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Biophys J, April 1999, p. 1723-1724, Vol. 76, No. 4
Max-Planck-Institut für Kolloid-und Grenzflächenforschung, Teltow-Seehof, Germany
Molecular transmembrane asymmetry plays a major
role in such diverse cellular aspects as morphology, adhesion, and
signaling. In general, surface receptor proteins have a different
molecular architecture on their cytoplasmic and extracellular side
corresponding to the respective function. Likewise, lipid asymmetry
between the two monolayers of eukaryotic plasma membranes and membranes from organelles, bacteria, and viruses is well established (Devaux, 1991 As a morphological response to a transbilayer asymmetry in lipid
species and/or different aqueous environments, a cellular membrane
generally adopts a curved equilibrium configuration. During the last
decade, an elegant description of the morphology of membranes and in
particular of free vesicle shapes has emerged from a general theory of
bending elasticity (Seifert, 1997 In an excellent paper published in this volume, Heinrich et al. apply
the area-difference elasticity model or the generalized bilayer-couple
model, as the authors call it, to the analysis of vesicle deformations
by an axial load. The authors describe in amazing detail the
morphological changes of a vesicle as it is strained axially with
micropipettes. It is well known that a sufficiently large point force
acting on a fluid membrane leads to the formation of a long tubular
appendix. The authors' main finding is that the precise morphological
scenario with which such a tether is pulled out of a vesicle in fact
depends quite delicately on membrane asymmetry. It is therefore
possible to measure membrane asymmetry with this technique.
Tether-pulling experiments are complementary to morphological
measurements on fluctuating vesicles (Döbereiner et al., 1999 Heinrich et al. provide a profound theoretical basis for the analysis
of such tether-pulling experiments, which in the past have relied on a
number of ad hoc assumptions and approximations. The authors have
developed a numerical technique that allows computation of complex
membrane morphologies characterized by strongly varying curvature
(e.g., a sphere with a thin tether) with very high precision. The
calculation of such shapes has not been possible before now. Tether-pulling experiments yield important mechanical characteristics of membranes, like bending moduli, in addition to the effective differential area encoding membrane asymmetry. Further, it is possible
using this technique to acquire information on dynamic features,
e.g., driven lipid flip-flop and intermonolayer friction. This
versatile usage of tether-pulling is nicely summarized in the rather
complete historical account the authors give in their Introduction. The
elastic interactions of the plasma membrane with cytoskeletal proteins
may be studied more quantitatively than has been possible so far using
the theoretical analysis of Heinrich et al. For instance, it is now
feasible to perform a comprehensive analysis of the buckling
instability of microtubules pushing against membranes (Kuchnir Fygenson
et al., 1997 In particular, the determination of membrane asymmetry by
tether-pulling and observations of fluctuating vesicles will deepen our
knowledge about cellular processes. One may speculate about a
cross-coupling of the morphology of a membrane with its signaling function via molecular asymmetry. One example of such a relationship may be the inositol phospholipid pathway or another biochemical cascade
involving lipid-derived second messengers (Ghosh et al., 1997
ARTICLE
; Dolis et al., 1997).
). Within the area-difference
elasticity model, asymmetry is quantified via both the area difference
between the two monolayers of a vesicle and the spontaneous curvature
of the membrane. These quantities may be combined into an effective
differential area (Mui et al., 1995; Döbereiner et al., 1999).
).
Both techniques allow quantitative determination of the tendency of a
membrane to bend. Due to the relatively high forces involved in
tether-pulling, the regime of strong membrane asymmetry, which is not
easily accessible by monitoring freely fluctuating vesicles, may be studied.
). In summary, the results of Heinrich et al. open
far-reaching possibilities for further quantitative work on
tether-pulling. This technique will continue to play a major role in
the quest to understand the behavior of biomembranes.
). In the
former pathway, the phospholipase C cleaves off the head group of
inositol phospholipids to produce inositol triphosphate and
diacylglycerol. The latter lipid has quite a small head group and,
thus, membrane asymmetry clearly changes during this process. The
variation in membrane asymmetry could, in turn, trigger morphological transformations of the bilayer. It may also be used to couple distinct
biochemical reactions. An asymmetry-induced change in the mechanical
stress profile of the bilayer membrane could alter the activity of
enzymes or ion channels that are some distance apart (Keller et al.,
1993). It remains to be seen whether these effects are in fact
widely employed by nature.
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FOOTNOTES |
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Received for publication 20 January 1999 and in final form 21 January 1999.
Address reprint requests to Dr. Hans-Günther Döbereiner, Max-Planck-Institut für Kolloid-und Grenzflächenforschung, Kantstrase 55, 14513 Teltow-Seehof, Germany. Tel.: 49-3328-46-592; Fax: 49-3328-46-212; E-mail: hgd{at}regulus.mpikg-teltow.mpg.de.
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Biophys J, April 1999, p. 1723-1724, Vol. 76, No. 4
© 1999 by the Biophysical Society 0006-3495/99/04/1723/02 $2.00
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