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L2 Molecular Cell Biology > Membrane contact sites > Flashcards

Membrane contact sites Flashcards

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1
Q

What is the function of the rough ER?

A

Protein synthesis

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2
Q

What is the function of the smooth ER?

A

Site of lipid synthesis and calcium storage

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3
Q

What do electron dense areas suggest in electron microscopy?

A

Lots of protein in the area

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4
Q

What is CLEM?

A

Correlative light and electron microscopy

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5
Q

How does CLEM work? (4)

A
  • Proteins are labelled
  • Look using a microscope and mark where the protein is
  • Rapidly freeze and fix the cell for electron microscopy
  • Look for the marked protein and visualise the ultrastructure using EM
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6
Q

What techniques have been used to visualise ER membrane contact sites? (3)

A
  • CLEM
  • Live cell imaging
  • Electron microscopy
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7
Q

What is the structure of ER contact sites? (4)

A
  • Ribosomes are excluded (smooth ER)
  • Membranes very close together
  • ER contacts can be short or long-lived
  • Membranes are connected by long multi-domain proteins which stretch out and hold the membranes together
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8
Q

What is the molecular machinery of ER membrane contact sites?

A

Tether proteins

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9
Q

What are the features of tether proteins? (4)

A
  • Can be protein-protein
  • Can be protein-lipid
  • Distance usually 30nm
  • Inhibit fusion
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10
Q

How are membrane contact sites defined? (4)

A
  • Absence of fusion
  • Must fulfil a function
  • Have a defined protein and lipid composition
  • Raft-like, enriched in sterols (local rigidity)
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11
Q

What is osbp? (2)

A
  • Tether and lipid transfer protein
  • Transfers cholesterol into the ER
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12
Q

What are the functions of ER membrane contact sites? (4)

A

Provide platforms for:
- Calcium mobilisation
- Lipid transfer
- Signalling
- Organelle division

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13
Q

How do membrane contact sites facilitate calcium mobilisation? (5)

A
  • T-tubule is closely associated with sarcoplasmic reticulum in skeletal and cardiac muscle
  • Stim1 is an ER transmembrane protein which is monomeric at high ER Ca2+
  • Low ER Ca2+ levels cause oligomerisation of stim1 which interacts with orai1 channel on the plasma membrane
  • Orai1 channel is in the membrane where there are high pip2 levels
  • Allows transportation of Ca2+ into the ER to replenish ER Ca2+ levels
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14
Q

What is the calcium sensor in the ER?

A

Stim1

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15
Q

How are membrane contact sites important for lipid transfer? (4)

A
  • Lipids are synthesised in the ER and lipids are delivered to the ER in order to be trafficked to the plasma membrane
  • Membrane contact sites allow for non-vesicular transfer of lipids
  • Lipid transfer is unidirectional
  • Lipid transfer proteins (LTPs) have hydrophobic grooves and may use lipid/ion concentration gradients to promote lipid transfer
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16
Q

What is an example of a disease caused by disruption to lipid transfer? (4)

A
  • Niemann Pick Disease C
  • Mutation in a contact-site transmembrane protein which is important for transport of cholesterol from the lysosome into the ER
  • Causes accumulation of lipids in the spleen, liver, lungs, bone marrow and brain
  • Sphingomyelin accumulates in lysosomes
17
Q

How are membrane contact sites involved in organelle fission? (2)

A
  • ER surrounds a fission enzyme on mitochondria and contributes to mitochondrial fission
  • Mediated by membrane contact sites
18
Q

What diseases are linked to membrane contact sites? (3)

A
  • TDP43 regulates ER-mitochondria contact sites and is linked to ALS
  • REEP1 regulates ER-mitochondria contact sites and is linked to hereditary spastic paraplegia
  • Presenilins are involved in Ca2+ exchange between the ER and mitochondria and are mutated in Alzheimer’s
19
Q

How are membrane contact sites involved in signalling? (3)

A
  • EGFR signalling is attenuated by dephosphorylation and downregulation of the receptors
  • EGFR on the endosome is dephosphorylated by a protein tyrosine kinase on the ER membrane which causes EGFR to be incorporated into multi-vesicular bodies and degraded
  • Facilitated by contact sites between the ER and endosome

L2 Molecular Cell Biology (13 decks)

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