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

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- 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

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pÄÄÄÄ6

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2

4

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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

pierre.sens@curie.fr 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