Next Lesson - Biological Membranes
Core
Proteins need to be sent to the correct organelle within the cell, or sometimes out of the cell into the extracellular space. This lets them carry out their function. To make sure the proteins arrive at the correct destination, each polypeptide carries a molecular label (a specific amino acid sequence). This label ensures that the protein is delivered to a specific location. Within the cell itself, there are various systems that help to deliver the proteins, produced in the process of translation, to reach their final destination.
Overview of Cellular Protein Transportation Systems
- Proteins intended for the cytoplasm, or for delivery to organelles such as the nucleus or peroxisome after the protein has been made (post-translational delivery), are produced on free ribosomes.
- Most proteins intended for the plasma membrane or the exterior of the cell are produced on ribosomes attached to the rough endoplasmic reticulum (RER). So are most proteins intended for the endoplasmic reticulum itself, the Golgi apparatus or lysosomes. The RER is the part of the endoplasmic reticulum that is covered in ribosomes.
As a protein is being made, a signal sequence at the start of the chain is recognised as soon as it emerges from the ribosome. If it is a signal for the endoplasmic reticulum, the ribosome is taken to the rough endoplasmic reticulum and the rest of the protein is made there.
Proteins Targeted to a Cell Organelle
Proteins that are destined for organelles (e.g. the nucleus, mitochondria, peroxisomes) or for the cytosol (the fluid part of the cytoplasm) are first produced on free ribosomes in the cytoplasm. They are then delivered to their destination. Proteins for the endoplasmic reticulum, Golgi apparatus and lysosomes follow a different route, described below.
To be delivered to one of these organelles, the protein contains a signal sequence (address label). A receptor recognises this label and directs the protein to the correct organelle. Once at the organelle, there are mechanisms that help the protein across the membrane of the organelle. Energy is needed to drive this transport. Depending on the organelle, the energy comes from the breakdown of ATP or GTP, or from a membrane potential, as the sections below show. GTP (guanosine triphosphate) is an energy-carrying molecule similar to ATP.
Diagram - The pathway for proteins targeted for organelles or the cytosol
SimpleMed original by Dr. Peter Parkinson
Proteins Targeted to the Nucleus
The nucleus has many proteins that are involved in its biological processes. However, all the proteins found in the nucleus are produced in the cytoplasm, so they need to be transported to the nucleus.
For proteins to enter the nucleus, they have to pass through a nuclear pore. This is a large channel, made of many proteins, that runs through the nuclear envelope (the double membrane around the nucleus). Proteins targeted for the nucleus will contain a nuclear localisation sequence (NLS). The NLS is typically one or two short runs of basic amino acids (amino acids with a positive charge). These runs lie in the primary sequence (the order of amino acids) of the polypeptide chain.
A folded protein with the NLS enters the nucleus from the cytosol with the help of other proteins called importins. The protein with the NLS and importins form a complex, which then binds to the nuclear pore and is transported into the nucleus using energy. When inside the nucleus, the importins bind to a GTPase protein called Ran (a protein that breaks down GTP to GDP, guanosine diphosphate). Ran in the nucleus is bound to GTP, and Ran-GTP binding to the importins makes the nuclear protein separate from the complex. The importins are released out of the nucleus along with Ran, so the importins can be reused to transport more proteins into the nucleus. In the cytoplasm, the GTP bound to Ran is broken down to GDP, and in the nucleus Ran-GDP is reloaded with GTP. Together, these two steps keep Ran-GTP high in the nucleus and low in the cytoplasm, which sets the direction of nuclear transport.
Proteins Targeted to Mitochondria
Most proteins destined for the mitochondria are produced in the cytoplasm in an unfolded state. In humans, the mitochondria make only 13 of their own proteins, from mitochondrial DNA. The address label is an amphipathic sequence, typically made up of about 15 to 60 amino acids found at the N-terminal (the start of the polypeptide chain). In an amphipathic sequence, one face is hydrophobic (water-repelling) and the other face is positively charged. The unfolded protein is stabilised by interacting with molecular chaperones (proteins that bind it and delay its folding) in the cytoplasm.
At the mitochondrial outer membrane, the signal sequence is recognised by receptors, which pass the protein to a protein channel that allows its transport through the outer mitochondrial membrane. The receptors and channel together are called the translocase of the outer membrane or TOM (a translocase is a protein complex that moves a protein across a membrane).
If the protein is destined for the matrix of the mitochondria (the innermost compartment, enclosed by the inner membrane), it must cross the inner mitochondrial membrane. It does this through another protein channel called translocase of the inner membrane or TIM. Transport through TIM requires a membrane potential and energy.
Proteins that belong in the inner mitochondrial membrane carry additional signals in their primary sequence that stop them entering the matrix of the mitochondria.
When the protein has crossed the inner mitochondrial membrane, an enzyme called mitochondrial processing peptidase will, in most cases, remove the signal sequence from the protein. The protein will then fold into its final three-dimensional shape (its conformation) in a process requiring chaperone proteins and energy.
Diagram - The pathway for proteins targeted for mitochondria
SimpleMed original by Dr. Peter Parkinson
Proteins Targeted to Peroxisomes
Peroxisomes have a role in the cell by metabolising lipids and detoxifying chemical molecules.
Proteins targeted for peroxisomes are first produced in the cytosol and require a peroxisome targeting sequence (PTS) to direct them to peroxisomes. The PTS is the address label. In most peroxisomal proteins it is a run of three amino acids, serine-lysine-leucine (or a close variant), found at the C-terminal (the end of the polypeptide chain). A few peroxisomal proteins instead carry a second type of peroxisome targeting sequence (PTS2) at the N-terminal, which is usually cut off inside the peroxisome. The receptor that recognises the C-terminal signal is the PTS receptor, which binds to the protein in the cytoplasm of the cell.
