ORP5 controls the partitioning of phosphatidic acid between triacylglycerol and cardiolipin synthesis at mitochondria–ER–lipid droplet contact sites

Phosphatidic acid (PA) is a central metabolic intermediate that can fuel triacylglycerol (TAG) synthesis in lipid droplets (LDs) or cardiolipin production in mitochondria, but how cells partition PA between these competing fates has remained a fundamental unresolved question in lipid cell biology. We identify the lipid transfer protein ORP5 as a key regulator of PA partitioning at Mitochondria–Associated endoplasmic reticulum Membranes (MAM) that contact lipid droplets (LD), referred to as MAM–LD junctions. Cell imaging analysis shows that ORP5 stabilizes PA levels at MAM to promote TAG synthesis. On the other hand, loss of ORP5 causes PA accumulation on mitochondrial membranes, leading to excess cardiolipin synthesis and mitochondrial hyperfusion, while impairing triacylglycerol (TAG) synthesis and LD formation. Finally, reconstitution assays using liposomes or giant organelles further demonstrate that ORP5 can transfer PA from mitochondria to the ER via its ORD domain. Together, these findings reveal that ORP5 functions as a PA lipid transfer protein at tripartite MAM–LD contacts, where it balances LD formation with mitochondrial lipid metabolism, protecting mitochondria from cardiolipin overload.