Secretory protein
Encyclopedia
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.
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 acid
s 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).
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.
, the sugar chains are modified by adding or removing certain sugars.
The secretory protein leaves the Golgi apparatus by an uncoated vesicle.
Some vesicles don't fuse immediately and await a signal before starting the fusing. This is seen in vesicles carying neurotransmitter
in presynaptic
cells.
Antimicrobial peptides
Antimicrobial peptides are an evolutionarily conserved component of the innate immune response and are found among all classes of life. Fundamental differences exist between prokaryotic and eukaryotic cells that may represent targets for antimicrobial peptides...
.
Secretory proteins are synthesized in endoplasmic reticulum.
Production
The production of a secretory protein starts like any other protein. The mRNAMessenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
is produced and transported to the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
where it interacts with a free cytosolic ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
. The part that is produced first, the NH3-ter, contains a signal sequence consisting of 6 to 12 amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s with hydrophobic side chains. This sequence is recognised by a cytosolic protein, SRP (Signal Recognition Particle), which stops the translation
Translation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
and aids in the transport of the mRNA-ribosome complex to a translocon
Translocon
The translocon is the complex of proteins associated with the translocation of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior space of the endoplasmic reticulum from the cytosol...
in the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
of the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
.
When it arrives at the translocon
Translocon
The translocon is the complex of proteins associated with the translocation of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior space of the endoplasmic reticulum from the cytosol...
, this channel protein opens and the signal sequence is introduced to the ER
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
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 apparatusGolgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
, 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
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
in presynaptic
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
cells.