What is the purpose of the Endomembrane system?
Endomembrane system provides a means by which the cell can transport critical larger cargo to various locations within the cell, remove unwanted cargo from within the cell, and attain necessary cargo from outside of the cell.
What are some key organelles of the Endomembrane System?
The ER - A highway system because it carries molecules around the cell. Additionally, it acts as a factory warehouse because it makes proteins, as well as fats, for the cell
Golgi Apparatus -GA is the post office of the cell because it labels proteins and other molecules with different signals, similar to bar codes or postal codes, which directs them towards their final destination within the cell.
The Golgi can be divided into different sections based on their location in the cell and function, which are labelled in the diagram.
The three major sections of the Golgi are:
- Cis Golgi - facing the ER - within the CGN, sugar groups called oligosaccharides can be added to proteins. Existing oligosaccharides can also be modified
- Medial Golgi
- Trans Golgi - The trans-Golgi Network (TGN) performs the final packaging to send materials to different organelles, and sorts cargo to specific destinations
Endosomes - hold content coming into the cell from the extracellular space through a process called endocytosis.
Lysosomes - break down protein, lipids, and sugars into their molecular building blocks.
Peroxisomes - break down molecules, specifically those that generate hydrogen peroxide as a by-product. This way, cells can neutralize hydrogen peroxide as it is produced to prevent cell damage.
The proteasome is not an organelle, but a group of enzymes required for initiation of protein breakdown. T
What is vesicle mediated transport?
- These small vesicles bud off from lipid membranes of organelles or the cell as a whole.
- These vesicles can transport cargo into or out of the cell, or just to a different location within the cell.
- Once reaching their final destination, they fuse with other lipid membranes to deposit their cargo
How do specific proteins reach their targeted destinations?
Unique signal sequences (or lack of a sequence) are used to target proteins to different areas of the cell.
Proteins in the endomembrane system - Proteins targeted to the E R, Golgi complex, endosomes, lysosomes, the plasma membrane, or other organelles contain target signal sequences or are tagged within the endomembrane system.
Proteins destined for the cytosol - Proteins destined for the cytosol lack signal peptides, since the free ribosomes will translate them in the cytosol
Proteins that need to be secreted from the cell - Proteins that are to be secreted out of the cell also have distinct signal sequences.
Define the steps of protein transport into the ER
Step 1 - Transport of proteins into the E R begins during translation. This is accomplished by the presence of a signal termed ER Signal sequence
Step 2 - ER signal sequence interacts with a receptor, termed the signal recognition particle (SRP), which binds to the ribosome that is translating the protein. This pauses translation.
Step 3 - During this pause in translation, the SRP docks onto the ER membrane by interacting with translocon, that allows and facilitates the translocation of the growing protein into the ER.
Step 4 - Translation restarts, and once the protein has translocated into the ER, the signal peptide sequence is cleaved off the protein, translation finishes, and the protein folds inside the ER lumen.The finished protein is destined to be soluble, and not attached to a membrane.
How does transporting a protein to the cytoplasm?
Thee translocon for transmembrane proteins recognizes a signal termed the transmembrane signal anchor sequences.
This sequence not only acts as a signal to direct these proteins to the E R but will also be the transmembrane domain of the protein once it is a functional mature protein within a membrane.
What are some examples of post-translational modifications?
Post-translational modification is the covalent modification of the protein after translation. These modifications may include:
- The addition of proteins, sugars, lipids, and new functional groups like phosphates and methyl groups, which can change the final target location, structure, or function of the protein.
- The cleaving (cutting) of the peptide bonds in the protein.
- Most proteins have the signal sequence, which is typically located at the N-terminus, removed
What are the 2 enzymes involved in protein folding?
PDI -PROTEIN DISULPHIDE ISOMERASE
Folding of a protein is often facilitated by the formation of a disulfide bond between cysteine amino acids. Enzymes called protein disulfide isomerases (PDI) help form disulfide bonds between cysteines in the protein to help it fold properly.
BiP -BINDING PROTEINAnother protein, termed chaperonins, help fold the polypeptide by binding hydrophobic patches in recently translated proteins. One type of chaperonin is called binding protein (BiP)
Describe Vesicle mediated transport and all its steps.
- Cargo Selection and Coat Proteins
Signal sequences and receptors on the membrane help gather the required cargo to the area of the membrane that will be turned into a vesicle.
Coat proteins, originally present in the cytosol, bind to the area on the membrane which will become the outside of the vesicle.
three common coat proteins:
- The ER uses the protein C O P I I to shuttle to the Golgi.
- Proteins that need to be returned to E R use C O P Ito coat vesicles that are recycled back to theER.
- Clathrinis used by the Golgi to form vesicles for exocytosis, endocytosis, and transport to the endosomes.
