Lecture 3: Physical Properties of Lipids Flashcards
What are the 5 physical properties of cell membranes?
surface charge, fluidity, width, curvature, lateral pressure
describe 5 differences between the ER membrane and PM
1) ER has lots of unsaturated phospholipids
2) ER is not well packed
3) PM has negatively charged membrane
4) PM is thicker
5) Proteins at the ER supposedly have shorter transmembrane domains
how is the ER a biogenic membrane?
it synthesises new lipids
what % of phospholipids are anionic?
30%
why is charge important for protiens and lipids?
for electrostatic interactions that can determine membrane associations
What does MARCKs do?
stabilise interactions with the membrane via polybasic stretch of amino acids. the membrane association is regulated by calcium ions and calmodulin and protein phosphorylation in the polybasic region
what factors affect membrane fluidity?
length of acyl chain, saturation of acyl chain, cholesterol, temperature
what is the effect of longer hydrocarbon chains?
more van der vaals so more rigid than shorter chains
what is the effect of saturated hydrocarbon chains?
they are straight, well packed so more rigid than double bonds
what is the effect of cholesterol in the membrane?
can make sphingolipids more fluid but can rigidify membranes too
How does cholesterol interact with the membrane?
ring structure interacts with the first few carbons of the lipid tails. has a small OH head group so is triangle shaped
what happens to the acyl chains of E.coli grown at high temps?
higher degree of acyl chain saturation
what have experiments shown in response to changing temperatures?
changing lipid composition
What is DesK and what organism is it present in?
Thermosensing histidine kinase, B. subtilis
Explain how the membrane of B. subtilis is remodelled in response to temperature.
DesK is usually inactive so won’t phosphorylate the TF DesR. when temps decrease, the membrane becomes more rigid, packing closer and the transmembrane domains become more straight so the kinase domain becomes active as is free from autoinhibition. it activates DesR which causes the transcription of a gene encoding deltaS desaturase which desaturates the phospholipid chains and increases fluidity. this causes autoinhibition of DesK kinase domains again.
Give an example of membrane fluidity allowing intermolecular interactions
EGFR dimerisation upon ligand binding
what don’t form bilayers by themselves?
non-bilayer lipids
what do all membranes contain at least one of?
non-bilayer lipid
what shape can non-bilayer lipids form when pure?
cone or wedge
what are micelles and why are they thermodynamically stable?
all the hydrophobic tails point inwards in a sphere
what is an inverted micelle?
the hydrophobic tails point outwards so isn’t thermodynamically stable
what are the possible functions of non-bilayer lipids?
membrane fusion, fission, packing properties, membrane curvature
how can non-bilayer lipids affect the packing properties?
they have small head groups so other proteins more likely to be able to fit between them for higher packing density and lateral pressure
Explain how PKC is activated
phospholipase cleaves PIP2 to DAG, releasing IP3 causing release of Ca ions from the ER. this causes PKCBII to translocate to the cell membrane. Ca binds regions of its C2 domain to mediate bridging bridging to the phosphates. Binding of DAG to C1A domain causes C1A disengagement and a pseudosubstrate is removed from the catalytic cleft. Binding of DAG to C1B domain results in downstream signalling
how may non-bilayer lipids in a local concentration influence membrane curvature?
can cause negative and positive curvature
Why are flippases essential?
for assembly and regulation of the membrane in PM, golgi and endosomes and maintain the asymmetric transbilayer distribution
What does the exoplasmic leaflet of the bilayer have that makes it inert?
Cholesterol and PC
where can phospholipids freely flip flop with no ATP?
from the ER to the cis golgi
what could PLD derived phosphatidic acid do?
induce negative curvature on the inner leaflet
What could ATP driven local translocation of lipids do?
help create high local conc of aminophospholipid on the cytosolic leaflet favourable for recruitment of clathrins, and ARF for example.
