Endoplasmic reticulum

Question 1

What is the primary role of the secretory pathway?

Answer 1

Primarily the transport of modified proteins from the inside of the cell to the outside.

Question 2

What are the three mechanisms of protein transport?

Answer 2

Nuclear pores
Vesicles
Transmembrane

Question 3

What is the ER made of?

Answer 3

Reticulum: network of membrane tubes
Plus outer nuclear envelope
Closed membrane: contents separate from cytoplasm.

Question 4

What are the key aspects of ER structure?

Answer 4

Cisternae are long flattened, sac like, unbranched tubules.
Fluid filled lumen or the cisternal lumen where biochemical reactions occur.
It is a closed organelle that provide a unique environment.

Question 5

What are the main differences between the rough and smooth ER

Answer 5

RER: important for secretary pathway and protein biosynthesis - folding proteins, regulating misfolded proteins and responding to cellular stress. Tissue types with high protein demand will have cells with lots of RER.
SER: main role is the biosynthesis of phospholipids and cholesterol and the repair of membranes.
Hepatocytes for example have a lot of SER

Question 6

What is translocation?

Answer 6

Movement of proteins across a membrane.

Question 7

What are the different components of the secretory pathway / endomembrane system?

Answer 7

1. ER
2. Golgi
   Then either transport vesicles through cytosol to plasma membrane
   Or to endosomes and lysosomes

Question 8

What organelle is required in larger quantities in cells that require the manufacturing and secretion of a lot of proteins, why?

Answer 8

Mitochondria, to provide the necessary energy.

Question 9

What is co-translocation?

Answer 9

When proteins are translated and translocated at the same time at the RER

Question 10

Which ribosomal subunit is attached to the RER, whats the other one doing?

Answer 10

The large subunit is attached and the small subunit exists separately and are recruited as they needed for translation.

Question 11

How is the correct ribosome get targeted to the RER for a specific process.

Answer 11

A signal sequence of amino acids on the RER act as a ‘postcode’ for transport destinations. The ribosome is connected to an mRNA transcript that recognises the signal sequence.

Question 12

What is a translocon?

Answer 12

A channel protein complex that transverses ER the membrane and exports and imports proteins to the ER. It has a roughly hour glass shape with a pore in the middle for storing proteins. The is a ‘plug’ region of the complex at the ER interface to prevent the leakage of ions during translocation of a protein substrate.

Question 13

What do the signal recognition particle and receptor proteins do?

Answer 13

They act together during co translocation
The SRP interacts with the newly synthesised polypeptide/ribosome to transport the protein being translated to the translocon. The SRP briefly pauses translation while it finds and binds to the SRP receptor in the ER membrane. The receptor recruits the translocation channel and translation continues.

Question 14

How does a protein pass through a channel/translocon during co translational translocation?

Answer 14

The signal sequence of a soluble protein attaches to the channel and then the protein passes through with the aid of a chaperone protein (eukaryotes: BIP, prokaryotes: Kar2) then an enzyme (signal peptidase) cleaves the signal sequence off the protein releasing the protein into the ER lumen. Thee protein can now be folded for its function.

Question 15

How does a protein pass through a channel/translocon during post translational translocation?

Answer 15

This is done independently of SRPs and receptors. Chaperone proteins still pull the protein through but due to the lack of the aid of SRP and receptor proteins, there are other transmembrane proteins that help the process occur.

Question 16

In what conditions can disulfide bonds form between which amino acids? Where is this found commonly?

Answer 16

In an oxidative environment such as the ER lumen between cysteines.

Question 17

What is the protein disulfide isomerase?

Answer 17

An ER lumenal oxidoreductase (enzyme) that catalyses redox reactions that alter the position of disulfide bonds on proteins.

Question 18

What is glycosylation?

Answer 18

A method of modifying proteins in the ER lumen by adding oligosaccharides (carbohydrates) as the protein passes from the cytosol to the ER lumen. Only extracellular parts of the protein are glycosylated. Glycosylated proteins function as cell receptors.

Question 19

How does glycosylation occur? What type of proteins does it make?

