STARD3 mediates non-vesicular cholesterol transport in Caenorhabditis elegans

Cholesterol transport plays a pivotal role in regulating development and metabolism in Caenorhabditis elegans, a sterol-auxotrophic organism. Here, we identify the nematode cholesterol-binding protein STARD3 and provide structural and functional evidence for its role in non-vesicular sterol mobilization. Using biophysical and high-resolution structural methods, we show that the START domain (Ce-START) of C. elegans STARD3 binds cholesterol with high affinity and adopts a fold conserved with its human ortholog. Crystal structures of Ce-START in both apo and cholesterol-bound forms reveal key determinants of sterol recognition and conformational changes upon ligand binding. Functional analysis of a C. elegans stard3 knockout strain demonstrates that STARD3 is essential for cholesterol trafficking under sterol-limited conditions and that it genetically interacts with the NPC1/NPC2 pathway to sustain cholesterol mobilization. Collectively, these results establish STARD3 as a crucial cholesterol transporter in C. elegans and underscore the evolutionary conservation of START-domain proteins, reinforcing the utility of C. elegans as a model for studying intracellular cholesterol dynamics.