1. Transcription:
* The DNA sequence encoding the protein is transcribed into messenger RNA (mRNA) by the enzyme RNA polymerase. This mRNA carries the genetic code for the protein.
2. Translation:
* The mRNA travels from the nucleus to the cytoplasm, where it binds to ribosomes.
* Ribosomes read the mRNA code and use it to assemble amino acids into a polypeptide chain, which is the primary structure of the protein.
3. Targeting and Translocation:
* Signal sequence: Proteins destined for secretion have a special "signal sequence" at their N-terminus. This sequence acts as a tag, directing the ribosome to the endoplasmic reticulum (ER), a network of membranes within the cell.
* ER translocation: As the ribosome moves along the mRNA, the growing polypeptide chain is threaded through a protein channel in the ER membrane. This process is called translocation.
4. Folding and Modification:
* Chaperones: Inside the ER, the protein folds into its correct three-dimensional structure with the help of chaperone proteins.
* Glycosylation: Many secreted proteins undergo glycosylation, the addition of sugar molecules, which can modify their function and stability.
* Other modifications: Other modifications, such as phosphorylation, can also occur within the ER.
5. Transport through the Golgi Apparatus:
* Vesicles: Once folded and modified, the protein is packaged into small membrane-bound vesicles that bud off from the ER.
* Golgi apparatus: These vesicles travel to the Golgi apparatus, a stack of flattened sacs within the cell.
* Further modification and sorting: Within the Golgi, proteins can undergo further modifications and are sorted based on their destination.
6. Secretion:
* Secretory vesicles: Finally, the protein is packaged into secretory vesicles that bud off from the Golgi apparatus.
* Exocytosis: The secretory vesicles fuse with the plasma membrane, releasing the protein outside the cell.
In summary, the production of secreted proteins involves:
* Transcription and translation of the gene coding for the protein.
* Targeting to the ER and translocation across its membrane.
* Folding and modification within the ER.
* Transport through the Golgi apparatus.
* Packaging into secretory vesicles and release from the cell by exocytosis.
