Lecture 14 - Molecular contact sites

Question 1

Do organelles always maintain their identity?

Answer 1

Yes - however don’t exist completely separate from each other. They can come together, at MEMBBRANE CONTACT SITES

Question 2

What is the function of membrane contact sites?

Answer 2

Provide platforms for:
- intracellular signalling
- lipid metabolism
- organelle biogenesis
- organelle division
- motor protein-mediated membrane dynamins

Question 3

When were membrane contact sites first identified?

Answer 3

in 1957 - Porter and Palade
- EM (electron microscopy) showed contact sites that are electron dense

Question 4

Where is contact between?

Answer 4

Contact isn’t only between ER, but also between mitochondria and chloroplasts in plant cells, or mitochondria and vacuoles.

There is no fusion & mixing of contents

Question 5

What is in contact with the ER?

Answer 5

• mitochondria
• endosomes
• plasma membrane
• peroxisomes
• lipid droplets
• golgi apparatus
• ribosomes

Question 6

What is rough ERs function?

Answer 6

Protein synthesis

Question 7

What is smooth ERs function?

Answer 7

sites of lipid synthesis & roles in calcium storage

Question 8

Describe the structure of smooth ER membrane contact sites

Answer 8

• ribosomes excluded from contact sites
• membranes very close (3-15nm)
• ER contact can be short- or long-lived

Question 9

What role does MCS play?

Answer 9

intracellular signalling calcium

Question 10

What are sarcoplasmic reticulum of muscle cells?

Answer 10

specialized ER for handing Ca2+ transients required for muscle contraction

Question 11

What is Stim 1?

Answer 11

calcium sensor in the ER

Question 12

How does calcium sensing in the ER work?

Answer 12

When calcium levels are high in ER, Stim1 is dispersed in the membrane. When there is a release of calcium, the Stim 1 comes together and aggregates, as the sensing mechanism detects low levels and forms macro-molecular complex. This will migrate to particular part of ER, allowing interaction with plasma membrane and formation of contact site. This allows interaction between Stim1 and plasma membrane, leading to recruitment of calcium channel, and the opening of channel leads to replenishment of calcium levels. This is useful as it prevents a rise in cytosolic calcium.

Question 13

What are phosphoinositides?

Answer 13

• low abundance lipids
• important in cell signalling and organelle identity
• Phosphoinositides - diversity, location and recognition

Question 14

Does the plasma membrane have one type of phosphoinositide?

Answer 14

No - they have more than 1

Question 15

Describe how the ER tubule interacts with late endosome

Answer 15

Late endosome also called ‘multi-vesicular body’, as the membrane buds inwards to form intra-luminal vesicles, and material that enters these vesicles is always targeted for degradation.

A contact site between the ER and the late endosome is important for type 1 tyrosine kinase signalling.

Question 16

What can endosomes do?

Answer 16

Endosomes can drag section of the ER

Question 17

Describe membrane contact sites (MCS) in lipid transport

Answer 17

ER: site of membrane lipid synthesis
Contact sites: non vesicular transfer of lipid
Lipid transfer is unidirectional
Lipid transfer proteins (LTPs) may use concentration gradients of lipid to promote lipid transfer

Question 18

How does a gradient of P14P allow transfer of cholesterol from the ER to the Golgi?

Answer 18

There is a higher concentration of cholesterol in the Golgi than ER, however it is made in the ER, so it needs to be transported against its concentration gradient.

OSBP combines with cholesterol and delivers it to the golgi, where it picks up P14P, before it transports it back to the ER, where it is hydrolyzed, loses its phosphate and becomes Pi (inorganic phosphate). Always a high concentration of P14P in the golgi. This lipid exchange ensures that cholesterol can be moved from the ER to the Golgi.

Question 19

Why are protein components often extended?

Answer 19

To allow bridges to form between membrane contact sites

Question 20

What does the structure of protein do?

Answer 20

Tethers for lipid transfer

Question 21

What can protein tethers be visualised by?

Answer 21

Electron Microscopy

Question 22

What are the features of Neimann Pick Disease C?

Answer 22

Arises from defects in proteins found in the lysosomal membrane that are important for cholesterol release from lysosomes. Cholesterol can be taken up from environment via endocytosis or synthesized. These patients take up a lot of cholesterol, however the sensors in the ER that indicate that cholesterol levels are low are always activated, so that there is still a lot of cholesterol synthesis. This can lead to a lot of digestive issues.

Question 23

Describe membrane contact sites in organelle fission

Answer 23

ER is constantly breaking and dividing, along with mitochondria

Advances in microscopy allow us to understand structure/function

Question 24

What does liver cell imaging suggest?

Answer 24

This suggested that MCS might be important in organelle fission

Question 25

Describe how membrane contact sites are involved in organelle fission

Answer 25

The tubules of ER contribute and surround mitochondria as it breaks apart. Experiments have shown that proteins are important in fission.