Decoding of lipoprotein – receptor interactions; Properties of ligand binding modules governing interactions with ApoE

摘要

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 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 exists in the lipid bound form. The combination of avidity effects with this distinct recognition process likely governs the ApoE-LDL receptor interaction.