A two-module region of the low-density lipoprotein receptor sufficient for formation of complexes with apolipoprotein E ligands

The low-density lipoprotein (LDL) receptor transports two different classes of cholesterol-carrying lipoprotein particles into cells: LDL particles, which contain a single copy of apolipoprotein B-100 (apoB-100), and beta-migrating very low-density lipoprotein (beta-VLDL) particles, which contain multiple copies of apolipoprotein E (apoE). The ligand-binding domain of the receptor lies at its amino-terminal end within seven adjacent LDL-A repeats (LA1-LA7). Although prior work clearly establishes that LA5 is required for high-affinity binding of particles containing apolipoprotein E (apoE), the number of ligand-binding repeats sufficient to bind apoE ligands has not yet been determined. Similarly, uncertainty exists as to whether a single lipid-activated apoE receptor-binding site within a particle is capable of binding to the LDLR with high affinity or whether more than one is required. Here, we establish that the LA4-5 two-repeat pair is sufficient to bind apoE-containing ligands, on the basis of binding studies performed with a series of LDLR-derived "minireceptors" containing up to four repeats. Using single chain multimers of the apoE receptor-binding domain (N-apoE), we also show that more than one receptor-binding site in its lipid-activated conformation is required to bind to the LDLR with high affinity. Thus, in addition to inducing a conformational change in the structure of N-apoE, lipid association enhances the affinity of apoE for the LDLR in part by creating a multivalent ligand.