Answer 19

Transmembrane dolichol proteins that are attached to a lipid linked oligosaccharide protruding into the ER lumen will transfer the oligosaccharide to the growing peptide chain attached to the translocon. The enzyme oligosaccharide protein transferase facilitates the transfer.
Glycoproteins that mostly act as cell receptors.

Question 20

How is protein export from the ER regulated?

Answer 20

Misfolded proteins are held in the ER by chaperone proteins until they are either correctly folded or degraded.

Question 21

How in vague terms do protein quality control checks occur

Answer 21

Chaperone cycle checks -> trimming proteins trim the oligosaccharide -> checks for functionality -> if not there is more trimming or degradation.

Question 22

What is ERAD and how does it work?

Answer 22

ER associated degradation, misfolded proteins either from ER lumen or cytosol will be recognised. From the ER the misfolded protein will be exported via retro translocation by the translocon. Ubiquitin ligase complexes that are associated with the ER lumen will attach ubiquitin to the misfolded protein which targets the protein for degradation by a proteosome .

Question 23

What is the UPR and what does it do in the ER?

Answer 23

The unfolded protein response is mechanism which deals with a backlog of unfolded proteins either by signaling apoptosis or fixes the backlog. The build up of unfolded proteins will trigger the upregulation of chaperone proteins to fold the proteins and it will trigger expansion of the ER to create a dilution effect to deal with the excess proteins. Another part of the response is the triggering of transmembrane receptors which signal there is a ‘problem’. This is a feedback response.

Question 24

What is Ire1 and what does it do?

Answer 24

A transmembrane protein that is part of the UPR. In its inactive state it is monomeric and unphosphorylated, it also has BIP protein bound to it. When its activated by unfolded proteins, BIP dissociates allowing it to dimerises and undergo trans autophosphorylation (where 1 protein phosphorylates the other). When active it splices an intron of a specific mRNA transcript turning into a protein that is a transcriptional activator of chaperone protein genes and it also destroys certain mRNAs ultimately reducing the load of misfolded proteins.

Question 25

What is ATF6 and what does it do?

Answer 25

A transmembrane protein that is part of the UPR. In its inactive state it is bound to BIP. When active it binds to unfolded proteins and BIP is released. The BIP is transported to the golgi and cleaved into a transcriptional activator of genes to help deal with the unfolded proteins.

Question 26

What is PERK and what does it do?

Answer 26

A transmembrane protein that is part of the UPR. In its inactive state it is monomeric and unphosphorylated. When its activated it dimerises and undergoes trans autophosphorylation (where 1 protein phosphorylates the other). When active it will inhibit protein synthesis and can signal apoptosis.

Question 27

What is the other role of the SER other than the synthesis of phospholipids and cholesterol?

Answer 27

Metabolism of drugs and hormones. (liver)

Question 28

How does the ER respond to low intracellular levels of Ca2+?

Answer 28

The ER stores Ca ions and has several Ca ion channels which are responsible for releasing the ions into the cytosol. Depolarisation of t-tubules will trigger conformational changes in the voltage gated ions channels releasing Ca2+. This is very important for muscular functions.

Question 29

What is the sarcoplasmic reticulum?

Answer 29

A specialised part of the SER. It’s a very dense tubular network primarily found in myofibrils (muscles). When an action potential arrives from to the sarcolemma (muscle fibre membrane) the impulse will travel down the t tubules connected to the sarcolemma. The depolarisation of the t tubules stimulates receptors on the sarcoplasmic reticulum leading to the release of Ca2+. The ions can now bind to troponin to allow muscle contractions.

Question 30

What is SERs role in the nervous system?

Answer 30

Calcium ions are essential for action potentials and so the SER is important for the storage and release of Ca2+.

Question 31

What is a cortical reaction and how is signaled?

Answer 31

After a sperm has entered an egg the cortical reaction follows to ensure another sperm wont enter the egg. This reaction signaled Ca ions.

Question 32

What is autophagy and the role the ER plays in it?

Answer 32

Autophagy is the degradation of the components of a cell that is no longer needed. A phagosome will form around the destructe followed by a lysosome attaching before the degradation occurs. An ER transmembrane protein mediates the degradation via activating sarcoplasmic reticulum CA2+ channels.