CGI-58 is the defective gene in the human neutral lipid storage disease called Chanarin-Dorfman syndrome. This disorder causes intracellular lipid droplets to accumulate in nonadipose tissues, such as skin and blood cells. Here, disruption of the homologous CGI-58 gene in Arabidopsis thaliana resulted in the accumulation of neutral lipid droplets in mature leaves. Mass spectroscopy of isolated lipid droplets from cgi-58 loss-of-function mutants showed they contain triacylglycerols with common leaf-specific fatty acids. Leaves of mature cgi-58 plants exhibited a marked increase in absolute triacylglycerol levels, more than 10-fold higher than in wild-type plants. Lipid levels in the oil-storing seeds of cgi-58 loss-of-function plants were unchanged, and unlike mutations in β-oxidation, the cgi-58 seeds germinated and grew normally, requiring no rescue with sucrose. We conclude that the participation of CGI-58 in neutral lipid homeostasis of nonfat-storing tissues is similar, although not identical, between plant and animal species. This unique insight may have implications for designing a new generation of technologies that enhance the neutral lipid content and composition of crop plants.compartmentation | plant lipid metabolism P lants synthesize and store neutral lipids such as triacylglycerols (TAGs) primarily in cytosolic lipid droplets of maturing seeds (1, 2). In domesticated oilseeds, these stored TAGs represent a major source of calories for human and animal nutrition, an excellent feedstock for diesel fuels, and a reservoir for the deposition of industrially important fatty acids used as chemical feedstocks (3-6). Although not commonly appreciated, TAGs also are synthesized in nonseed tissues (7,8), but their abundance in these tissues is low, in part because of the metabolism of the cell and perhaps as a consequence of the continuous recycling of fatty acids for energy and membrane synthesis. Indeed, vegetative cells can incorporate radiolabeled precursors into TAG (7, 8), they express diacylglycerol acyltransferases [the only enzyme in the "Kennedy pathway" unique to TAG production (9)], and they can accumulate TAGs in β-oxidation mutants (2) and in some floral (10) and fruit (7) tissues. Thus, although plant vegetative cells appear to have the metabolic machinery to synthesize and accumulate neutral lipids, there are likely underlying regulatory mechanisms in place to minimize this process, none of which are understood.Chanarin-Dorfman syndrome is a neutral-lipid storage disease (11) caused by a defect in the protein CGI-58 (comparative gene identification-58, also called ABHD5 for α/β hydrolase-5). CGI-58 is a soluble enzyme that associates with cytosolic lipid droplets under certain metabolic conditions and appears to play a role in hydrolysis of stored lipids (11)(12)(13)(14). Several different mutations in this protein, including amino acid substitutions, premature stop codons, and defects in mRNA splicing, have been identified in various Chanarin-Dorfman patients, all of which result in a hyperac...

An expanding appreciation for the varied functions of neutral lipids in cellular organisms relies on a more detailed understanding of the mechanisms of lipid production and packaging into cytosolic lipid droplets (LDs). Conventional lipid profiling procedures involve the analysis of tissue extracts and consequently lack cellular or subcellular resolution. Here, we report an approach that combines the visualization of individual LDs, microphase extraction of lipid components from droplets, and the direct identification of lipid composition by nanospray mass spectrometry, even to the level of a single LD. The triacylglycerol (TAG) composition of LDs from several plant sources (mature cotton (Gossypium hirsutum) embryos, roots of cotton seedlings, and Arabidopsis thaliana seeds and leaves) were examined by direct organelle mass spectrometry and revealed the heterogeneity of LDs derived from different plant tissue sources. The analysis of individual LDs makes possible organellar resolution of molecular compositions and will facilitate new studies of LD biogenesis and functions, especially in combination with analysis of morphological and metabolic mutants. Furthermore, direct organelle mass spectrometry could be applied to the molecular analysis of other subcellular compartments and macromolecules. LDs4 are organelles that are specialized for the storage of neutral lipids and as such provide energy-rich reserves in all cellular organisms (1). Understanding LD ontogeny is of major importance to human physiology; on the one hand, seed oils, packaged in LDs, make up a growing proportion of daily caloric intake in most diets around the world, and on the other hand, the regulation of lipid storage and mobilization underlies significant human health issues: obesity, diabetes and cardiovascular disease.Although storage is considered the principal role of neutral lipids, LDs in nonfat storing tissues recently have become more appreciated for their dynamic nature and functional roles independent of storage. These roles include acyl reserves for phospholipid recycling (2), lipid signaling (3), membrane trafficking (2, 4), inflammation and cancer (5), and hostpathogen interactions (6, 7). These various functions attributed to LDs vary with cell type and likely are manifested by differences in droplet composition. The basic structural model of LDs in plant seeds provides a thermodynamically stable organization that is thought to be conserved throughout eukaryotes, although the nature of the lipids and proteins associated with droplets varies with cell/tissue type. The structure describes a neutral lipid core (triacylglycerols in plant seeds and/or steryl esters in other organisms or cell types) surrounded by a phospholipid monolayer with specific proteins associated with the LD surface (8). Although the endoplasmic reticulum is considered by most to be the major cellular location for LD biogenesis, droplets associate frequently with other subcellular compartments, presumably to carry out unique functions (9).The recent emphasis...