Abstract
Storage of cellular triacylglycerols (TAGs) in lipid droplets (LDs) has been linked to the progression of many metabolic diseases in humans, and to the development of biofuels from plants and microorganisms. However, the biogenesis and dynamics of LDs are poorly understood. Compared with other organisms, bacteria seem to be a better model system for studying LD biology, because they are relatively simple and are highly efficient in converting biomass to TAG. We obtained highly purified LDs from Rhodococcus sp. RHA1, a bacterium that can produce TAG from many carbon sources, and then comprehensively characterized the LD proteome. Of the 228 LD-associated proteins identified, two major proteins, ro02104 and PspA, constituted about 15% of the total LD protein. The structure predicted for ro02104 resembles that of apolipoproteins, the structural proteins of plasma lipoproteins in mammals. Deletion of ro02104 resulted in the formation of supersized LDs, indicating that ro02104 plays a critical role in cellular LD dynamics. The putative α helix of the ro02104 LD-targeting domain (amino acids 83-146) is also similar to that of apolipoproteins. We report the identification of 228 proteins in the proteome of prokaryotic LDs, identify a putative structural protein of this organelle, and suggest that apolipoproteins may have an evolutionarily conserved role in the storage and trafficking of neutral lipids.
Highlights
Storage of cellular triacylglycerols (TAGs) in lipid droplets (LDs) has been linked to the progression of many metabolic diseases in humans, and to the development of biofuels from plants and microorganisms
Lipid-TOX staining showed that RHA1 cells cultivated in nutrient broth (NB) formed small LDs, whereas in cells cultivated in mineral salt medium (MSM), LDs were much larger (Fig. 1A)
TLC analysis verified this finding by showing that the amount of TAG in cells cultivated in MSM was significantly higher than that in cells cultivated in NB (Fig. 1B)
Summary
Storage of cellular triacylglycerols (TAGs) in lipid droplets (LDs) has been linked to the progression of many metabolic diseases in humans, and to the development of biofuels from plants and microorganisms. We report the identification of 228 proteins in the proteome of prokaryotic LDs, identify a putative structural protein of this organelle, and suggest that apolipoproteins may have an evolutionarily conserved role in the storage and trafficking of neutral lipids.—Ding, Y., L. LDs have been found in almost all eukaryotic organisms from yeast to mammals [4] They interact with other cellular organelles [5,6,7,8], and their dynamics is closely related to the progression of metabolic diseases, such as obesity, fatty liver, type 2 diabetes mellitus, and atherosclerosis [9]. LDs are highly dynamic organelles involved in many cellular activities, especially lipid metabolism, the molecular mechanisms that govern LD formation remain largely unknown. Many studies have attempted to unravel how LDs form and grow, but this hypothesis still lacks direct evidence and the molecular mechanism
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