Ube4a liver-specific knockout effect on fasted liver

Ectopic deposition of free fatty acids (FFAs) causes lipotoxicity in metabolic disorders such as obesity/diabetes. How intracellular FFAs are sensed and detoxified remains unclear. We identify a cytosol-facing FA-sensing machinery that synchronizes lipid droplet (LD) dynamics with FFA abundance. Specifically, an Ubxn4-Ube4a E3 complex located at the ER-LD interface restricts LD growth via non-degradative ubiquitylation of the acyl-CoA synthetases FATP1/4, thereby disrupting Fatp interaction with diacylglycerol O-acyltransferase 2 (Dgat2) to avoid premature triglyceride synthesis and storage into LD. Unsaturated FA binds to Ubxn4’s UAS domain, competitively displacing the E3 ligase Ube4a to autonomously promote Fatp-Dgat2 complexation and LD growth. Cellular UBE4A deletion accelerates LD growth, whereas mice with hepatocyte-specific Ube4a deletion are predisposed to hepatic steatosis, fibrosis, and hepatocellular carcinoma. Thus, FFA levels and LD expansion are coordinately monitored via ubiquitylation-modulated coupling between FA activation and acyltransfer, thereby maintaining lipid homeostasis to avoid metabolic anomalies.