Organelle Ecology - the ER and Self-Organisation Flashcards
1
Q
Structure
A
- The fluid mosaic model and membrane composition
- Membrane specificity
- Lateral aggregation
- Membrane curvature
- ER architecture
- Tubule formation
- Cisternae formation
- Steady-state models
- Super-resolution microscopy
2
Q
The Fluid Mosaic Model
A
- Singer and Nicolson, 1973
- PLB self-assembly is stabilised by interfacial hydrophobic interactions, balancing head-group repulsion
- 7-8nm
3
Q
Membranes differ in their lipid composition
A
- cholesterol
- phosphotidylethanolamine
- sphingomyelin (SM)
- phosphatidylcholine (PC)
- phosphatidylserine (PS)
- glycolipids
4
Q
What are some potential implications of differing membrane lipid composition?
A
phosphate starvation in plants
5
Q
outer PM leaflet
A
higher SM and cholesterol
6
Q
inner PM leaflet
A
higher PS (acidic head group)
7
Q
ER membrane
A
- few anionic lipids
- 4-5nm
8
Q
cis-Golgi membrane
A
- loosely packed
- 4-5nm
9
Q
lipid distribution reflects […] between membranes and organelles
A
- local synthesis
- selective packaging
- vesicular transfer
10
Q
regulation of PC in the ER membrane
A
- CCTalpha is rate limiting
- unfolded M domain is inactive
- PC depletion results in an increase in membrane conical and ionic lipids
- M domain folds into an amphipathic helix
- increased PC synthesis
11
Q
cholesterol
A
- synthesised in the ER
- enriched in the trans-Golgi and PM
- requires molecular crane (OSBP) for counter-exchange against conc. gradient
12
Q
CCTalphha
A
- phosphocholine cytidylyltransferase
- controls cholesterol levels
- drives nuceloplasmic reticular tubular invagination (via AHs)
13
Q
OSBP
A
- oxysterol binding protein
- binds PI4P + Arf1-GTP @ trans-Golgi
- forms a bridge w/ VAP @ contact sites
- cholesterol transfer
- PI4P retrotranslocation to ER
- PI4P -(Sac1)-> PI (energy!)
- Nir2 binds VAP; transfers PI -> trans-Golgi
- phosphorylation!
14
Q
What provides membrane specificity?
A
the balance of hydrophilic and -phobic interactions
15
Q
AH domains
A
- IM: folding
- hydrophobic strips embed
- hydrophilic groups: surface
- sufficient for targeted localisation under heterologous expression
16
Q
cis-Golgi/ER
A
- coiled-coil tethers use Amphipathic Lipid Packing Sensor (ALPS) domains
17
Q
trans-Golgi/PM
A
- AH w/ higher charged groups (e.g. synuclein)
18
Q
lateral aggregation
A
- microns
- lipid rafts
- different lipids favour liquid-(dis)ordered lamellar phases
19
Q
Raft hypothesis
A
- extends the fluid mosaic to functionally distinct fluid domains
- selective of lipid + protein components
20
Q
nanoscale domains
A
exist transiently
21
Q
how do proteins partition?
A
TMD length
22
Q
Hydrophobic mismatch
A
is reduced by proteins w/ long TMDs associating with a SM/cholesterol raft
23
Q
Membrane curvature is required for
A
1) endocytosis
2) tubular structures
3) NEs
4) fission
5) filopodial protrusion (actin skeletal emergence)
24
Q
How to induce curvature
A
1) domain insertion
2) lipid modifications
3) lipid interaction + clustering
4) wedge TM proteins into the bilayer
25
BAR proteins
- Bin, Amphiphysin, Rvs
- curved domains
- bind to membrane via concave face
- cause membrane curvature in retromer coats
26
COPI proteins
- Exit Site
- cause vesicle formation
- not necessary for maintenance
27
CCVs
- PM/trans-Golgi
28
ER architecture
- largest organelle
- 30% of all proteins pass through
- acts as a single compartment
- continuous w/ NE, tubules, cisternae(, plasmodesmata)
- exists at an energy-dependent steady-state between formation and disassembly
29
What is the significance of the ER being continuous with the plasmodesmata?
...
30
reticulons
- 2x wedge-shaped TMDs + C-terminal AH
- induce and maintain tubule curvature
31
atlastins
- dynamin-like GTPases
- drive homotypic tubule fusion via conformational change
- form 3-way junctions and polygonal networks (complex architecture)
32
tubule extension
driven by cytoskeleton-motor-tubule interactions
33
animals
microtubules
34
plants
microfilaments
35
Reconstitution experiments reveal the minimum kit for tubule formation:
- reticulons
- atlastins
- vesicles
- GTP
36
nucleoplasmic reticulum
- tubular invaginations in mammalian nuclei
- cell-type specific (highest in cancer cells)
- driven by CCTalpha AHs
37
Cisternae
- high SA
38
Cisternae formation mechanism
1) ribosome docking
2) co-translational insertion of a series of protein: reticulons, climp63, p180, lunapark
39
reticulons in the forming cisternae
stabilise the curved edges
40
climp63 in the forming cisternae
- lumenal spacer protein
- keeps width constant
41
p180 in the forming cisternae
- rod-like scaffold protein
- keeps surface flat
42
lunapark in the forming cisternae
- LNPARK motif
- localises to 3-way junctions
- wedge-shaped helices
- favours concave membrane curvature
43
Steady-state models
predict ER morphology using ER-shaping proteins
44
Expanding steady-state models
- to capture complex ER architecture
- the type and amount of ER-shaping proteins simulates multiple ER morphologies
- the energetically favoured ER morphology varies with Ctot and Phi
45
Ctot
the total concentration of curvature-stabilising proteins
46
Phi
the fraction of Lunapark protein that favours concavity
47
Super-resolution microscopy
- breaks light diffraction barrier
- cisternae may be collapsed tubules
48
ER networks are
a dynamic balance of tubule extension and ring closure
49
nanoholes
- left by ring closure collapse of the tubular network back into the cisternae
- homotypic fusion = recovery
