A FIT2-BLTP1 module sustains lipid droplet phosphatidic acid and restrains overgrowth

Lipid droplets (LDs) are assembled at discrete endoplasmic reticulum (ER) subdomains whose local phospholipid composition, rather than their protein content alone, determines whether a LD nucleates and how large it grows. Phosphatidic acid (PA) is the pivotal lipid of this environment, yet how PA reaches a forming LD has remained unknown. Here we show that the bridge-like lipid transfer protein BLTP1 is required to maintain PA in LD-associated membranes. BLTP1 depletion impaired LD formation and produced fewer but enlarged LDs across mammalian cell types, and lowered LD accumulation during adipogenic differentiation. At the ER, BLTP1 engaged the LD emergence factor FIT2 through its N-terminal transmembrane segment, an interaction that strengthened upon induction of LD formation and was strongly reduced by the disease-associated FIT2 G232R substitution. Co-depletion of FIT2 relieved the LD overgrowth caused by loss of BLTP1. Wild-type BLTP1, but neither a FIT2-binding-defective chimera nor a lipid-transfer-groove mutant, rescued LD formation in BLTP1-depleted cells. Lipidomics of purified LDs showed that BLTP1 loss most strongly lowered PA, most prominently the species PA 36:2, whereas PA and the major phospholipid classes of bulk ER membranes were unchanged. Collectively, we propose that local PA supply by BLTP1 coordinates LD assembly and growth.