Chen Yuxing and Zhou congzhao, professors of China University of science and technology, used single particle cryoelectron microscopy to analyze the three-dimensional structure of human cholesterol efflux transporter ABCG1 and the molecular complex of substrate cholesterol, adenosine triphosphate and sphingomyelin, and revealed the molecular mechanism of cholesterol efflux through ABCG1 transmembrane, Cholesterol receptors and membrane sphingomyelin molecules were found to be necessary for efflux activity. Relevant results were published online in cell report a few days ago.

As the main component of cell membrane, cholesterol plays a key role in maintaining membrane fluidity, stability and synthesis of bile acids. Reverse cholesterol transport is the main way of excess cholesterol excretion. When cholesterol reverses abnormal transport function, intracellular cholesterol will accumulate excessively and eventually lead to atherosclerosis. This disease is currently the world's first fatal disease. ABCG1 is a key membrane protein responsible for the extracellular transport of cholesterol in the cholesterol reverse transport pathway. Its abnormal function is highly correlated with major diseases such as atherosclerosis.

In this study, the research group found that ABCG1 presents a three-dimensional structure of intracellular opening conformation, identified the amino acid residues of ABCG1 binding to cholesterol substrate, explained the protein substrate interaction mode, and found that there is a group of phenylalanine residues at the outlet of substrate binding pocket, which is very important for substrate transport. The corresponding biochemical experiments revealed that ABCG1 activity was also dependent on cholesterol receptor - high density lipoprotein. In addition, the researchers also found sphingomyelin components that specifically bind near the substrate binding pocket. Based on the results of structural analysis and functional experiments, the sphingomyelin molecule is necessary for ABCG1 mediated cholesterol transport.

Based on the experimental results, the research group proposed a model of ABCG1 mediated cholesterol efflux mechanism: sphingomyelin assists in the binding of cholesterol with ABCG1 and participates in the formation of a complete binding pocket to stabilize substrate molecules, and extracellular bound high-density lipoprotein promotes the efflux of cholesterol from ABCG1 substrate pocket. This study not only reveals the interaction mode between ABCG1 and cholesterol and the importance of HDL for cholesterol efflux and transport, but also further improves the regulation network of cholesterol metabolism, which has important theoretical guiding significance for the prevention and treatment of diseases related to cholesterol metabolism disorders.