The site of synthesis of secretory proteins

Most of the proteins that make up organisms are synthesized on ribosomes in the cytoplasm. After synthesis, various proteins must be transported to different parts of the cell to ensure the normal progress of cell life activities. Some proteins need to pass through the endoplasmic reticulum membrane and enter the endoplasmic reticulum cavity to become secretory proteins; some proteins need to pass through the membranes of various organelles and enter the organelles to form organelle proteins.

Secretory proteins are proteins that are synthesized inside cells and then secreted outside the cells to function. For example: salivary amylase, pepsin, digestive enzymes, antibodies and some hormones. Note: For example, respiratory enzymes are not secretory proteins.

Secretory proteins synthesized on ribosomes must pass through the endoplasmic reticulum and Golgi apparatus instead of being transported directly to the cell membrane.

Further research has shown that after the proteins secreted from the ribosomes enter the endoplasmic reticulum cavity, they must undergo some processing, such as folding, assembly, and adding some sugar groups, before they can become relatively mature proteins. Then, the endoplasmic reticulum cavity expands and buds to form membrane-bound vesicles, which wrap the proteins and transfer them to the Golgi apparatus, transporting the proteins to the Golgi apparatus cavity for further processing. Next, the edge protrusions of the Golgi apparatus form vesicles, which wrap the proteins in the vesicles and transport them to the cell membrane. The vesicles fuse with the cell membrane and release the proteins to the outside of the cell.

Although protein transport is relatively complex, it can be explained by a relatively simple model in biological systems. Each polypeptide that needs to be transported contains an amino acid sequence, called a signal peptide sequence, which guides the polypeptide to different transport systems.