Decoding of Lipoprotein−Receptor Interactions: Properties of Ligand Binding Modules Governing Interactions with Apolipoprotein E

摘要

Clusters of complement-type ligand binding repeats in the LDL receptor family are thought tonmediate the interactions between these receptors and their various ligands. Apolipoprotein E, a key ligand forncholesterol homeostasis, has been shown to interact with LDLR, LRP, and VLDLR, through these clusters.nLDLR and VLDLR each contain a single ligand binding repeat cluster, whereas LRP contains three largenclusters of ligand binding repeats, each with ligand binding functions. We show that within sLRP3 the threerepeatnsubcluster CR16-18 recapitulated ligand binding to the isolated receptor binding portion of ApoEn(residues 130-149). Binding experiments with LA3-5 of LDLR and CR16-18 showed that a conservednW25/D30 pair appears to be critical for high-affinity binding to ApoE(130-149). The triple repeat LA3-5nshowed the expected interaction with ApoE(1-191) 3DMPC, but surprisingly CR16-18 did not interact withnthis form of ApoE. To understand these differences in ApoE binding affinity, we introduced mutations ofnconserved residues from LA5 into CR18 and produced a CR16-18 variant capable of binding ApoE-n(1-191) 3DMPC. This change cannot fully be accounted for by the interaction with the proposed ApoEnreceptor binding region; therefore, we speculate that LA5 is recognizing a distinct epitope on ApoE that maynonly exist in the lipid-bound form. The combination of avidity effects with this distinct recognition processnlikely governs the ApoE-LDL receptor interaction.