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Biophys J, June 1999, p. 2951-2965, Vol. 76, No. 6

Lipid Flow through Fusion Pores Connecting Membranes of Different Tensions

Yuri A. Chizmadzhev,* Dimetry A. Kumenko,* Peter I. Kuzmin,* Leonid V. Chernomordik,# Joshua Zimmerberg,# and Fredric S. Cohen§

 *Frumkin Institute of Electrochemistry, Moscow, Russia;  #Laboratory of Cellular and Molecular Biophysics, National Institutes of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892 USA, and  §Department of Molecular Biophysics and Physiology, Rush Medical College, Chicago, Illinois 60612 USA

When two membranes fuse, their components mix; this is usually described as a purely diffusional process. However, if the membranes are under different tensions, the material will spread predominantly by convection. We use standard fluid mechanics to rigorously calculate the steady-state convective flux of lipids. A fusion pore is modeled as a toroid shape, connecting two planar membranes. Each of the membrane monolayers is considered separately as incompressible viscous media with the same shear viscosity, eta s. The two monolayers interact by sliding past each other, described by an intermonolayer viscosity, eta r. Combining a continuity equation with an equation that balances the work provided by the tension difference, Delta sigma , against the energy dissipated by flow in the viscous membrane, yields expressions for lipid velocity, upsilon , and area of lipid flux, Phi . These expressions for upsilon  and Phi  depend on Delta sigma , eta s, eta r, and geometrical aspects of a toroidal pore, but the general features of the theory hold for any fusion pore that has a roughly hourglass shape. These expressions are readily applicable to data from any experiments that monitor movement of lipid dye between fused membranes under different tensions. Lipid velocity increases nonlinearly from a small value for small pore radii, rp, to a saturating value at large rp. As a result of velocity saturation, the flux increases linearly with pore radius for large pores. The calculated lipid flux is in agreement with available experimental data for both large and transient fusion pores.

Biophys J, June 1999, p. 2951-2965, Vol. 76, No. 6
© 1999 by the Biophysical Society   0006-3495/99/06/2951/15  $2.00


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