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Biophys J, April 1999, p. 2056-2071, Vol. 76, No. 4
i
,*
a
Svetina,*§ andtjan
ek*§
*Institute of Biophysics,
A sufficiently large force acting on a single point of
the fluid membrane of a flaccid phospholipid vesicle is known to cause the formation of a narrow bilayer tube (tether). We analyze this phenomenon by means of general mathematical methods allowing us to
determine the shapes of strongly deformed vesicles including their
stability. Starting from a free vesicle with an axisymmetric, prolate
equilibrium shape, we consider an axial load that pulls (or pushes) the
poles of the vesicle apart. Arranging the resulting shapes of strained
vesicles in dependence of the axial deformation and of the area
difference of monolayers, phase diagrams of stable shapes are presented
comprising prolate shapes with or without equatorial mirror symmetry.
For realistic values of membrane parameters, we study the
force-extension relation of strained vesicles, and we demonstrate in
detail how the initially elongated shape of an axially stretched
vesicle transforms into a shape involving a membrane tether. This
tethering transition may be continuous or discontinuous. If the free
vesicle is mirror symmetric, the mirror symmetry is broken as the
tether forms. The stability analysis of tethered shapes reveals that,
for the considered vesicles, the stable shape is always asymmetric
(polar), i.e., it involves only a single tether on one side of the main
vesicle body. Although a bilayer tube formed from a closed vesicle is
not an ideal cylinder, we show that, for most practical purposes, it is
safe to assume a cylindrical geometry of tethers. This analysis is
supplemented by the documentation of a prototype experiment supporting
our theoretical predictions. It shows that the currently accepted model
for the description of lipid-bilayer elasticity (generalized bilayer
couple model) properly accounts for the tethering phenomenon.
Biophys J, April 1999, p. 2056-2071, Vol. 76, No. 4eva 2,
© 1999 by the Biophysical Society 0006-3495/99/04/2056/16 $2.00
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