Bsc2 is a novel regulator of triglyceride lipolysis that demarcates a lipid droplet subpopulation.

Cells store lipids in the form of triglyceride (TG) and sterol-ester (SE) in lipid droplets (LDs). Distinct pools of LDs exist, but a pervasive question is how proteins localize to and convey functions to LD subsets. Here, we show the yeast protein Bsc2 localizes to a subset of TG-containing LDs, and reveal it negatively regulates TG lipolysis. Mechanistically, Bsc2 LD targeting requires TG, and LD targeting is mediated by hydrophobic regions (HRs). Molecular dynamics simulations reveal these Bsc2 HRs interact with TG on modeled LDs, and adopt specific conformations on TG-rich LDs versus SE-rich LDs or an ER bilayer. Bsc2-deficient yeast display no defect in LD biogenesis, but exhibit elevated TG lipolysis dependent on lipase Tgl3. Remarkably, Bsc2 abundance influences TG, and over-expression of Bsc2, but not LD protein Pln1, promotes TG accumulation without altering SE. Finally, we find Bsc2-deficient cells display altered LD mobilization during stationary growth. We propose Bsc2 regulates lipolysis and localizes to subsets of TG-enriched LDs.