Tld1 is a novel regulator of triglyceride lipolysis that demarcates a lipid droplet subpopulation

Abstract

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.

A metabolically controlled contact site between lipid droplets and vacuoles

“…This indicates that Pdr16 might limit lipolytic LD breakdown when lipids become more precious due to restrictions in nutrient availability, a function that may also apply for vegetative haploid cells exposed to glucose restriction. Consistently, Bsc2, a protein that we previously identified as a further resident of the LDO-and Pdr16-enriched LD subpopulation of cells in logarithmic growth phase (Eisenberg-Bord et al, 2018), was recently shown to counteract triacylglycerol lipolysis (Speer et al, 2023), suggesting that the LD subpopulation engaged in vCLIP during logarithmic phase might be protected from lipolysis. Similar to what we observe for metabolic regulation of vCLIP abundance, the pool of Bsc2-positive LDs expands when cells run out of glucose (Speer et al, 2023).…”

“…Consistently, Bsc2, a protein that we previously identified as a further resident of the LDO-and Pdr16-enriched LD subpopulation of cells in logarithmic growth phase (Eisenberg-Bord et al, 2018), was recently shown to counteract triacylglycerol lipolysis (Speer et al, 2023), suggesting that the LD subpopulation engaged in vCLIP during logarithmic phase might be protected from lipolysis. Similar to what we observe for metabolic regulation of vCLIP abundance, the pool of Bsc2-positive LDs expands when cells run out of glucose (Speer et al, 2023). Collectively, this suggests that LDs engaged in vCLIPs may be preserved for longer-term lipid storage particularly during nutrient deprivation.…”

Molecular dynamics simulations of intracellular lipid droplets: a new tool in the toolbox

“…2B). These "voids," that in lipid bilayers are generated by the exposure of the hydrocarbon atoms belonging to the lipid acyl chains to the surface [22,73], affect LD surface properties [22] and, consequently, protein targeting [45,72,[74][75][76]. To this end, all-atom simulations have suggested that LPDs are possibly more persistent in LDs, compared with phospholipid bilayers [72,75], and, as such, can constitute specific binding sites for the preferential targeting of proteins [24,72].…”

“…For instance, CYTOLD proteins have been proposed to bind to the monolayer following a multi-stage pathway: from their initial unfolded configuration in solution, proteins can fold upon anchoring to LPDs thanks to hydrophobic residues [72]. Indeed, specific sequence "signatures," in particular bulky hydrophobic residues, have been demonstrated by MD simulations to be crucial for specific binding to LDs [45,72,76].…”

“…All atom simulations were also used to identify specific sequence features of hydrophobic membrane motifs that mediate LD targeting, identifying a prominent role for deeply inserted tryptophan residues that promote subtle conformational changes of the protein structure in the bilayer versus a LD-like environment [78]. We caution, however, that all protein-LD interactions described using all-atom MD so far in this context, both for CYTOLD and ERTOLD proteins [45,72,76,78], have been carried out using the old, more hydrophilic, CHARMM parameters for TG molecules. Future studies, possibly in combination with experimental methods that probe protein-membrane interactions with high resolution, will certainly help shed light on this fascinating molecular mechanism.…”