Most animals store lipid intracellularly in protein-coated droplets. The protein coat usually contains at least one member of the PAT (perilipin, adipose differentiation-related protein, and TIP47) family. Evidence suggests that PAT proteins control access to the lipid they enclose. The protein S3-12, which has sequence similarity to the PAT proteins, was found in a screen for adipocyte-specific proteins. The adipocyte expression of S3-12 and its similarity to the PAT proteins suggest that S3-12 is involved in adipocyte lipid storage. To test this hypothesis, we supplemented 3T3-L1 adipocytes with fatty acids and assessed the distribution of S3-12 by immunofluorescence microscopy. Prior to fatty acid incubation, S3-12 was distributed diffusely throughout the cytoplasm on punctate structures of heterogeneous size. After 10 min of lipid loading, S3-12 localized to 500-nm structures concentrated at the adipocyte periphery. After longer incubations, S3-12 coated the surface of lipid droplets up to several micrometers in diameter. Initially, these droplets were distinct from those droplets surrounded by perilipin; but by 240 min, most perilipin-coated droplets had some S3-12 on the surface as well. We additionally report that the formation of S3-12-coated droplets 1) required glucose and fatty acids that can be incorporated into triacylglycerol, 2) was blocked by an inhibitor of triacylglycerol synthesis, and 3) was insulin-dependent. This study reports for the first time the early morphological events in the genesis and maturation of adipocyte lipid droplets.
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