Translocon
Encyclopedia
The translocon is the complex of protein
s associated with the translocation
of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior (cisternal or luminal) space of the endoplasmic reticulum
(ER) from the cytosol
. This process requires the protein to cross a hydrophobic lipid bilayer
. In prokaryotes, a similar protein complex transports polypeptides across the plasma membrane. Bacterial pathogen
s can assemble translocons in their host membranes, allowing them to export virulence factor
s into their target cells.
Translocators can also move polypeptides (such as damaged proteins targeted for proteasome
s) from the cisternal space of the endoplasmic reticulum to the cytosol. This requires the expenditure of energy in the form of GTP
.
Proteins due to be translocated to the endoplasmic reticulum are recognized by the signal-recognition particle (SRP), which halts translation
of the polypeptide by the ribosome
while it attaches the ribosome to the SRP receptor on the endoplasmic reticulum. This recognition event is based upon a specific N-terminal signal sequence that is in the first few codons of the polypeptide to be synthesised.
The current model of protein translocation is that the translocon (translocator) acts as a channel through the hydrophobic membrane of the endoplasmic reticulum (after the SRP has dissociated and translation is continued). The emerging polypeptide is threaded through the channel as a string of amino acids. Once translation is finished a signal peptidase cleaves off the short signal peptide from the nascent protein leaving the polypeptide free in the interior of the endoplasmic reticulum.
The translocon can also translocate and integrate membrane proteins in the correct orientation into the membrane of the endoplasmic reticulum. The mechanism of this process is not fully understood, but involves the recognition and processing by the translocon of hydrophobic stretches in the amino acid sequence which are destined to become transmembrane helices
.
An important translocon is known as Sec61
.
Translocons are not always unidirectional but also can be bidirectional. This allows polypeptides to leave and enter from a location. An example is Sec61.
The translocon protein complexes are formed from Sec proteins.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s associated with the translocation
Protein targeting
Protein targeting or protein sorting is the mechanism by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its exterior via secretion...
of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior (cisternal or luminal) space 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...
(ER) from 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....
. This process requires the protein to cross a hydrophobic lipid bilayer
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
. In prokaryotes, a similar protein complex transports polypeptides across the plasma membrane. Bacterial pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s can assemble translocons in their host membranes, allowing them to export virulence factor
Virulence factor
Virulence factors are molecules expressed and secreted by pathogens that enable them to achieve the following:* colonization of a niche in the host...
s into their target cells.
Translocators can also move polypeptides (such as damaged proteins targeted for proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
s) from the cisternal space of the endoplasmic reticulum to the cytosol. This requires the expenditure of energy in the form of GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
.
Proteins due to be translocated to the endoplasmic reticulum are recognized by the signal-recognition particle (SRP), which halts translation
Translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. Whereas interpreting undoubtedly antedates writing, translation began only after the appearance of written literature; there exist partial translations of the Sumerian Epic of...
of the polypeptide by the 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....
while it attaches the ribosome to the SRP receptor on the endoplasmic reticulum. This recognition event is based upon a specific N-terminal signal sequence that is in the first few codons of the polypeptide to be synthesised.
The current model of protein translocation is that the translocon (translocator) acts as a channel through the hydrophobic membrane of the endoplasmic reticulum (after the SRP has dissociated and translation is continued). The emerging polypeptide is threaded through the channel as a string of amino acids. Once translation is finished a signal peptidase cleaves off the short signal peptide from the nascent protein leaving the polypeptide free in the interior of the endoplasmic reticulum.
The translocon can also translocate and integrate membrane proteins in the correct orientation into the membrane of the endoplasmic reticulum. The mechanism of this process is not fully understood, but involves the recognition and processing by the translocon of hydrophobic stretches in the amino acid sequence which are destined to become transmembrane helices
Transmembrane helix
Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein. It is called a "domain" because an alpha-helix in a membrane can fold independently from the rest of the protein, similar to domains of water-soluble proteins...
.
An important translocon is known as Sec61
Sec61
Sec61 is an endoplasmic reticulum membrane protein translocator . It is a doughnut shaped pore through the membrane with 3 major subunits . It has a region called the plug that blocks transport into or out of the ER...
.
Translocons are not always unidirectional but also can be bidirectional. This allows polypeptides to leave and enter from a location. An example is Sec61.
The translocon protein complexes are formed from Sec proteins.