Organization of the yeast Seipin complex reveals differential recruitment of regulatory proteins

Lipid droplets (LD) are neutral lipid storage organelles that emerge from the endoplasmic reticulum (ER). Their assembly occurs in ER regions enriched with seipin, which, through its homooligomeric ring-like structure, facilitates neutral lipid nucleation. In yeast, Seipin (Sei1) partners with Ldb16, Ldo45 (yeast homologue of human LDAF1), and Ldo16, which regulate LD formation and consumption. How the molecular architecture of the yeast seipin complex and its interaction with regulatory proteins adapt to different metabolic conditions remains poorly understood. Here, we show that multiple Ldb16 regions contribute differently to recruiting Ldo45 and Ldo16 to the seipin complex. Using an in vivo site-specific photo-crosslinking approach, we further show that Ldo45 resides at the center of the seipin ring both in the absence and presence of neutral lipids. Interestingly, neutral lipid synthesis leads to the recruitment of Ldo45 but not Ldo16 to the complex. Our findings suggest that the seipin complex serves as a preassembled scaffold for lipid storage that can be remodeled in response to increased neutral lipid availability.