Pierre SensInstitut Curie - Paris. France
& Institut Charles Sadron - Strasbourg. France
“Gating-by-Tilt”Mechano-sensitivity of Biomembrane transport
APS - March 2004 - MontrealAPS - March 2004 - Montreal
Matthew TurnerWarwick University. England
Membrane Transport Proteins
20% of identified genes in E-Coliare associated with membrane transport processes
Very important for the cell
Outline of the Talk New gating mechanism for mechano-sensitive channels Membrane tension influences (may inhibit ?) transport
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From Alberts etal. Molecular Biology of the CellFrom Alberts etal. Molecular Biology of the Cell
Lipid bilayer(5nm)
Lipid molecules
Proteins
Types of transportTypes of transport
Mechano-sensitive channelsMechano-sensitive channels
Function: osmotic regulation
Structure Structure Two states
Low tension - closed High tension - open
OpenClosed
Traditional pictureTraditional picture
Gating-by-dilation
open
closed
Gating-by-tilt
““New” pictureNew” picture
Influence of Membrane Tension
Active TransportActive Transport
Function: creates/maintains concentration gradientsRequires Energy consumption
(ATP hydrolysis - use of ionic gradient)
ATP ADP + Pi
Very asymmetric ionic concentrationsVery asymmetric ionic concentrations
K+K+ Na+ [mM]Na+ [mM]
insideinside
outsideoutside
140140 1010
55 145145
Examples that involve a structural change
Protein Tilt -> Membrane Energy -> Protein Conformation Energy
Membrane tension influences transport rateMembrane tension influences transport rate
Receiving state Receiving state For some transporters, it involvesFor some transporters, it involves
A change of protein tiltA change of protein tilt
Releasing stateReleasing state
Origin of Membrane Energy
Energy (Helfrich - 70s’)
Deformation
Energy
Bending rigidity vs. Membrane tension
Minimization
:Increasing
Membrane displacement(small)
a
Mechano-sensitive channelsMechano-sensitive channels Example: MsCl
Chang Science, 1998 (side) K. Schulten (top)
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.K+ Channel (M. Sansom)
Bottom view
2-state model(+ possible intermediate, metastable states)
states
EN
ER
GY
closed open
Energy barriers Energy difference(1)
(2)
Energy difference decreases with tension
Membrane
Low tension
High tension
statesEN
ER
GY
Total Energy
Equilibrium population
Transition rates
Channel
Traditional pictureTraditional picture
Gating-by-dilation
open
closed
Gating-by-tilt
““New” pictureNew” picture
High (membrane) Gating energy = High Channel Sensitivity
Membrane contribution to the energy
Adds to (dominates) the sensitivity
Requires large dilation for high sensitivity
Channel sizeChannel size
Channel opening byasymmetrical lipid addition
New Feature of Gating-by-Tilt !
Effect of Membrane asymmetry Reduction of tension (bad for gating-by-dilation)Increase of spontaneous curvature (good for gating-by-tilt)
Observed in vitro (by EPR spectroscopy)Perozo: Nature Struct. Biol. 9, 696 (2002)
MEMBRANE TRANSPORTMEMBRANE TRANSPORT
Locher, Bass, & Rees, Science 301, 603 (2003)
Active transport (uses ion gradient)
Involves a change of protein tilt
2-State model
(1)(2)
(0)
Active transition Active transition
Passive (thermal) transition Passive (thermal) transition
Membrane energy Membrane energy
May destabilize the metastable stateand inhibit membrane transport
- pÄÄÄÄÄÄÄ12
- pÄÄÄÄ6
- pÄÄÄÄ4
pÄÄÄÄÄÄÄ12
pÄÄÄÄ6
pÄÄÄÄ4
2
4
6
8
10
12
14
-π/4 -π/6 -π/12 0 π/12 π/6 π/4
- pÄÄÄÄÄÄÄ12
- pÄÄÄÄ6
- pÄÄÄÄ4
pÄÄÄÄÄÄÄ12
pÄÄÄÄ6
pÄÄÄÄ4
2
4
6
8
10
12
14
(1)
(0)(2)
-π/4 -π/6 -π/12 0 π/12 π/6 π/4
Summary
Physical consequences of tilted protein conformationPhysical consequences of tilted protein conformation
Membrane tilt may have a dominant contributionMembrane tilt may have a dominant contributionto the gating energy of mechano-sensitive channelsto the gating energy of mechano-sensitive channels
Gating-by-Tilt explains the channel sensitivity to membrane asymmetryGating-by-Tilt explains the channel sensitivity to membrane asymmetry
Membrane elasticity plays an important role in Active transportMembrane elasticity plays an important role in Active transport
Active transport may - too - be mechano-sensitiveActive transport may - too - be mechano-sensitiveAnd may be inhibited under high membrane tensionAnd may be inhibited under high membrane tension
M.S. Turner & P. Sens. Gating-by-tilt of mechanosensitive membrane channels. cond-mat/0311574
[email protected] http://perso.curie.fr/Pierre.Sens/
hydrophobic
hydrophilic
Cell membrane are composed of amphiphilic moleculesCell membrane are composed of amphiphilic moleculeswhich self-assemble into fluid bilayerswhich self-assemble into fluid bilayers
Mechano-sensitive channelsMechano-sensitive channels
Example: MsCl
Sukharev Nature 409, 771 (2001)
Patch clamp measurements
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Frans Maathuis, York
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•Open area from conductivity
•Energy G estimated fromopen area x tension
Corresponding tilt angles
Channel 100 (degrees)
[100% of gating Gdilate]
10 (degrees)
[10% of gating Gdilate]
MscL 39 12
MscS 24 8
MscA1 21 7
MscA2 31 10
MscMJ 16 5
MscMJR 35 11