Human apolipoprotein CIII (apoCIII) is a surface component of chylomicrons, very low density lipoproteins, and high density lipoproteins. ApoCIII inhibits lipoprotein lipase as well as binding of lipoproteins to cell surface heparan sulfate proteoglycans and receptors. High levels of apoCIII are often correlated with elevated levels of blood lipids (hypertriglyceridemia). Here, we report the three-dimensional NMR structure and dynamics of human apo-CIII in complex with SDS micelles, mimicking its natural lipidbound state. Thanks to residual dipolar coupling data, the first detailed view is obtained of the structure and dynamics of an intact apolipoprotein in its lipid-bound state. ApoCIII wraps around the micelle surface as a necklace of six ϳ10-residue amphipathic helices, which are curved and connected via semiflexible hinges. Three positively charged (Lys) residues line the polar faces of helices 1 and 2. Interestingly, their three-dimensional conformation is similar to that of the low density lipoprotein receptor binding motifs of apoE/B and the receptor-associated protein. At the C-terminal side of apoCIII, an array of negatively charged residues lines the polar faces of helices 4 and 5 and the adjacent flexible loop. Sequence comparison shows that this asymmetric charge distribution along the solvent-exposed face of apoCIII as well as other structural features are conserved among mammals. This structure provides a template for exploration of molecular mechanisms by which human apoCIII inhibits lipoprotein lipase and receptor binding.Apolipoproteins consist of five major classes, apo-A 2 through -E, and several subclasses. They are designed for lipid transport in blood and are attached to lipid droplets (lipoproteins) through amphipathic helices that intercalate into the single layer of phospholipids and cholesterol covering the droplet (1-3). The apolipoproteins regulate blood lipid levels by interaction with specialized endocytotic receptors and by modulating enzyme activities and lipid exchange reactions (4). They are therefore in focus with regard to disturbances in blood lipid metabolism (dyslipidemias), which in turn are associated with metabolic disorders like obesity, insulin resistance, diabetes type 2, and cardiovascular disease.Apolipoprotein CIII (apoCIII) is the most abundant C-apolipoprotein in humans (2-4). It is found on very low density lipoproteins, chylomicrons, and high density lipoproteins (HDL). ApoCIII is predominantly expressed in liver and intestine from a gene cluster important for lipid regulation (ApoAI, -AIV, -AV, and -CIII) (4, 5). ApoCIII inhibits lipoprotein lipase (LPL) and receptor-mediated endocytosis of lipoprotein particles by competing for space at the surface of the lipoproteins and by interfering with their binding to endothelial proteoglycans and to specific lipoprotein receptors (2, 4, 6). Overexpression of apoCIII in transgenic mice leads to severely increased plasma triglyceride levels due to accumulation of very low density lipoprotein-like lipoprotein remnants w...