The ER Flashcards
What is the primary function of the endoplasmic reticulum (ER)?
The ER is involved in protein modifications, folding, and quality control.
What process allows soluble proteins to be released into the ER lumen?
Co-translational translocation.
What is the role of the Sec61 translocator in protein insertion?
It opens a channel for the polypeptide to be threaded through as translation occurs.
Fill in the blank: The _______ is cleaved by signal peptidase, releasing the protein into the ER lumen.
signal sequence
What stabilizes the folded structure of proteins in the ER?
Disulfide bonds formed by oxidation of cysteine side chains.
True or False: N-glycosylation occurs on serine residues in the ER.
False
What is the consensus sequence for N-glycosylation?
Asn-X-Ser or Asn-X-Thr (X = any residue except Proline).
List three functions of N-glycosylation.
- Assists protein folding
- Modified to create mannose-6-phosphate tags which act as a lysosome sorting signal
- Acts as a ligand for specific cell-cell recognition events
What type of proteins does the ER quality control retain?
Misfolded proteins.
What triggers the Unfolded Protein Response (UPR) in the ER?
Build-up of misfolded proteins in the ER lumen.
What is the role of molecular chaperones in the ER?
They assist in the proper folding of newly synthesized polypeptide chains.
How do transmembrane proteins anchor in the lipid bilayer?
Through additional hydrophobic sequences.
What is the significance of the ER being the entry point for the secretory pathway?
It is where proteins destined for the Golgi, lysosomes, and plasma membrane are synthesized.
Fill in the blank: The ER lumen is _______ which aids in disulfide bond formation.
oxidizing
What happens to proteins with a signal sequence when they encounter the ER?
They are targeted and imported into the ER.
What method is used to analyze radioactive proteins to confirm signal cleavage?
SDS-PAGE.
True or False: The orientation of membrane proteins is fixed after insertion.
True
What happens to proteins that cannot fold correctly in the ER?
They are retained by ER quality control mechanisms.
What role does BiP play in the ER?
It binds exposed hydrophobic residues and prevents aggregation to allow proteins more time to get into the right conformation
What type of vesicles transport proteins along the secretory pathway?
Membrane-bound transport vesicles.
How do hydrophobic sequences anchor transmembrane proteins in the lipid bilayer?
- Signal sequence binds Sec61 = channel opens
- Hydrophobic stop-transfer sequence stops the movement of the polypeptide through the channel
- Stop-transfer sequence is released from the channel into the bilayer, forming a transmembrane domain
- Signal sequence is cleaved
- Protein is inserted into the bilayer, orientation is fixed at the N-terminus in the lumen
What do multi-pass transmembrane proteins have?
An internal ER signal sequence instead of an N-terminus sequence
Known as the start-transfer sequence
Role of start-transfer sequence
- Start-transfer sequence acts as an ER targeting signal
- Binds to Sec61 and opens the channel to start translocation
- The polypeptide moves through the channel
- Stop-transfer sequence enters the channel and stops translocation
- Both hydrophobic sequences are released from the channel, forming transmembrane domains
- Alternating start and stop transfer sequences generate complex multi-pass membrane proteins
- Signals do not get cleaved
Ways membrane protein can associate with the lipid bilayer
- transmembrane
- monolayer associated
- lipid linked
- protein attached
What is BiP?
An ATPase (heat shock protein) that binds to exposed hydrophobic residues
ATP is used as fuel for binding and release of BiP
What is Calnexin?
A chaperone protein that binds N-glycosylated proteins
How may proteins be modified in the ER?
- Signal sequence cleavage = proteolysis reaction
- Disulfide bond formation via oxidation = occurs primarily in the ER
- Glycosylation = covalent attachment of carbohydrates to proteins once they’ve entered the lumen of the ER
Describe Disulfide bond formation
- Formed by oxidation of cysteine side chains
- Stabilises folded structure of proteins
- Catalysed by protein Disulfide isomerase (POi) inside the ER lumen = ensures bonds are formed in the right place
- ER lumen is oxidising
How does N-glycosylation occur?
- Occurs in lumen of ER
- N = Asparagine
- Oligosaccharide is transferred to the protein from a special lipid donor, dolichol
- Catalysed by oligosaccharyl transferase (OST)
- OST only glycosylates Asn residues in a specific consensus sequence
Role of N-glycosylation in the glycocalyx
- Eukaryotic cells are coated in carbohydrates attached to proteins and lipids called the glycocalyx
- Forms a protective layer outside the cell
- This layer attracts water & provides a hydrated layer on top of the cells (e.g. stops white and red blood cells sticking to each other)
- Made at the ER and Golgi before delivery to the plasma membrane
- Ultimately end up present on the cell surface
How is exit from the ER controlled?
Quality control by chaperone proteins —> bind to misfolded proteins & stop them leaving the ER
How is ER size and function controlled?
A build up of misfolded proteins in the ER lumen will trigger the unfolded protein response (UPR)
