Protein Secretion

Many cells secrete proteins into their surrounding extracellular fluid. Some cells, for example, secrete regulatory molecules such as hormones and neurotransmitters. Others release digestive enzymes, antibodies, or mucus. And throughout the body, cells such as fibroblasts secrete collagen and other structural proteins to provide strength and hold the body together. A full list of secreted proteins would be very long.

All proteins, whether they are used in a cell or secreted, are synthesized at cellular structures called ribosomes. If the protein is to be used with the cell, the ribosome remains in the cytosol and the completed protein is released into the cytosol. But if a protein is destined for secretion, the very first amino acids in the polypeptide chain are a special sequence called the signal sequence. Shortly after the synthesis of this sequence is completed, the synthesis stops until the ribosome docks at the rough endoplasmic reticulum.

Once docked, the synthesis continues, with the new protein threading into the rough endoplasmic reticulum. The signal sequence is then cleaved from the polypeptide chain. Often, too, further enzymes cut the protein in other places. Most secreted proteins are modified before secretion.

Next vesicles containing the protein bud from the rough endoplasmic reticulum and move too nearby the nearby Golgi apparatus. This is comprised of a stack of large, flattened vesicles. It is often likened to a stack of pita breads. The vesicles from the rough endoplasmic reticulum fuse with one end, adding their proteins to the first flattened vesicle. In turn, small vesicles bud from this structure and transfer the protein to the next layer of the stack. This continues until the protein winds up at the opposite end of the Golgi apparatus.

Further modification of the protein typically occurs within the Golgi apparatus. Here, for example, is where carbohydrate is added (glycosylation) if the secreted protein is to be a glycoprotein. This makes the secreted protein much more polar. Some secreted proteins are heavily glycosylated so that they take up and hold much water. Mucus is an example.

Once the protein has moved through the entire Golgi apparatus, secretion vesicles containing the protein bud off. These vesicles move to the plasma membrane, attach to the membrane, and then release their contents into the extracellular fluid through the process of exocytosis. Often a signal is required to initiate the release.

While not mentioned above, membrane proteins, which function while embedded in the plasma membrane, pass through the above sequence. The key difference is that the protein threads into the membrane as it is being synthesized in the rough endoplasmic reticulum. Notice how the membrane of the endoplasmic reticulum moves through the Golgi apparatus and to the plasma membrane during the secretion process.

But in many important physiological processes, membrane proteins are either added or removed from the plasma membrane in a relatively short time. Membrane proteins are removed from the plasma membrane through the formation of vesicles by endocytosis. Inside the cell, these vesicles fuse with a larger, membranous structure called an endosome. Vesicles budding from the endosome can then fuse with the plasma membrane, adding the proteins once again to the plasma membrane. Endosomes are a quick means of forming new secretion vesicles.