期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2021
卷号：118
期号：34
DOI：10.1073/pnas.2110483118
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Cholesterol is an essential component of animal cell membranes whose level in cells is maintained within a narrow range. Cholesterol is actively excreted from cells by two ATP-binding cassette (ABC) transporters, ABCG5–ABCG8 (G5G8) in the liver and gut and ABCG1 (G1) in macrophages. The mechanism(s) by which these proteins translocate rigid, planar sterol molecules across the membrane bilayer remain unknown. Here, we report the structure of human G1 and G5G8 in their unbound and cholesterol-bound states. We also determined the structure of G1 bound to ATP. These structures, together with functional studies in model organisms and biochemical studies, identify the binding site for cholesterol and provide the basis for a model of cholesterol transport by ABC transporters.
The ABCG1 homodimer (G1) and ABCG5–ABCG8 heterodimer (G5G8), two members of the adenosine triphosphate (ATP)–binding cassette (ABC) transporter G family, are required for maintenance of cellular cholesterol levels. G5G8 mediates secretion of neutral sterols into bile and the gut lumen, whereas G1 transports cholesterol from macrophages to high-density lipoproteins (HDLs). The mechanisms used by G5G8 and G1 to recognize and export sterols remain unclear. Here, we report cryoelectron microscopy (cryo-EM) structures of human G5G8 in sterol-bound and human G1 in cholesterol- and ATP-bound states. Both transporters have a sterol-binding site that is accessible from the cytosolic leaflet. A second site is present midway through the transmembrane domains of G5G8. The Walker A motif of G8 adopts a unique conformation that accounts for the marked asymmetry in ATPase activities between the two nucleotide-binding sites of G5G8. These structures, along with functional validation studies, provide a mechanistic framework for understanding cholesterol efflux via ABC transporters.
