5 research outputs found
Targeting sequences of UBXD8 and AAM-B reveal that the ER has a direct role in the emergence and regression of lipid droplets
Lipid droplets are sites of neutral lipid storage thought to be actively
involved in lipid homeostasis. A popular model proposes that droplets are
formed in the endoplasmic reticulum (ER) by a process that begins with the
deposition of neutral lipids between the membrane bilayer. As the droplet
grows, it becomes surrounded by a monolayer of phospholipid derived from the
outer half of the ER membrane, which contains integral membrane proteins
anchored by hydrophobic regions. This model predicts that for an integral
droplet protein inserted into the outer half of the ER membrane to reach the
forming droplet, it must migrate in the plane of the membrane to sites of
lipid accumulation. Here, we report the results of experiments that directly
test this hypothesis. Using two integral droplet proteins that contain unique
hydrophobic targeting sequences (AAM-B and UBXD8), we present evidence that
both proteins migrate from their site of insertion in the ER to droplets that
are forming in response to fatty acid supplementation. Migration to droplets
occurs even when further protein synthesis is inhibited or dominant-negative
Sar1 blocks transport to the Golgi complex. Surprisingly, when droplets are
induced to disappear from the cell, both proteins return to the ER as the
level of neutral lipid declines. These data suggest that integral droplet
proteins form from and regress to the ER as part of a cyclic process that does
not involve traffic through the secretory pathway
Evidence that Mono-ADP-Ribosylation of CtBP1/BARS Regulates Lipid Storage
Mono-ADP-ribosylation is emerging as an important posttranslational modification that modulates a variety of cell signaling pathways. Here, we present evidence that mono-ADP-ribosylation of the transcriptional corepressor C terminal binding protein, brefeldin A (BFA)-induced ADP-ribosylated substrate (CtBP1/BARS) regulates neutral lipid storage in droplets that are surrounded by a monolayer of phospholipid and associated proteins. CtBP1/BARS is an NAD-binding protein that becomes ribosylated when cells are exposed to BFA. Both endogenous lipid droplets and droplets enlarged by oleate treatment are lost after 12-h exposure to BFA. Lipid loss requires new protein synthesis, and it is blocked by multiple ribosylation inhibitors, but it is not stimulated by disruption of the Golgi apparatus or the endoplasmic reticulum unfolded protein response. Small interfering RNA knockdown of CtBP1/BARS mimics the effect of BFA, and mouse embryonic fibroblasts derived from embryos that are deficient in CtBP1/BARS seem to be defective in lipid accumulation. We conclude that mono-ADP-ribosylation of CtBP1/BARS inactivates its repressor function, which leads to the activation of genes that regulate neutral lipid storage