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Biochemistry. 2010 Feb 16;49(6):1207-16. doi: 10.1021/bi9017208.

Decoding of lipoprotein-receptor interactions: properties of ligand binding modules governing interactions with apolipoprotein E.

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Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0378, USA.


Clusters of complement-type ligand binding repeats in the LDL receptor family are thought to mediate the interactions between these receptors and their various ligands. Apolipoprotein E, a key ligand for cholesterol homeostasis, has been shown to interact with LDLR, LRP, and VLDLR, through these clusters. LDLR and VLDLR each contain a single ligand binding repeat cluster, whereas LRP contains three large clusters of ligand binding repeats, each with ligand binding functions. We show that within sLRP3 the three-repeat subcluster CR16-18 recapitulated ligand binding to the isolated receptor binding portion of ApoE (residues 130-149). Binding experiments with LA3-5 of LDLR and CR16-18 showed that a conserved W25/D30 pair appears to be critical for high-affinity binding to ApoE(130-149). The triple repeat LA3-5 showed the expected interaction with ApoE(1-191).DMPC, but surprisingly CR16-18 did not interact with this form of ApoE. To understand these differences in ApoE binding affinity, we introduced mutations of conserved residues from LA5 into CR18 and produced a CR16-18 variant capable of binding ApoE(1-191).DMPC. This change cannot fully be accounted for by the interaction with the proposed ApoE receptor binding region; therefore, we speculate that LA5 is recognizing a distinct epitope on ApoE that may only exist in the lipid-bound form. The combination of avidity effects with this distinct recognition process likely governs the ApoE-LDL receptor interaction.

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