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HIV Life Cycle and the Properties of Membranes
1. Difference between viruses and cellsa. HIV anatomy and genome
2. Overview of HIV life cycle and targets for therapeuticsa. Host cell recognitionb. Entry into host cell
3. Membrane structure and propertiesa. Phospholipidsb. Lipid properties and behavior in aqueous solutionc. Membrane fluidityd. How we know: FRAP and green fluorescent proteine. Influence of fatty acid structure on fluidityf. Influence of cholesterol on membrane properties
Alberts 221-222, 365-388;McMurry 744-767, 808-810
Lecture Readings
The Anatomy of HIV
HIV viralparticle
capsid
viral RNA moleculecoated with
structural protein
glycoproteins
membrane
3
Cells Can Replicate AutonomouslyBut Viruses Cannot
WHY?
HIV Genome and Proteins
HIV genome
HIV viralparticle
capsid
viral RNA moleculecoated with
structural protein
4
HIV Life Cycle
1 2 3 4
recognition of host cell
entry intocell
viralamplification/
replication
viral assemblyand exit from cellHIV
host cellnucleus
1 2 3 4
recognition of host cell
entry intocell
viralamplification/
replication
viral assemblyand exit from cellHIV
host cell
HIV Targets for Therapeutics
reverse transcriptaseinhibitors
fusioninhibitors
protease inhibitors
5
Recognition of Appropriate Host Cell
macrophageT-cell
HIV
CD4
chemokine receptor
chemokine
membrane
nucleus
CHEMOKINERECEPTOR
BINDING
Recognition of Appropriate Host Cell
HIV
membrane
gp41
gp120CD4
FREE VIRUS
host cell membranechemokine
receptor
CD4ATTACHMENT
6
Entry into Host Cell
membrane
membrane
glycoproteinsgp120gp41
host cell
diameter >10 µm
diameter~100 nm
Cell Membranes
cytoplasm
outside of cell
plasmamembrane
electron micrograph
~10 nm long
(covalent bond ~ 0.1 nm)
7
Phospholipids: Phosphatidylcholine
saturated unsaturated
O
O
OPO
O
N
O
O
cis double bond
O
Hydrophobic Effect
8
Packing of Lipids in Aqueous Solutions
conical
cylindrical
Spontaneous Closure to Form a Sealed Compartment
10
How Do We Know?: An Experiment
fluorescence recovery afterphotobleaching (FRAP)
fluorescentmolecule
protein inmembrane
cell
Green Fluorescent Protein (GFP) Fusions
fluorescentproteins CFP
GFPYFP
jellyfish
cell
nucleus
11
FRAP With Membrane Protein-GFP Fusion
• Tm (melting temperature) is a phase transition, a changefrom a more rigid solid-like state to a fluid-like state
Measures of Membrane Fluidity: Melting Temperature
12
unsaturated fatty acid chains
Membrane Composition Influences MembraneFluidity: Fatty Acid Structure
saturated fatty acid chains
lower Tm higher Tm
cis double
bond
oleic acid
OH
O O
OH
stearic acid
17 carbons
olive oil candle wax
Membrane Composition Influences MembraneFluidity: Cholesterol Content
HO
H
H
H
H
H
13
Cholesterol Influences Membrane Fluidity
Summary of Main Points
• HIV needs host cells to replicate because its genome does not encodeall of the proteins required for living systems
• HIV recognizes host cells by interacting with specific protein receptorsfound on the surface of those cell types
• Cell membranes are bilayers composed of amphipathic phospholipidscontaining charged head groups and hydrophobic tails
• The hydrophobic effect drives the packing of lipids into structures whichminimize exposed hydrophobic groups
• Membranes are fluid because phospholipids and proteins can move inthe plane of the bilayer; fatty acid structure and cholesterol contentinfluence fluidity