ER, Golgi, nuclear envelope and membrane trafficking

Question 1

What is a rough outline of protein synthesis to secretion?

Answer 1

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

Question 2

How are proteins synthesised and folded correctly in the ER lumen?

Answer 2

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

Question 3

Step 1: docking to the ER

Answer 3

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

Question 4

What do docked ribosomes synthesise?

Answer 4

Membrane, luminal or secretory proteins

Question 5

What is a signal recognition particle

Answer 5

Ribonuclear protein complex - RNA molecule with several proteins

Question 6

In step 1 where the ribosome is docked onto the ER membrane when is the signal receptor particle displaced and recycled?

Answer 6

When the ribosome is moved into the translocation channel

Question 7

Step 2: translocation through an ER membrane

Answer 7

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.

Question 8

Integral membrane proteins/translocation channels

Answer 8

Start or stop transfer sequences are hydrophobic so remain in the membrane

Question 9

What determines the number of transmembrane domains?

Answer 9

The number of internal hydrophobic transfer sequences

Question 10

What happens when the internal signal sequence which binds the SRP is not at the start of the polypeptide?

Answer 10

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.

Question 11

What are the different types of integral membrane proteins?

Answer 11

Type I
Type II
Type III
Type IV

Question 12

Type I integral membrane protein

Answer 12

N terminus in the lumen and C terminus in the cytoplasm

Question 13

Type II integral membrane protein

Answer 13

N terminus in the cytoplasm and C terminus in the Lumen

Question 14

Type III integral membrane protein

Answer 14

N terminus in the lumen and C terminus in the cytoplasm

Question 15

Type IV integral membrane protein

Answer 15

N terminus in the lumen and C terminus in the cytoplasm

Question 16

Insertion of type I single pass transmembrane protein

Answer 16

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 .

Question 17

Nascent

Answer 17

Just coming into existence

Question 18

Insertion of type II single pass trans-membrane proteins

Answer 18

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.

Question 19

Insertion of type III single pass transmembrane proteins

Answer 19

Insertion is similar to that of type II except the C terminus is in the cytosol and N terminal in the lumen.

Question 20

What determines the number of trans-membrane domains?

Answer 20

The number of internal hydrophobic transfer sequences

Question 21

Insertion of type IV transmembrane proteins

Answer 21

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.