Pierre Sens Institut Curie - Paris. France & Institut Charles Sadron - Strasbourg. France

1. “Gating-by-Tilt” Mechano-sensitivity of Biomembrane transport Pierre Sens Institut Curie - Paris. France & Institut Charles Sadron - Strasbourg. France Matthew Turner Warwick University. England APS - March 2004 - Montreal

2. Lipid bilayer (5nm) Lipid molecules Proteins From Alberts etal. Molecular Biology of the Cell Membrane Transport Proteins 20% of identified genes in E-Coli are associated with membrane transport processes Very important for the cell Types of transport Outline of the Talk New gating mechanism for mechano-sensitive channels Membrane tension influences (may inhibit ?) transport

3. Low tension - closed High tension - open closed open Gating-by-dilation Gating-by-tilt Mechano-sensitive channels Function: osmotic regulation Structure Two states Closed Open Traditional picture “New” picture Influence of Membrane Tension

4. ADP + Pi ATP K+ Na+ [mM] inside 140 10 5 145 outside Releasing state Receiving state Active Transport Very asymmetric ionic concentrations Function: creates/maintains concentration gradients Requires Energy consumption (ATP hydrolysis - use of ionic gradient) Examples that involve a structural change For some transporters, it involves A change of protein tilt Membrane tension influences transport rate Protein Tilt -> Membrane Energy -> Protein Conformation Energy

5. a Deformation : Increasing Energy Minimization Origin of Membrane Energy Membrane displacement (small) Energy (Helfrich - 70s’) Bending rigidity vs. Membrane tension

6. Mechano-sensitive channels Example: MsCl Chang Science, 1998 (side) K. Schulten (top) K+ Channel (M. Sansom)

7. Equilibrium population Transition rates High tension ENERGY states Low tension 2-state model (+ possible intermediate, metastable states) Bottom view closed open Channel ENERGY (2) (1) Energy barriers Energy difference states Membrane Energy difference decreases with tension Total Energy

8. closed open Gating-by-dilation High (membrane) Gating energy = High Channel Sensitivity Membrane contribution to the energy Traditional picture “New” picture Gating-by-tilt Requires large dilation for high sensitivity Adds to (dominates) the sensitivity Channel size

9. 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) Channel opening by asymmetrical lipid addition Observed in vitro (by EPR spectroscopy) Perozo: Nature Struct. Biol. 9, 696 (2002)

10. MEMBRANE TRANSPORT Locher, Bass, & Rees, Science301, 603 (2003) Active transport (uses ion gradient)

11. (0) (2) (1) -π/4 -π/6 -π/12 0 π/12 π/6 π/4 Active transition 2-State model (0) (2) Passive (thermal) transition (1) -π/4 -π/6 -π/12 0 π/12 π/6 π/4 Involves a change of protein tilt Membrane energy May destabilize the metastable state and inhibit membrane transport

12. Summary Physical consequences of tilted protein conformation Membrane tilt may have a dominant contribution to the gating energy of mechano-sensitive channels Gating-by-Tilt explains the channel sensitivity to membrane asymmetry Membrane elasticity plays an important role in Active transport Active transport may - too - be mechano-sensitive And may be inhibited under high membrane tension M.S. Turner & P. Sens. Gating-by-tilt of mechanosensitive membrane channels. cond-mat/0311574

13. pierre.sens@curie.fr http://perso.curie.fr/Pierre.Sens/

15. hydrophilic hydrophobic Cell membrane are composed of amphiphilic molecules which self-assemble into fluid bilayers

16. Mechano-sensitive channels Example: MsCl Sukharev Nature409, 771 (2001)

17. Patch clamp measurements • Open area from conductivity • Energy DG estimated from • open area x tension Frans Maathuis, York

18. q Corresponding tilt angles