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The receptor-binding domain of apolipoprotein E, the first important protein involved in 
cholesterol metabolism to be solved by x-ray crystallography. This protein is responsible for 
targeting plasma lipoproteins such as chylomicrons, VLDL ( very low density lipoprotein ), 
and HDL (high-density lipoprotein) to their appropriate cellular receptors (the LDL 
receptor). 
1. Electron density map showing two a-helices from the recently determined crystal 
structure of apolipoprotein E. 
2. Diagram showing that apolipoprotein E is an unusually elongated, antiparallel, four- 
helix bundle, with an additional short helix linking helices 1 and 2. The structure is built 
from repeating sequence elements shown by different colored regions. 
3- A cutaway surface revealing the hydrophobic interface between helices 1 and 4. The 
structure is substantially stabilized by a leucine zipper motif. The juxtaposed leucines from 
one helix are shown in gold, the other in orange. 
Research of David A. Agard. 
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