Trends in Biochemical Sciences
ReviewMechanisms of lipid-body formation
Section snippets
Seeds and fruits
Intracellular storage-lipid bodies in plants are particularly abundant in oil-rich fruit and seed tissues, which can contain as much as 50–75% (w/w) lipid. As in animals, such cytosolic lipid bodies are believed to arise from specific microdomains of the endoplasmic reticulum (ER) membrane that contain the full complement of TAG-biosynthesis enzymes1 (Fig. 1). Data from several labeling studies suggest that these enzymes channel intermediates towards lipid-body formation and, hence, segregate
Hepatocytes and enterocytes
In mammalian liver and intestine, TAG and cholesteryl esters can be stored in the cytosol as lipid droplets or, instead, secreted into the circulation as very low-density lipoproteins (VLDLs) from the liver or as chylomicrons from the intestine (Fig. 2). These plasma lipoproteins deliver water-insoluble lipids from the liver and intestine to other tissues in the body for use as an energy source. Plasma lipoproteins all have a common spherical structure in which a surface monolayer that consists
Yeast
Most yeasts produce small numbers of cytosolic lipid bodies, but the oleagenous yeasts can accumulate up to 25% (w/w) storage lipid in response to a high carbon : nitrogen ratio34. Lipid bodies in Saccharomyces cerevisiae contain almost equal amounts of TAGs and sterol esters35. As in other eukaryotes, yeast lipid bodies probably arise from the ER. However, several enzymes involved in yeast lipid metabolism, including glycerophosphate acyltransferases, sterol Δ24-methyltransferase and squalene
Conclusions and future directions
The major, and perhaps unique, site of lipid-body assembly (except in prokaryotes and plastids) is in specialized regions of the ER, where biosynthetic enzymes might be grouped functionally (e.g. as metabolons). Most lipid bodies accumulate with specific surface-bound proteins that are also found on the ER. The tiny nascent lipid bodies normally undergo a series of highly regulated fusions and can alter their surface proteins before reaching their mature size and composition. Targeting of lipid
Acknowledgements
We thank C. Londos, J. E. Thompson and T. W. Keenan for their advice, and C. Londos and M. J. Hills for the provision of unpublished data. We thank D. E. Vance, M. J. Hills and Z. Poghosyan for critically reading the manuscript, T. Bown for the graphics, and the Heart and Stroke Foundation of Alberta and BBSRC for funding (to J. E. V. and D. J. M., respectively).
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