Secretory protein

A secretory protein is any protein, whether it be endocrine or exocrine, which is secreted by a cell. Secretory proteins include many hormones, enzymes, toxins, and antimicrobial peptides. Secretory proteins are synthesized in endoplasmic reticulum.

Production
The production of a secretory protein starts like any other protein. The mRNA is produced and transported to the cytosol where it interacts with a free cytosolic ribosome. The part that is produced first, the NH3-ter, contains a signal sequence consisting of 6 to 12 amino acids with hydrophobic side chains. This sequence is recognised by a cytosolic protein, SRP (Signal Recognition Particle), which stops the translation and aids in the transport of the mRNA-ribosome complex to a translocon in the membrane of the endoplasmic reticulum. When it arrives at the translocon, this channel protein opens and the signal sequence is introduced to the ER lumen through this opening. The signal sequence is removed and the translation continues while the produced chain moves through the translocon (cotranslational translocation).

Modification
After the production of the protein is completed, it interacts with several other proteins to gain its final state.

Endoplasmic Reticulum
After translation proteins within the ER make sure that the protein is folded correctly. If after a first attempt the folding is unsuccessful, a second folding is attempted. If this fails too the protein is exported to the cytosol and labelled for destruction. Aside from the folding, there is also a sugar chain added to the protein. After these changes, the protein is transported to the Golgi apparatus by a coated vesicle using coating protein COPII.

Golgi Apparatus
In the Golgi apparatus, the sugar chains are modified by adding or removing certain sugars. The secretory protein leaves the Golgi apparatus by an uncoated vesicle.

Excretion
Membrane proteins with functional areas on the cytosolic side of both the vesicle and cell membrane make sure the vesicle associates with the membrane. The vesicle membrane fuses with the cell membrane and so the protein leaves the cell. Some vesicles don't fuse immediately and await a signal before starting the fusing. This is seen in vesicles carying neurotransmitter in presynaptic cells.