Lecture 16- Membrane contact sites

Question 1

What occurs at the rough ER?

Answer 1

Protein synthesis

Question 2

What occurs at the smooth ER?

Answer 2

Lipid synthesis and important for calcium storage

Question 3

What is an intermembrane contact site?

Answer 3

Areas of close proximity between heterologous membranes

Question 4

How does the level of calcium change between the newly formed endosome and the early/late endosome?

Answer 4

Low calcium in extracellular space which increases in the early and late endosome as calcium gets pumped in

Question 5

How does calcium levels in the cytosol compare to the levels in the extracellular space and ER lumen?

Answer 5

Calcium levels are higher in the cytosol compared to the extracellular space, but lower than in the ER lumen

Question 6

What is the specialised term given to the ER of muscle cells?

Answer 6

Sarcoplasmic reticulum

Question 7

What is the purpose of the sarcoplasmic reticulum?

Answer 7

Handles calcium transients required for muscle contraction

Question 8

How can you visualise the contact between the PM and the sarcoplasmic reticulum?

Answer 8

Using electron microscopy

Question 9

What is the role of Stim1?

Answer 9

Calcium sensor in the ER

Question 10

How are ER calcium stores replenished?

Answer 10

1. When calcium is released from the ER, it is sensed by the sensing unit and forms a complex with Stim1
2. The complex migrates within the ER membrane and associates via a PIP2 MCS with an area of the PM enriched with lipids
3. This allows interaction with the Orai1 calcium channel on the PM which comes together to form a complex
4. Calcium outside the cell can be transferred directly through the ER channel membrane and into the ER lumen

Question 11

Where and in what form is Stim1 found when there is plenty of calcium in the ER?

Answer 11

Stim1 is located at the end of the ER tubules and is inactive

Question 12

How is material transferred from the ER to other organelles in the cell?

Answer 12

• Requires a lipid and calcium transfer
• Absence of membrane insertion of fusion so material must be transferred from one organelles to the next (using lipid and calcium transfers)

Question 13

Which technique can be used to follow the movement of tubules and how they contact and interact with mitochondria?

Answer 13

Live cell imaging

Question 14

Outline the structure of ER contact sites

Answer 14

1. Ribosomes excluded from contact sites
2. Membranes are very close in proximity
3. ER contacts can be short or long lived

Question 15

What functions/platforms does ER contact sites allow for?

Answer 15

1. Calcium mobilisation
2. Lipid transfer
3. Signalling

Question 16

How to bridges form between membranes?

Answer 16

Protein components (tethers) extend to link 2 membranes together

Question 17

What is a MSC tether?

Answer 17

A protein/complex of proteins that simultaneously bind 2 opposing membranes at contact sites

Also important for maintaining contact sites

Question 18

What can defects in MSC tethers lead to?

Answer 18

Muscular myopathies

Question 19

How are MCS important on the endocytic pathways?

Answer 19

MSCs help regulate the movement of signalling proteins (EGFR) into MVBs (multi-vesicular bodies)

Question 20

What is the role of PTP1b?

Answer 20

Regulated signalling and the level of phosphorylation

Question 21

Which direct does lipid transfer occur in?

Answer 21

Unidirectional

Question 22

Given an example of a disease associated with MCS and defects in NPC1/2 proteins

Answer 22

Neimann Pick Disease C

Question 23

What causes Neimann Pick Disease C and what are the symptoms?

Answer 23

Defect in NPC1/2 protein which leads to the accumulation of sphingomyelin in lysosomes.

This movement is though to occur via MCS

Symptoms: poor muscle growth, difficulty swallowing etc

Question 24

Explain organelle fission

Answer 24

1. Protein is recruited to the point of fission when mitochondria divide
2. The ER is recruited along with a protein to the point of fission and a ER tubule is thought to promote fission
3. Endosomes also recruit ER tubule and associate with the endosomal tubule
4. This is an alternative to vesicular transport