Cideb controls sterol‐regulated ER export of SREBP/SCAP by promoting cargo loading at ER exit sites

SREBPs are master regulators of lipid homeostasis and undergo sterol‐regulated export from ER to Golgi apparatus for processing and activation via COPII‐coated vesicles. While COPII recognizes SREBP through its escort protein SCAP, factor(s) specifically promoting SREBP/SCAP loading to the COPII machinery remains unknown. Here, we show that the ER/lipid droplet‐associated protein Cideb selectively promotes the loading of SREBP/SCAP into COPII vesicles. Sterol deprivation releases SCAP from Insig and enhances ER export of SREBP/SCAP by inducing SCAP‐Cideb interaction, thereby modulating sterol sensitivity. Moreover, Cideb binds to the guanine nucleotide exchange factor Sec12 to enrich SCAP/SREBP at ER exit sites, where assembling of COPII complex initiates. Loss of Cideb inhibits the cargo loading of SREBP/SCAP, reduces SREBP activation, and alleviates diet‐induced hepatic steatosis. Our data point to a linchpin role of Cideb in regulated ER export of SREBP and lipid homeostasis.