The PTS receptor-protein complex binds to a docking complex of proteins in the peroxisome membrane. There the receptor becomes part of a protein channel that allows the passage of the protein into the matrix of the peroxisome. Once in the matrix of the peroxisome, the C-terminal PTS sequence remains attached to the polypeptide chain. The PTS receptor is returned to the cytosol, a step that needs energy from the breakdown (hydrolysis) of ATP.
Proteins Targeted to the Endoplasmic Reticulum and Secretory Pathway
The endomembrane system is the group of compartments that pass material to each other in vesicles: the endoplasmic reticulum, Golgi apparatus, lysosome and plasma membrane. When a protein is intended for any part of this system, or for secretion out of the cell, the polypeptide is first taken to the endoplasmic reticulum (ER) during translation.
Diagram - The pathway for proteins targeted for secretion and the endoplasmic reticulum
SimpleMed original by Dr. Peter Parkinson
To be transported to the ER, the polypeptide chain must have a signal sequence. This is a run of hydrophobic amino acids found at the N-terminal of the chain.
As soon as the signal sequence is produced by the ribosome, the signal-recognition-particle (SRP) recognises it and binds to it. The SRP transports the ribosome with the polypeptide chain to the ER, where the SRP binds to a receptor on the ER membrane. The ribosome and polypeptide chain are handed over to a protein channel in the ER membrane. Hydrolysing GTP (splitting it to release energy) then makes the SRP and its receptor dissociate (come apart). This lets the SRP bind to another signal sequence of a different polypeptide chain. The ribosome continues to translate the mRNA, with the polypeptide chain being fed into the ER lumen. As the chain enters the lumen, an enzyme called signal peptidase removes the signal sequence from it. Once translation is finished, the ribosome returns to the cytosol to translate further mRNA chains. The polypeptide chain is folded and modified in the ER. Most proteins then leave the ER in vesicles for the Golgi apparatus, but proteins that work in the ER stay there (see below).
For membrane proteins destined for the plasma membrane, the signal sequence or another region of hydrophobic amino acids will cause the protein to be embedded in the ER membrane. The protein is anchored in the membrane as it is made, and is later carried to the plasma membrane in vesicles (see below).
Proteins Targeted to Lysosomes
Proteins destined for lysosomes will be modified in the Golgi by the addition of a mannose-6-phosphate group (mannose, a sugar, with a phosphate group attached). The group is added to mannose residues (mannose units) on N-linked oligosaccharides (sugar chains attached to the protein at a nitrogen atom of an asparagine amino acid). The reaction needs two enzymes. Proteins only receive the addition of the mannose-6-phosphate group if they contain a signal patch. This is a site on the surface of the folded protein, made up of specific amino acids that come from different parts of the polypeptide chain. They are brought together when the chain folds into its three-dimensional shape. An enzyme called a phosphotransferase recognises this patch and starts the process of adding the mannose-6-phosphate group.
The mannose-6-phosphate groups are recognised by a mannose-6-phosphate receptor on the trans Golgi apparatus (the side of the Golgi apparatus from which vesicles leave). This leads to the protein being packaged into a vesicle, which then pinches off and is transported to lysosomes. When the vesicle containing the lysosomal protein reaches an endosome (a compartment that matures into a lysosome), the acidic environment causes the protein to dissociate from the receptor. The receptor then travels back to the Golgi apparatus to be reused. In most cell types, once the protein has arrived in the lysosome, enzymes called acid phosphatases remove the phosphate group, and the protein carries out its function in the lysosome.
Diagram - The pathway for proteins targeted for lysosomes from the Golgi apparatus
SimpleMed original by Dr. Peter Parkinson
Proteins That Remain in the Endoplasmic Reticulum
Some proteins work inside the ER itself, for example by helping other proteins to fold, so they must stay there. Soluble proteins of this kind (those that lie free in the ER lumen) contain a retention signal at the C-terminal of the polypeptide chain. This signal is a sequence of four amino acids, which is Lysine-Aspartate-Glutamate-Leucine (KDEL).
A folded protein containing the KDEL signal may be transported to the cis-Golgi apparatus (the side of the Golgi apparatus nearest the ER) via a vesicle. There, the KDEL signal will bind to a KDEL receptor in the Golgi. This binding is enhanced by the low pH (acidic) environment of the Golgi. The Golgi is more acidic than the ER. The ER protein-KDEL receptor complex is transported back to the ER. Due to the neutral pH environment of the ER, the protein dissociates from the receptor and the KDEL receptor returns to the Golgi apparatus.
Proteins Targeted to the Plasma Membrane and Secretion
Proteins destined for the plasma membrane or secretion out of the cell are taken into the ER by their signal sequence (or a hydrophobic region). They do not carry a retention signal such as KDEL or a lysosomal tag such as mannose-6-phosphate. Without a retention signal or lysosomal tag, these proteins are packaged in a vesicle that is taken to the plasma membrane. There they either become part of the plasma membrane or are secreted into the exterior environment via exocytosis. This continuous delivery is called constitutive secretion. Some specialised cells instead store proteins in secretory vesicles and release them only when stimulated (regulated secretion). Regulated secretion needs extra sorting into secretory vesicles, which constitutive secretion does not.
Edited by: Dr. Thomas Burnell
Reviewed by: Dr. Marcus Judge
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