Lipid droplet

Fatty acids are also used to produce triacylglycerols (TGs), which store energy in structures called lipid droplets.

They add an acyl group from a fatty acid that has been activated with coenzyme A (FA-CoA) to diacylglycerol (DG), forming TG.

[10] When nutrients become available, the yeast cells enter the exponential growth phase (EXP) to grow quickly.

It has been shown that during the EXP phase, Lro1-GFP is localized in the endoplasmic reticulum (ER) to synthesize triacylglycerols (TG), which are essential for phospholipid synthesis.

Furthermore, this movement is influenced by signals from the cell cycle and nutrient availability, and it stops when the nucleus grows larger.

It has been shown that when the Lro1's N-terminal domain was enlarged with one, two, or three copies of the maltose-binding protein (MBP), its ability to target the nucleolus was significantly reduced.

FRB-GFP construct contains GFP fused to the FRB domain but without Lro1, to show the specific effects of Lro1's presence and used as a control.

[9] Upon the addition of rapamycin, FRB-GFP quickly (within 30 minutes) changed from a diffuse distribution to a ring-like pattern, indicating that it had been recruited to the INM by Heh1-FKBP12.

In the strain expressing FRB-Lro1-GFP, rapamycin treatment caused a loss of Lro1's cortical ER localization and its accumulation at a perinuclear ring, which is characteristic of INM proteins.

In contrast, when Lro1's N-terminal domain was enlarged by adding 3xMBP, the fusion protein retained its localization at the cortical ER even after rapamycin treatment.

[9] To test Lro1's activity, the researchers expressed it in a yeast strain that couldn't produce any neutral lipids on its own.

When Lro1 is reintroduced as the only enzyme capable of producing TG, it rescues the cells, allowing them to survive better during the stationary phase by forming lipid droplets.

[9] These findings[9] suggest that Lro1's activity in the nucleus creates a local site for TG synthesis, which helps reshape the nuclear membrane as needed.

[9] When the K/R residues in the N-terminal domain are mutated to alanines, the nucleolar enrichment is partially compromised but not completely lost, indicating that other regions of Lro1 are also contributing to its proper localization.

[17] The budding of lipid droplets is promoted by an asymmetric accumulation of phospholipids that decrease surface tension towards the cytosol.