Endoplasmic Reticulum Flashcards
Describe what the endoplasmic reticulum is?
Continuous network of tubules forming a net structure
What other structure is the ER continuous with?
The Nucleur membrane
What are the major functions of the ER?
Protein synthesis - folding & assembly of long peptide chains
Glycosylation - attachment of sugars to proteins
Multiprotein Complexes
Lipid Synthesis - initiated in ER, (choletserol, phospholipids etc.)
Ca2+ sequestration - storage of ca2+ in lumen
Detoxification - by cytochrome P450 enzymes
Explain how Detoxification works?
Cytochrome P450 enzymes convert (inactive) prodrugs -> active drugs
How does calcium sequestration occur?
Ca2+ conc. very low in cytoplasm
cell has to actively pump ca2+ out
acts as a secondary messenger
What equipment can be used to view Pancreatic Acinar Cells?
Electron microscope
Where are the enzymes stored in acinar cells?
In Secretory Vesicles
What is the function of the Acinar cells?
secrete enzymes which enter small intestine (food digestion etc.)
Describe the structure of the RER?
Bilayers with Ribosomes on the membrane with the lumen inside
Where are Acinar Cells found in the body?
Exocreatine Pancreas
What is the RER?
Rough endoplasmic reticulum
How and where are the enzymes released from acinar cells?
Vesicles fuse with membrane in lumen of ER to secrete enzymes
How are SER and RER interlinked?
They are both part of the same network of membranes - all one organelle
What is the role of Insulin?
Regulates Blood Glucose levels
Outline the stages of Co translational Targetting to the ER
- mRNA encodes protein requiring secretion from cell
- when translation occurs, ribosome has to assemble on mRNA and begin protein synthesis at AUG initiation methionine
- At beginning of protein, a signal sequence is present (v. rich in hydrophobic amino acids)
- the signal sequence is a distinctive amino terminal signal
- Signal recognised by Signal recognition Particle (SRP)
- SRP bind sto its receptor (SRP receptor)
- Binding causes SRP and its receptor to dissociate following GTP hydrolysis
- Ribosome now able to bind to Peptide translocation complex Sec61
- Translation occurs
- Newly synthesised peptide is threaded into ER lumen
- Signal sequence cleaved off by Signal Peptidase
- mature protein produced
Why do disulphide bonds only form in the ER and not the cytoplasm ?
Cytoplasmic proteins don’t have S-S bonds due to insufficient enzymes present to catalyse their formation
Which proteins have their signal sequence cleaved?
All proteins synthesised in ER have their signal sequence cleaved off
How are proteins transported from one organelle to another?
via Vesicular Transport
Which proteins would require to be exported from the cell?
Golgi apparatus
Lysosomes
etc.
How is the final structure of insulin produced?
Via cotranslational protein targetting (requiring further modifications)
How does vesicular transport work?
Vesicles fuse with the largest membrane and secrete the protein contents
Why are some proteins not exported from the nucleus?
If the protein fail to correctly undergo :
- Proteolysis
- Glycosylation
- Disulfide bond formation
- Folding
- assembly into protein complex
they will not be exported out of the nucleus
What happens to mRNA encoding a protein that needs to be exported to somewhere else in the secretory pathway?
Requires cotranslational targetting to the ER
Where does the central dogma occur?
Takes place in nucleus and products (RNA) exported to cytoplasm
