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First published online 19 September 2006
doi: 10.1242/jcs.03191
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Research Article |
1 Leibniz Institute for Arteriosclerosis Research, University of Münster, Domagkstr. 3, 48149 Münster, Germany
2 Department of Medicine, Division of Cardiology, Pulmonary Diseases and Angiology, Heinrich Heine University, Düsseldorf, Germany
3 National Heart and Lung Institute, Imperial College, London, UK
* Author for correspondence (e-mail: robenek{at}uni-muenster.de)
Accepted 25 July 2006
The prevailing hypothesis of lipid droplet biogenesis proposes that neutral lipids accumulate within the lipid bilayer of the ER membrane from where they are budded off, enclosed by a protein-bearing phospholipid monolayer originating from the cytoplasmic leaflet of the ER membrane. We have used a variety of methods to investigate the nature of the sites of ER–lipid-droplet association in order to gain new insights into the mechanism of lipid droplet formation and growth. The three-dimensional perspectives provided by freeze-fracture electron microscopy demonstrate unequivocally that at sites of close association, the lipid droplet is not situated within the ER membrane; rather, both ER membranes lie external to and follow the contour of the lipid droplet, enclosing it in a manner akin to an egg cup (the ER) holding an egg (the lipid droplet). Freeze-fracture cytochemistry demonstrates that the PAT family protein adipophilin is concentrated in prominent clusters in the cytoplasmic leaflet of the ER membrane closely apposed to the lipid droplet envelope. We identify these structures as sites at which lipids and adipophilin are transferred from ER membranes to lipid droplets. These findings call for a re-evaluation of the prevailing hypothesis of lipid droplet biogenesis.
Key words: Lipid droplet formation and growth, Freeze-fracture immunocytochemistry, PAT proteins
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