Neutral lipids regulate amphipathic helix affinity for model lipid droplets

Abstract

Cellular lipid droplets (LDs) have a neutral lipid core shielded from the aqueous environment by a phospholipid monolayer containing proteins. These proteins define the biological functions of LDs, and most of them bear amphipathic helices (AH), which can selectively target to LDs, or to LD subsets. How such binding preference happens remains poorly understood. Here, we found that artificial LDs made of different neutral lipids but presenting equal phospholipid packing densities differentially recruit AHs. Varying the phospholipid density shifts the binding levels, but the differential recruitment is unchanged. We found that the binding level of AHs is defined by their interaction preference with neutral lipids and ability to decrease surface tension. The phospholipid packing level regulates mainly the amount of neutral lipid accessible. Therefore, it is the hydrophobic nature of the phospholipid packing voids that controls the binding level of AHs. Our data bring us a major step closer to understanding the binding selectivity of AHs to lipid membranes.

Triacylglycerols sequester monotopic membrane proteins to lipid droplets

“…The monolayer of a droplet contiguous with a bilayer is less packed with phospholipids than the bilayer leaflets 9 . In a bilayer, HD peptides would be in contact with phospholipid acyl chains along their full length while, in a monolayer, a significant fraction of the peptide would be in contact with TG.…”

“…In conclusion, our data indicate that HDs prefer mixing with TG instead of being in contact with membrane phospholipids. Because phospholipid packing is less compact in a monolayer compared with a bilayer 9 , and because a monolayer thickness is half the thickness of a bilayer, partitioning toward monolayers is favored as they expose HD proteins to TG (Fig. 4c).…”

“…We chose an oil phase very different from TG, namely silicone oil, in an attempt to trigger major changes in oil-protein affinity. Silicone oil is chemically very different from TG but they both have a high surface tension with water 9 .…”

“…For instance, the presence of charged residues in a HD may hamper HD-TG interaction, since embedding charges in a low dielectric milieu such as of a TG phase is unfavorable, and generally requires conformational changes to the protein or interaction with a protein of opposite charge. More generally, as we recently proposed for AHs 9 , there could exist sequence motifs tailored with an optimal affinity with TG.…”

The surface of lipid droplets constitutes a barrier for endoplasmic reticulum residential integral membrane spanning proteins

“…These cues are likely lipid-dependent and it has been proposed that perilipins and particularly their repeats of amphipathic helices detect lipid packaging defects (Bulankina et al, 2009;Copic et al, 2018;Prévost et al, 2018;Chorlay and Thiam, 2020). Such lipid packing defects could be induced by the presence of low levels of neutral lipids, including TAG, STE but possibly also DAG, within the LD monolayer but maybe also within the ER bilayer.…”