Mechanism for oil-phase separation by the lipid droplet assembly complex

Area of Science:

• Cellular biology
• Biophysics
• Structural biology

Background:

• Cells store energy as triglyceride (TG) oils in lipid droplets (LDs), which originate from the endoplasmic reticulum.
• The precise mechanism by which the lipid droplet assembly complex (LDAC), comprising seipin and LDAF1, initiates organized oil phase formation in membranes remains unclear.

Purpose of the Study:

• To elucidate the catalytic role and structural basis of the LDAC in initiating lipid droplet formation.
• To investigate how the LDAC facilitates the formation of an oil phase from triglycerides within the endoplasmic reticulum membrane.

Main Methods:

• Reconstitution of lipid droplet formation *in vitro* using purified LDAC and artificial membranes with physiological triglyceride levels.
• Structural studies of the LDAC complex.
• Molecular dynamics simulations and biochemical assays to analyze TG and phospholipid interactions within the LDAC structure.

Main Results:

• The purified LDAC was found to be necessary and sufficient to catalyze oil-phase formation below the threshold of spontaneous phase separation.
• Structural analysis revealed the LDAC forms a toroidal, membrane-spanning structure with LDAF1 at the center within a seipin cage.
• This structure creates a selective compartment that permits TG access while excluding phospholipids, facilitating TG self-interaction and oil phase initiation.

Conclusions:

• The LDAC functions as a protein catalyst essential for initiating the oil-phase separation required for lipid droplet biogenesis.
• The study reveals a fundamental mechanism for how cells organize oil storage within the hydrophilic environment of the endoplasmic reticulum.