Endoplasmic Reticulum Flashcards
What percentage of the cells volume does the ER take up?
10%
What percentage of the cells membranes does the ER take up?
60%
What is the function of the ER?
The site of phospholipid synthesis = by membrane bound enzymes of the outside of the ER
What is scramblase responsible for?
The translocation of phospholipids between the two monolayers of a lipid bilayer
How is the rough ER different from the smooth ER?
It has attached ribosomes
What is the function of the ribosomes attached to the rough ER?
Protein sythesis
- secretory proteins
- transmembrane proteins
- ER and golgi proteins
- lysosomal proteins
How is the location of protein synthesis linked to where the protein will go once its been synthesised?
If proteins are destined to end up in the cytosol they will be synthesised in the cytosol
If proteins are destined to end up being excreted they will be synthesised on the rough ER
Describe the process by which proteins are targeted to the rough ER to be synthesised.
- mRNA binds to a ribosome
- Translation begins
- When the signal peptide is translated a SRP (signal recognition particle) will bind to it, temporarily pausing translation
- The ribosome travels to the rough ER where the SPR binds to a SPR recpetor on the membrane of the ER
- The ribosome interacts with the ER membrane
- The SRP and SRP receptor are displaced and recycled
- Translation resumes with the elongating chain being ‘pushed’ into the ER lumen through a translocator protein
- The signal peptide detaches from the polypeptide
What does SRP stand for?
Signal recognition particle
What is the translocator protein called that ‘pushes’ the polypeptide into the ER lumen?
Sec61 translocon
What is glycosylation?
The modification of proteins by the addition of a 14 sugar sequence
Which proteins are glycosylated?
Almost all proteins synthesised on the rough ER
What is the 14 sugar sequence?
Oligosaccharide
Where on the protein do sugars attach?
To a specific amino acid sequence starting with asparginine
How many glucose molecules are there within the oligosaccharide?
3
What is the molecule that the precursor oligosaccharide is produced on and where is it found?
Phosphodolichol
On the ER membrane
What is the enzyme that transfers the oligosaccharide onto the elongating polypeptide?
Membrane bound oligosaccharyltransferase enzyme
Found in the ER lumen
What does glycolysation also act as?
A folding tag
Describe the process by which glycolysation acts as a folding tag.
- During the initial phase of protein folding 2 of the 3 glucose molecules are removed
- Calnexin recognises the single glucose molecule on the incompletely folded protein and prevents it leaving the golgi
- The final glucose is removed
What happens if the final glucose is removed before the protein has completed folding?
Glucosyl transferase added another glucose to prevent it from leaving the golgi
What happens if the final glucose is removed from misfolded proteins?
Misfolded protein is transported back to the dislocation complex and is sent to the cytoplasm to be degraded
What happens if the glucose is removed from a correctly folded protein?
The protein is exported to the golgi
What is the smooth ER a site for?
Lipid synthesis
Steroid hormone synthesis
How does the smooth ER change in order for it to be able to synthesis cholesterol?
It expands in order to accomodate the enzymes which synthesise and modify cholesterol
Where does the detoxification of drugs occur?
In hepatocytes in the liver
Describe the process of drug detoxification.
- Lipophilic drugs are found on the ER membrane
- Drugs are oxidised which increases their water solubility
- The drug leaves the ER membrane where it further modified in the cytoplasm
- The drug is excreted in the bile and urine
