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The ER
Endoplasmic Reticulum
| Question | Answer |
|---|---|
| What is the ER? | Labyrinthine-like membrane bound organelle that is the major site of protein synthesis & assembly |
| What is made in the ER? (2) | 1. Golgi & ER resident proteins 2. Plasma membrane, endosomal & lysosomal proteins |
| How are proteins targeted to the ER? | N-terminal signal sequences |
| What is a signal sequence? | Short hydrophobic sequence ('molecular postcode') |
| What does a Signal Recognition Particle (SRP) do? | Recognises signal sequences & pauses translation, preventing signal proteins from folding in the cytosol. |
| How does an SRP prevent early translation? (2) | 1. SRP54 subunit binds to signal sequence 2. SRP RNA's Alu domain binds to ribosome elongation factor binding site |
| 1st part of protein to appear from the ribosome? | The Signal Sequence |
| How does the SRP deliver the ribosome to the ER? (2) | 1. SRP binds SRP receptor (SRPR) on ER membrane 2. SRP displaced & ribosome docks onto SEC61 |
| Function of the SEC61 Translocon? | Forms an aqueous pore across ER membrane |
| What is the Binding Immunoglobulin Protein (BiP)? | An ER luminal HSP70 chaperone required for protein translocation into ER |
| BiP function?(2) | 1. Binds proteins as they are inserted into the ER 2. Helps proteins fold by binding to unfolded regions (ATP dependent mechanism) |
| What kind of environment is the ER? | An oxidising environment - favouring disulphide bond formation |
| WHere does N-linked Glycosylation take place? | In the ER |
| What is calnexin? | Chaperone molecule associated with protein ERp57 |
| What does calnexin and ERp57 do? | bind to oligosaccharides & free cysteines preventing unfolded proteins aggregating |
| What is the retrieval motif? | KKXX |
| What does the KDEL motif do? | ER retention signal - keeps protein localised to the ER |
| What is ERAD? | ER-associated Degradation |
| What does ERAD do? | Prevent accumulation of misfolded proteins in the ER |
| How does ERAD pathway? (4) | 1. EDEMs catalyse removal of mannose residues from N-linked chains 2. Mannose trimmed N-glycans recognised by adaptors 3. Protein is ubiquinated & translocated into cytosol 4. Ubiquitinated protein recognised by cytosolic proteasome & hydrolysed |
| Diseases affecting ERAD pathway? (3) | 1. Cystic Fibrosis 2. HCMV (Human Cytomegalovirus) 3. Cholera toxin |
| What does cystic fibrosis affect? | The CFTR ion channel |
| What is lumacaftor? | Chemical chaperone that increases ER exit of the mutant & Cl- transport in cells. |
| What does HCMV do? | Hijacks ERAD pathway to escape immune recognition |
| What does Cholera toxin do? | Promotes electrolyte & water mvt into intestinal lumen |
Created by:
rubyroo
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