Sec14L6 Is a Phosphoinositide Transporter That Regulates Phosphoinositide Homeostasis and Biogenesis of Lipid Droplet

Lipid droplets (LDs) are evolutionarily conserved organelles crucial for cellular metabolism. Their biogenesis and growth occur in the endoplasmic reticulum (ER) and rely on lipid transfer between the ER and LDs. However, the molecular mechanisms remain poorly understood. In this study, we identified Sec14L6, a unique Sec14 protein family member, as a lipid transporter that regulates phosphoinositide homeostasis and biogenesis of LDs, and is required for the differentiation of adipose-derived mesenchymal stem cells. Sec14L6 directly binds to ACSL3, a known LD biogenesis factor, which facilitates the association of Sec14L6 with LD surface. Furthermore, we identify PGRMC1, an ER membrane protein, as an adaptor that recruits Sec14L6 to the ER, specifying a role for Sec14L6 at ER-LD interface. Targeted lipidomics revealed profound dysregulation of PIP homeostasis: residual LDs from Sec14L6-KO cells exhibited aberrant accumulation of PI4P and PI(4,5)P2, concomitant with a reduction of these PIPs within the ER compartment. In vitro assays demonstrate that Sec14L6 preferentially transports phosphoinositide-4-phosphate (PI4P) and PI(4,5)P2. Sec14L6 knockout (KO) severely impaired nascent LD formation, a defect rescued by wild-type Sec14L6 but not by lipid-transfer-deficient mutants. Our study identified Sec14L6 as a new factor that regulates PIP homeostasis and biogenesis of LDs via lipid transfer at ER-LD interface.