ER, Golgi, nuclear envelope and membrane trafficking Flashcards
What is a rough outline of protein synthesis to secretion?
Secreted proteins are synthesised by ribosomes attached to the ER
N-terminus is synthesised first during translation of mRNA
Proteins are passed into the ER lumen
Modification
Passage to the golgi apparatus which has a series of enzymes and compartments
Further modification and transport out of the cell
How are proteins synthesised and folded correctly in the ER lumen?
New proteins from the RER are fed into the ER lumen
Chaperones mediate folding (BiP binds the hydrophobic regions and prevents aggregation)
Disulphide bonds are added by protein disulphide isomerase (PDI)
Oligomerisation occurs - assembly of multi-protein complexes
Step 1: docking to the ER
Ribosomes start reading mRNA in the cytosol
Signal peptide is synthesised at the NH2 terminal
The signal peptide is recognised by a signal recognition particle
Signal recognition particle docks onto a signal recognition particle in the ER membrane
Ribosome is not attached to the ER membrane
Ribosome is transferred to a traslocation channel which opens
Protein synthesised through translocation channel into ER lumen
What do docked ribosomes synthesise?
Membrane, luminal or secretory proteins
What is a signal recognition particle
Ribonuclear protein complex - RNA molecule with several proteins
In step 1 where the ribosome is docked onto the ER membrane when is the signal receptor particle displaced and recycled?
When the ribosome is moved into the translocation channel
Step 2: translocation through an ER membrane
The hydrophobic initiator sequence is stuck in the translocation channel.
Once the mRNA has been translated the signal peptides are cleaved by signal peptidase to release the protein from the translocon.
Mature protein is separated from the signal peptide and released into the cell.
Integral membrane proteins/translocation channels
Start or stop transfer sequences are hydrophobic so remain in the membrane
What determines the number of transmembrane domains?
The number of internal hydrophobic transfer sequences
What happens when the internal signal sequence which binds the SRP is not at the start of the polypeptide?
The signal sequence is not cleaved and remains in the membrane creating a transmembrane protein that is present in the membrane in more that one place.
What are the different types of integral membrane proteins?
Type I
Type II
Type III
Type IV
Type I integral membrane protein
N terminus in the lumen and C terminus in the cytoplasm
Type II integral membrane protein
N terminus in the cytoplasm and C terminus in the Lumen
Type III integral membrane protein
N terminus in the lumen and C terminus in the cytoplasm
Type IV integral membrane protein
N terminus in the lumen and C terminus in the cytoplasm
Insertion of type I single pass transmembrane protein
The nascent polypeptide chain-ribosome complex becomes associated with the translocon in the ER membrane. The N-terminal signal sequence is cleaved. The chain is elongated until the hydrophobic strop transfer sequence is synthesised and enters the translocon. This prevents the nascent chain from extruding further into the ER lumen. The stop transfer sequence moves laterally through a hydrophobic cleft between translocon subunits and becomes anchored in the phospholipid bilayer.
Translocon closes.
Synthesis continues and the elongating chain may loop out into the cytosol.
When synthesis is complete the ribosomal subunits are released .
Nascent
Just coming into existence
Insertion of type II single pass trans-membrane proteins
After the internal signal-anchor sequence is synthesised it is bound by an SRP which binds the SRP receptor on the ER membrane.
The nascent polypeptide chain becomes orientated in the translocon with its N terminal in the cytosol.
The chain is elongated into the lumen and the internal signal anchor sequence moves laterally through a hydrophobic cleft between translocon subunits and anchors the chain in the phospholipid bilayer.
Once protein synthesis is complete the C terminus is released into the lumen and the ribosomal subunits are released into the cytosol.
Insertion of type III single pass transmembrane proteins
Insertion is similar to that of type II except the C terminus is in the cytosol and N terminal in the lumen.
What determines the number of trans-membrane domains?
The number of internal hydrophobic transfer sequences
Insertion of type IV transmembrane proteins
The number of internal hydrophobic sequences determines the number of trans-membrane domains. The signal sequence is not at the start of the polypeptide so is not cleaved. Depending on how many hyrdrophobic sequences there are this controls how many trans-membrane domains there are.