- Budding
the coat proteins interact with proteins termed cytosolic adaptor proteins. Together they form a mesh-like network that acts like a basket to pull the membrane into a bulging shape, which will become the vesicle
- Scission
Scission (like cutting with scissors) requires another set of proteins to pinch the newly formed vesicle off the membrane
- Uncoating
Now that the coat and adaptor proteins are disassembled, uncoating the vesicle. This allows the cell to reuse the coat and adaptor proteins.
- Transport
Vesicles are typically attached to a type of protein, termed motor proteins, in the cytosol, which allow for the movement of vesicles throughout the cell.
- Tethering
Once the vesicle has reached its destination, the vesicle uses a tether protein to attach to a receptor on the acceptor membrane. - Docking
If the target proteins are found, the vesicle then docks on the surface, allowing for fusion of the vesicle with the membrane. - Fusion
Fusion entails the lipid membrane becoming continuous with that of the acceptor membrane. This allows the cargo contained within the vesicle to be exuded into the acceptor organelle. - Disassembly
Finally, the remaining tether proteins and other receptors are disassembled and recycled back to the initial organelle.
What is exocytosis?
Exocytosis refers to the exportation of cargo out of the cell.
What are the 2 ways exocytosis is regualated?
- Constitutive secretion is uncontrolled, and always happening.Examples include extracellular matrix proteins such as collagen, which is produced and excreted continuously.
- Regulated secretion in cells requires a signal to stimulate the cell to secrete it. Cells will often have specialized storage granules that are released in response to specific signals. Examples of regulated secretion include neurotransmitters in neurons, or insulin from the pancreas.
What is endocystosis?
Endocytosis is the import of cargo from outside the cell into the cell
What are the steps of the endocytosis pathway?
- Clathrin is a protein involved in vesicle formation during endocytosis. It initiates the invagination of the plasma membrane that will form the vesicle that will take the cargo to the early endosome.
- The early endosome is made up of vesicles from the plasma membrane and vesicles from the TGN. Once these vesicles fuse together, the cargo and receptors dissociate in the slightly acidic early endosome. Lowering the pH initiates dissociation of proteins. Cargo is retained in the endosome, while dissociated receptors are typically recycled back to the plasma membrane. Some cargo and receptors may be transported elsewhere in the cell, depending on their signal sequences.
- The early endosome matures to become the late endosome. Further lowered pH by the proton pumps. Cargo will be degraded as the pH continues to drop. This is similar to how the stomach uses acid to help digest food. The low pH will unfold proteins.
- When the pH becomes more acidic, enzymes including proteases are activated, leading to conversion to the lysosome.A protective layer of proteins forms to prevent the degradation of the organelle.
How are mis-folded proteins or non functional proteins sent to the endosome?
Cargo is targeted to the lysosome by a specific mannose-6-phosphate (M6P) sugar tag. Enzymes that degrade these proteins, or proteases, are also directed to the lysosome with the same M6P tag.
How do lysosomes break down molecules?
The lysosome contains high concentrations of proteases, which cleave both membrane proteins and proteins contained in the lysosome.
The lysosome also contains enzymes that cleave and digest fats and sugars, and can even engulf other organelles, like damaged mitochondria.
Once large molecules have been broken down into their basic parts (like proteins to amino acids), they are transported to the cytosol so the cell can reuse them.
Lysosomes are mainly responsible for the breakdown of proteins that are not endogenous to the cell
The lysosome has a special coat called the glycocalyx (shown in blue) that prevents it from digesting itself.
Which proteins are degraded by the proteasome?
Intracellular proteins (endogenous to the cell) from the cytosol and E R are degraded in the proteasome. Cytosolic proteins that have been mis-folded or damaged are tagged with a polyubiquitin chain. Ubiquitin is a small protein that is attached to another protein that has been selected for destruction.
Multiple ubiquitins are required for the protein to be degraded.
This process of ubiquitination targets the protein for destruction in the proteasome. Ubiquitination is another form of post-translational modification
What are the differences between proteasome, lysosomes, and peroxisomes?
Lysosome - small membrane bound organelle. Site of degradation for reactions that produce reactive oxygen species. Contains enzyme catalase to neutralize these reaction
Proteasome - small membrane bound organelle. Degrades protein, DNA and RNA
Peroxisome - degrade proteins using ubiquitin tag
What are peroxisomes?
Peroxisomes are small, membrane-enclosed organelles that contain enzymes that catalyze a variety of metabolic reactions.
Essential peroxisome proteins called peroxins, synthesized in the cytosol, are targeted to the peroxisome by specific peroxisomal targeting signals (P T S).
Oxidizing agents like peroxide are very hazardous to the cell. Peroxisomes serve as a place to keep and use these reactive oxygen species safely. These include ions, free radicals, and peroxides.
