Secretion

Secretion is the process of segregating, elaborating, and releasing chemicals from a cell, or a secreted chemical substance or amount of substance.

Eukaryotic cells have a highly evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum. As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Misfolded proteins are usually identified here and retrotranslocated by ER-associated degradation to the cytosol, where they are degraded by a proteasome. The vesicles containing the properly-folded proteins then enter the Golgi apparatus.

In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur. The proteins are then moved into secretory vesicles which travel along the cytoskeleton to the edge of the cell. More modification can occur in the secretory vesicles (for example insulin is cleaved from proinsulin in the secretory vesicles).

Eventually, the vesicle fuses with the cell membrane in a process called exocytosis, dumping its contents out of the cell's environment.

Strict biochemical control is maintained over this sequence by usage of a pH gradient: the pH of the cytosol is 7.4, the ER's pH is 7.0, and the cis-golgi has a pH of 6.5. Secretory vesicles have pHs ranging between 5.0 and 6.0; some secretory vesicles evolove into lysosomes, which have a pH of 4.8.

Nonclassical secretion
There are many proteins like FGF1 (aFGF), FGF2 (bFGF), interleukin1 (IL1) etc which do not have a signal sequence. They do not use the classical ER-golgi pathway. These are secreted through various nonclassical pathways.

Secretion in the other domains of life:
Secretion is not unique to eukaryotes alone, it is present in bacteria and archaea as well. ATP binding cassette (ABC) type transporters are common to all the three domains of life. The Sec system is also another conserved secretion system which is homologous to the translocon in the eukaryotic endoplasmic reticulum consisting of Sec 61 translocon complex in yeast and Sec Y-E-G complex in bacteria. Gram negative bacteria have two membranes, thus making secretion topologically more complex. So there are at least five specialized secretion system in bacteria:

1) Type I secretion system: It is same as the ATP binding cassette transporters mentioned above.

2) Type II secretion system: It depends on the Sec system for a protein to cross the inner membrane and another special system to cross the outer membrane. Bacterial pili use modifications of the sec system, but are different from type I system.

3) Type III secretion system (T3SS): It is homologous to bacterial flagellar basal body. It is like a molecular syringe through which a bacterium (e.g. Shigella or Yersinia) can inject proteins into eukaryotic cells. The low Ca2+ concentration in the cytosol opens the gate that regulates TTSS. The Hrp system in plant pathogens inject harpins through similar mechanisms into plants.

4) Type IV secretion system: It is homologous to conjugation machinary of bacteria (and archaeal flagella). It is capable of transporting both DNA and proteins. It was discovered in Agrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host which develops the crown gall (tumor). Helicobactor pylori uses a type IV secretion system to inject Cag A into gastic epithelial cells. Bordetella pertussis, the causative agent of whooping cough, secretes the pertussis toxin partly through the type IV system.

5) Type V secretion system, also called autotransporter system: This use the sec system for crossing the inner membrane. The proteins which uses this path have the capability to form a beta barrel in their C terminus and insert into the outer membrane to transport the rest of the peptide out. Finally the beta barrel may be cleaved and left back in the outer membrane. Some people believe these remnants of the autotransporters gave rise to the porins which are similar beta barrels.

Bacteria as well as mitochondria and chloroplasts also use many other special transport systems like twin arginine transporters (TAT) which may transport proteins without need for unfolding unlike the translocon. The name of the system comes from the requirement for two consecutive arginines in the signal sequence required for targeting to this system.