[The GTP-Bound form of Rab3D Promotes Lipid Droplet Growth in Adipocyte]

Rab GTPases are major regulators of membrane trafficking. Proteome profiling reveals dozens of Rab proteins associated with lipid droplets (LDs), but few of them have been functionally validated. Cell death activator CIDE-3 protein mediates LD fusion and growth. It is highly enriched at LD-LD contact sites. Here, we investigated the role of Rab3D in lipid storage in adipocyte. We verified that the adipose levels of Rab3D transcript were higher than that of other Rab3 family members; the differences were most pronounced in white adipose tissue. Moreover, we showed that Rab3D promotes LD growth in 3T3-L1 preadipocytes in a dose dependent manner, independently of the process of CIDE-3-mediated LD fusion. Finally, we confirmed that the GTP-bound form of Rab3D is its LD promoting form; it translocates from other vesicles to LDs during adipocyte differentiation. By contrast, the Rab3D-GDP form is retained in the cytoplasm and has no effect on LD sizes. Presented results provide evidence for the role of Rab3D in controlling formation of large LDs in adipocytes. We conclude that Rab3D is a novel LD regulator with characteristics differing from these of previously identified LD associated Rab proteins, such as Rab18 and Rab8a.