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J. Biol. Chem., Vol. 280, Issue 27, 25811-25819, July 8, 2005
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From the
Institut de Biologie Physico-Chimique, 13
rue Pierre et Marie Curie 75005 Paris, France and the
Instituto de Ciencia de los Materiales
Nicolás Cabrera, C-XVI Universidad Autónoma de Madrid, E-28049
Cantoblanco Madrid, Spain
The transbilayer diffusion of unlabeled ceramides with different acyl
chains (C6-Cer, C10-Cer, and C16-Cer) was
investigated in giant unilamellar vesicles (GUVs) and in human erythrocytes.
Incorporation of a very small percentage of ceramides (
0.1% of total
lipids) to the external leaflet of egg phosphatidylcholine GUVs suffices to
trigger a shape change from prolate to pear shape vesicle. By observing the
reversibility of this shape change the transmembrane diffusion of lipids was
inferred. We found a half-time for unlabeled ceramide flip-flop below 1 min at
37 °C. The rapid diffusion of ceramides in a phosphatidylcholine bilayer
was confirmed by flip-flop experiments with a spin-labeled ceramide analogue
incorporated into large unilamellar vesicles. Shape change experiments were
also carried out with human erythrocytes to determine the trans-membrane
diffusion of unlabeled ceramides into a biological membrane. Addition of
exogenous ceramides to the external leaflet of human erythrocytes did not
trigger echinocyte formation immediately as one would anticipate from an
asymmetrical accumulation of new amphiphiles in the outer leaflet but only
after
15 min of incubation at 20 °C in the presence of an excess of
ceramide. We interpret these data as being indicative of a rapid ceramide
equilibration between both erythrocyte leaflets as indicated also by electron
spin resonance spectroscopy with a spin-labeled ceramide. The late appearance
of echinocytes could reveal a progressive trapping of a fraction of the
ceramide molecules in the outer erythrocytes leaflet. Thus, we cannot exclude
the trapping of ceramides into plasma membrane domains.
Received for publication, October 25, 2004 , and in revised form, May 5, 2005.
* This work was supported in part by grants from the CNRS (Unité Mixte de Recherche 7099), the Université Paris 7, and the Universidad Autónoma de Madrid. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ Recipient of a fellowship from the Ministère de l'Education, de l'Enseignement Supérieur et de la Recherche Scientifique (Paris) and from the Fundación General de la Universidad Autonóma de Madrid.
|| To whom correspondence should be addressed: Tel.: 33-1-58-41-5105; Fax: 33-1-58-41-5024; E-mail: Philippe.Devaux{at}ibpc.fr.
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