Intracellular Processes Flashcards
What is the basic 3 steps of protein sorting?
- Protein synthesis
- Proteins dispatched to different locations in the cell according to specific ‘adress labels’ contained in their amino acid sequence.
- Once at adress, protein enters organelle.
What are the possible pathways for he protein if it stays in the cytosol(default)?
- Stays put
- Nucleus (if it has the NLS signal)
- Mitochondria (if has mitochondrial import sequence)
- Peroxisomes (if it has C-terminal tri-peptide (3 amino acids))
Why is it the default setting for proteins to stay in the cytosol?
Newly synthesised proteins lack a signal peptide
Where does the protein go if a signal peptide is present?
It will go to the ER then to the golgi before being dispatched to its final location.
What are the possible pathways for the protein if it goes to the ER/Golgi?
- Secretion
- Plasma membrane(if there is a stop translocation peptide)
- Lysosome(if M-6-P sugar side chains are added)
How would a protein enter a nucleus?
Nuclear pores- selective gates for nuclear proteins
What is the role of protein translocators?
Helps proteins moving from the cytosol to the ER, mitochondria or peroxisomes to enter their organelle.
How do ribosomes know to go to the ER?
Due to the signal peptide on the protein being made, this is a specific sequence on the N-terminal of the amino acid (NH2 Group)
How does the signal peptide guide the protein to the ER?
The SRP (signal recognition particle) in the cytosol will bind to the signal peptide and guide it to the SRP receptor embedded on the ER membrane.
What happens to the protein after it binds to the SRP receptor on the ER membrane?
Polypeptides then continue to be created as they are threaded through the protein channel ‘translocon’ in the ER membrane.
What happens to the signal peptide after binding?
It is cleaved off by signal peptidase
How do proteins from the ER reach the golgi?
Proteins in the ER lumen is encapsulated into a transport vesicle that buds off and is secreted from the ER. These vesicles fuse to become the cis cisterna of the golgi. (cis maturation model)
What happens to proteins in the golgi?
Protein continues to move through the stack and they are modified, labelling them for their final destination after the golgi.
How are proteins transported from the golgi?
The trans golgi network bud into vesicles which travel to their next destination in the cell, often specific receptors will meet them here and carry them to their destiantion.
What is exocytosis?
When proteins remain anchored to the ER, vesicles from the ER ultimately fuse with the plasma membrane.
A stop translocation signal at the C-terminal ensures this pathway.
What is endocytosis?
When proteins are directed from adress labels from golgi for lysosomes
This will be due to specific sugar chains such as M6P
Initially the proteins become endosomes before maturing into lysosomes.
Why are post-translational modifications important?
Necessary for correct delivery of protein to its final cellular location.
What are examples of post translational modification?
- Glycosylation
- Phosphorylation
- Acetylation
- Ubiqitination
- Farnesylation
What are the types of protein degradation?
- Lysosomal
- Proteosomal
Where and how does lysosomal degradation happen?
Activated by acidic environment inside lysosome.
Carried out by lysosomal enzymes. (eg.hydrolases, lipases, nucleases, proteases)
Where and how does proteosomal degradation happen?
Occurs at proteosomes, walls are formed from protease enzyme and the active site is inside the cyllinder.
This process is ATP dependant.
On what molecules does lysosomal degradation occur?
- Long half life (20hrs)
- membrane proteins brought via endocytosis
- extracellular proteins brought into cell via receptor mediated endocytosis
- Pathogenic proteins brought into cell via phagocytosis
On what molecules does proteosomal degradation occur?
- Short half life
- Key metabolic enzymes
- Defective proteins
How is proteosomal degradation mediated?
Mediated by ubiquitin tagging
What is the process of ubiquitin tagging?
- Shuttling proteins take ubiquitinated protein to proteosome, the tagged proteins are recognised, unfolded and translocated
- Protein is degraded inside proteosome to give peptides, peptides are extruded and digested by cytosolic peptidases
