesicles: Proteins can be packaged into small vesicles called exosomes or microvesicles, which are then released from the cell. These vesicles can carry a variety of molecules, including proteins, RNA, and lipids.
- Autophagy-mediated Secretion: Proteins can be sequestered into autophagosomes, which then fuse with lysosomes. In some cases, autophagosomes can also fuse with the plasma membrane, releasing their contents, including proteins, into the extracellular space.
Summary
Protein secretion is a fundamental cellular process that allows cells to communicate with their environment and deliver essential molecules. The conventional protein secretion pathway, involving the ER and Golgi, is the primary route, but non-classical pathways provide alternative mechanisms for protein export.
