Results from a field-theoreticapproach to membrane fusion

Michael Schick

Department of Physics

University of Washington

à mon cher maître

à mon cher maître

à mon cher maître

Maurice Ravel

Synaptic Vesicles

They are small R~15-25 nm

cf bilayer thickness t~4nm

They dock: nothing happens

They are triggered: fuse rapidly

Fusion on demand

Synaptic vesicles in C. Elegans

J.L. Bessereau: www.biologie.ens.fr

Questions

• Why does fusion occur and how?

• What are free energy barriers to fusion?

• What factors promote fusion?

Theoretical Procedure

Consider fusion of vesicles of block copolymer: universality

Theoretical Procedure

Consider fusion of vesicles of block copolymer: universality

Polymers->Random walk->Diffusion Eq.-> Quantum Mech.

Theoretical Procedure

Consider fusion of vesicles of block copolymer: universality

Polymers->Random walk->Diffusion Eq.-> Quantum Mech.

SCFT of interacting polymers-> Hartree Theory

Advantages: microscopic model->architecture

calculation of free energies

Bringing bilayers together removes water->

free energy per area increases-> bilayers are under tension

WHY DOES FUSION OCCUR?

Bringing bilayers together removes water->

free energy per area increases-> bilayers are under tension

Fusion eliminates area -> reduces the free energy

Fusion is one possible response to induced tension

Lysis (rupture) is another

WHY DOES FUSION OCCUR?

HOW DOES FUSION OCCUR?

one commonly accepted mechanism

Kozlov and Markin 1983

Profiles obtained by the theory

WHAT ARE THE FREE ENERGY BARRIERS TO FUSION?

WHAT ARE THE FREE ENERGY BARRIERS TO FUSION?

Consider the fusion of two bilayers

One component only

A lamellar former

Kozlov and Markin 1983

Profiles obtained by the theory

One component, f = 0.4 (DOPC), /0 = 0.2, equilibrium H, stalk

1 bilayer =4.3 Rg

One component, f = 0.4, /0 = 0.2, equilibrium H, stalk

One component, f = 0.4, /0 = 0.2, equilibrium H, stalk

One component, f = 0.4, /0 = 0.2, equilibrium H, stalk

One component, f = 0.4, /0 = 0.2, equilibrium H, pore

One component, f = 0.4, /0 = 0.2, equilibrium H, stalk & pore

One component, f = 0.4, /0 = 0.2, equilibrium H, stalk & pore

In lipids, barrier would be 50 kT !

WHAT FACTORS AFFECT FUSION?

Effect of Tension

Effect of Two Components and Asymmetry in Leaves

SMALL HEADS, LARGE TAILS FAVORED HERE IN PROXIMAL LEAF

Effect of Two Components and Asymmetry in Leaves

Average concentration of hex-former is 0.35

0.35 in cis

0.40 in cis

0.43 in cis

F/kbT

f=0.4 (DOPC) and f=0.294 (DOPE)

Effect of Two Components and Asymmetry in Leaves

Average concentration of hex-former is 0.35

0.35 in cis

0.40 in cis

0.43 in cis

F/kbT

Note that stalk becomes metastable. Its formation is now largest barrier

Effect of curvatureFusion of Bilayer and Vesicle:

1 bilayer =4.3 Rg60:40 mixture J Y Lee & M.S. BJ 2008

What should we expect the effect of curvature to be?

As vesicle radius decreases, effective tension increases, which decreases barrier

Fusion of two identical vesicles

60:40 mixture

H = 2.2 R_g, zero tension, 60:40 mixture

Control Fusion by Controlling the Interbilayer Spacing

H = 2.2, 2.7 R_g, zero tension

H = 2.2, 2.7, 3.2 R_g, zero tension

H = 2.2, 2.7, 3.2, 3.7 R_g, zero tension

H = 2.2, 2.7, 3.2, 3.7, 4.0 R_g, zero tension

Conclusions• Two barriers to fusion

Conclusions• Two barriers to fusion• Barrier to stalk formation linear in distance

->easy to prevent fusion

Conclusions• Two barriers to fusion• Barrier to stalk formation linear in distance

->easy to prevent fusion• Second barrier reduced by composition and

curvature

Conclusions• Two barriers to fusion• Barrier to stalk formation linear in distance

->easy to prevent fusion• Second barrier reduced by composition and

curvature• Consequently, when brought close, stalk

barrier is small, ~13kT, and fusion is quick• Fusion on demand!

Acknowledgements

Marcus MuellerKirill KatsovJae-Youn Lee

NSF Grant DMR 0503752