Triacylglycerols sequester monotopic membrane proteins to lipid droplets

Abstract

Triacylglycerols (TG) are synthesized at the endoplasmic reticulum (ER) bilayer and packaged into organelles called lipid droplets (LDs). LDs are covered by a single phospholipid monolayer contiguous with the ER bilayer. This connection is used by several monotopic integral membrane proteins, with hydrophobic membrane association domains (HDs), to diffuse between the organelles. However, how proteins partition between ER and LDs is not understood. Here, we employed synthetic model systems and found that HD-containing proteins strongly prefer monolayers and returning to the bilayer is unfavorable. This preference for monolayers is due to a higher affinity of HDs for TG over membrane phospholipids. Protein distribution is regulated by PC/PE ratio via alterations in monolayer packing and HD-TG interaction. Thus, HD-containing proteins appear to non-specifically accumulate to the LD surface. In cells, protein editing mechanisms at the ER membrane would be necessary to prevent unspecific relocation of HD-containing proteins to LDs.

ER-localized phosphatidylethanolamine synthase plays a conserved role in lipid droplet formation

“…Additionally, in yeast genetic backgrounds that support the formation of supersized LDs (such as ∆sei1 ∆pln1), the absence of ER-localized Psd1 may reduce the PE levels on the LD surface monolayer, attenuating the ability of small LDs to fuse with one another and generate supersized LDs (Fei et al, 2011). A non-mutually exclusive possibility is that altered PC/PE ratios at the ER in PSD-deficient cells influences LD protein targeting, which feeds back into influencing LD biogenesis or expansion (Caillon et al, 2020).…”

Co-translational biogenesis of lipid droplet integral membrane proteins

“…Hence, LD biogenesis at the ER results in the formation of a hydrophobic oil phase surrounded by a phospholipid monolayer derived from the cytosolic leaflet of the ER. Such an architecture distinguishes LDs from other intracellular membrane-enveloped organelles, which are surrounded by phospholipid bilayers, and has profound consequences for phospholipid packing and protein localisation to LDs ( Caillon et al, 2020 ; Dhiman et al, 2020 ; Kory et al, 2015 ; Krahmer et al, 2011 ; Olarte et al, 2020 ).…”

Liquid-crystalline lipid phase transitions in lipid droplets selectively remodel the LD proteome

“…As such, Type I LD proteins can translocate between the ER and LD monolayer via lipidic bridges connecting the two organelles (Wilfling et al, 2013). Elegant in vitro studies have suggested that LD localization promotes energetically favorable conformational changes within some proteins, and the movement of proteins to LDs from the ER network can even influence their enzymatic activities, or modulate their degradation (Caillon et al, 2020), (Chorlay and Thiam, 2020), (Leber et al, 1998), (Schmidt et al, 2013), (Ohsaki et al, 2006). A second mechanism of LD targeting occurs from the cytoplasm, where soluble proteins insert into the LD monolayer via a hydrophobic region, amphipathic helix, or lipid moiety.…”