8 Protein Sorting and Intracellular Traffic Flashcards
Summarise the ER
- ER is continuous with the outer membrane of the nuclear envelope
- It consists of a vast network of tubules which extend through the cytoplasm
- Rough ER has ribosomes for protein synthesis
- Smooth ER is for protein synthesis
Give all the purposes of the Smooth ER
1 - synthesis of lipids such as phospholipids and cholesterol which form cell membranes
2 - synthesis of steroid hormones like testosterone
3 - storage and release of Ca2+ (sarcoplasmic reticulum in muscles)
4 - detoxification (present in the liver)
What is the role of specifically the Rough ER
- The site of protein synthesis and processing of transmembrane proteins destined for:
ER, Golgi, Plasma membrane, Lysosomes, Endosomes
and also for the cell exterior
where are proteins synthesised if not on the RER
- On cytosolic ribosomes, which are free in the cytoplasm
what is co-translational translocation
The process by which proteins are synthesised on the ribosome at the RER
Describe the process of co-translational translocation
- A specific signal sequence on a protein destined for the ER is recognised by a signal recognition particle (SRP)
- The SRP also binds to the ribosome pausing synthesis of the polypeptide
- The SRP takes the ribosome and polypeptide to an SRP receptor on a membrane which is close to a protein translocator
- The SRP releases the ribosome and the protein translocator transports the polypeptide into the ER, lumen
- The SRP and SRP receptor dissociate and are recycled
Give a brief summary of co-translation translocation
- SRP recognises SSS on polypeptide and binds to it and the ribosome
- SRP takes the complex to an SRP receptor on ER membrane
- Polypeptide is transported through protein translocator into ER lumen
- SRP and ribosome are recycled
Where does the SRP bind
onto the Specific signal sequence on the polypeptide chains end terminus
proteins collect in the ER lumen and on the ER membrane why?
- proteins destined to be released out of the cell via exocytosis are fully transported into the ER lumen first
- Proteins destined to be a membrane protein will just get embedded within the ER membrane
once a protein has been translated and inside the ER how is it modified?
1) N-linked Glycosylation, where carbs groups are added
2) The protein is folded into its 3D shape via bond formation, often glycosylation helps this
what is N-linked glycosylation
- When a precursor oligosaccharide composed of 14 sugars is added to the protein via the N terminus (amine group) of an asparagine amino acid
When do these oligosaccharides bind to the protein
- The ER lumen is packed with oligosaccharides and these bind to the protein N terminus in the ER while the back end is still being translated outside of the ER by a ribosome
describe the structure of a regular oligosaccharide
- 14 sugars
- 3 glucose at 1 end
- 2 N-acetyl glucosamine at the other end
- 9 Mannose sugars in the middle which branch out
once the protein has folded correctly in the ER lumen what change occurs to the oligosaccharide
- 3 glucose and 1 mannose sugar is cleaved from the oligosaccharide
What is the importance of glycosylation
- the Oligosaccharide ensures the protein folds correctly
- this prevents build up of toxic proteins
describe the mechanism by which proteins are checked and altered to ensure they fold correctly
- When the protein is first folded 3 glucose and 1 mannose are cleaved from the oligosaccharide
- If not folded correctly glucosyl transferase enzymes add a glucose back onto the oligosaccharide
- a Chapperone protein then binds to this glucose structure and holds down the protein until it is refolded
- another enzyme called glucosidase then cleaves off the glucose
- if the protein is still folded incorrectly glycosyl transferase adds another glucose to repeat
What occurs when the unfolded protein response is triggered in a cell due to accumulation of too many misfolded proteins
- Protein synthesis is inhibited
- misfolded proteins are degraded
- More chaperone proteins are transcripted to bind onto the misfolded proteins until they fold correctly
- Apoptosis as a last resort
How does the pulse chase experiment discover the secretory pathway in cells
- A cell is fed radioactive labelled amino acids for 3 minutes
- The cell is then fed regular amino acids
- proteins which have been synthesised with the radioactive amino acids can be traced
- so the pathways of the proteins can be viewed and worked out
- this discovered the secretory pathway
what are the 3 main pathways involved in intracellular membrane traffic
1) Secretory pathway
2) Endocytic pathway
3) Retrieval pathway
Things are moved in the cell using vesicles, what types of vesicles are there?
1- Clathrin coated vesicles
2- COP1 coated vesicles
3- COP2 coated vesicles
Where do clathrin coated vesicles transport their content
- from the plasma membrane
- and between the Golgi and endosomes
where do COP1 and COP2 coated vesicles transport their content from
- COP1 - from the Golgi
- COP2 - from the ER
Describe how Rab is used to ensure specificity, meaning that the right vesicle finds the right membrane to fuse with
- Rab-GDP is inacitve
- Inactive Rab-GDP binds to GEF (guanin exchange factor) on a vesicle membrane,
- GEF exchanges a GDP for GTP, by adding a gaunine to the GDP. this produces Rab-GTP
- Rab-GTP is active and a rab effector protein on the vesicle binds to it if its the right type of Rab securing it to the vesicle
what is GEF
Guanine exchange factor, this causes inactive Rab-GDP to gain a guanine becomming active Rab-GTP
- basically GEF activates Rab
