Lipid droplet biogenesis from specialized ER subdomains

Abstract

Lipid droplets (LDs) are cellular compartments dedicated to the storage of metabolic energy in the form of neutral lipids, commonly known as "fat". The biogenesis of LDs takes place in the endoplasmic reticulum (ER), but its spatial and temporal organization is poorly understood. How exactly sites of LD formation are selected and the succession of proteins and lipids needed to mediate this process remains to be defined. In our current study we show that the yeast triacylglycerol (TAG)-synthases, Lro1 and Dga1 get recruited to discrete ER subdomains where they initiate TAG synthesis and hence LD formation (Choudhary et al. (2020), J Cell Biol). These ER subdomains are defined by yeast seipin, Fld1, and a regulator of diacylglycerol (DAG) production, Nem1. Both Fld1 and Nem1 are ER proteins which localize at contact sites between the ER and LDs. Interestingly, even in cells lacking LDs, Fld1 and Nem1 show punctate localization at ER subdomains independently of each other, but they are required together to recruit the TAG-synthases and hence create functional sites of LD biogenesis. Fld1/Nem1-containing ER subdomains recruit additional LD biogenesis factors, such as Yft2, Pex30, Pet10 and Erg6, and these membrane domains become enriched in DAG. In conclusion, Fld1 and Nem1 play a crucial role in defining ER subdomains for the recruitment of proteins and lipids needed to initiate LD biogenesis.

Keywords: ER subdomains; Nem1; Pex30; Yft2; diacylglycerol; lipid droplet; seipin.

Figure 1. FIGURE 1: LD biogenesis in the ER.

ER-localized triacylglycerol producing enzymes, Lro1, and Dga1 catalyze neutral lipid (NL, indicated in yellow) synthesis from opposite sides of the ER membrane. The NL then accumulate between the two-leaflets of the ER membrane leading to formation of lipid lenses. These NL lenses grow in size to become nascent LDs, which then emerge towards the cytoplasm where they further mature. Fld1 and Nem1 proteins show punctate localization at ER-LD contact sites. The acyltransferase Dga1, and the TAG lipase Tgl3 translocate onto the periphery of mature LDs. Finally, LD-marker proteins such as the perilipin ortholog Pet10 and the sterol biosynthetic enzyme Erg6 decorate the surface of the mature LD.

Figure 2. FIGURE 2: Model of LD biogenesis from specialized ER subdomains.

a) Fld1 and Nem1 show punctate ER distribution in yeast mutants lacking LDs. b) Nem1 gets activated at sites where Fld1 and Nem1 colocalize (Fld1/Nem1 sites). c) Activation of Pah1 and production of DAG at these sites. DAG enrichment drives Yft2 recruitment at Fld1/Nem1 sites. d) Pex30 localizes to Fld1/Nem1/Yft2-containing ER subdomains, and deforms the ER bilayer. e-g) Sites containing Fld1/Nem1/Yft2/Pex30 become functional in recruiting TAG producing enzymes: Lro1 and Dga1 are recruited (e), resulting in neutral lipid synthesis (f), and formation of neutral lipid lenses (g). h, i) Lenses grow into nascent LDs that are recognized by bona fide LD marker proteins, such as Pet10 and Erg6, facilitating nascent LDs to emerge towards the cytoplasm (h) where they further grow in size and mature (i). Mature LDs acquire additional LD surface proteins and remain connected to the ER to promote bidirectional transport of proteins and lipids between the two compartments. This figure has been modified from Choudhary et al (2020). J Cell Biol; doi: 10.1083/jcb.201910177