Human apolipoprotein E. Receptor binding activity of truncated variants with carboxyl-terminal deletions

The amino-terminal thrombolytic fragment (residues 1-191) of human apolipoprotein (apo) E was previously shown to be fully active in binding to the low density lipoprotein receptor. In this study, truncated apoE variants with progressive deletions at the carboxyl terminus were produced in Escherichia coli by linker-insertion mutagenesis to define the minimum amino-terminal structure necessary for full receptor binding. These truncated forms of apoE, comprising residues 1-166, 1-170, 1-174, or 1-183, were combined with the phospholipid dimyristoylphosphatidylcholine and tested for their ability to bind to low density lipoprotein receptors on human fibroblasts. All of the truncated variants formed typical discoidal particles when combined with the phospholipid, and the particles could be isolated by density gradient ultracentrifugation. The 1-166 and 1-170 variants had very little receptor binding activity (1%), whereas the 1-183 variant had nearly full activity (85%). The 1-174 variant had 19% activity. We conclude that the 171-183 region of apoE is important for receptor binding, either by contributing one or more residues essential for receptor binding or, more likely, by stabilizing or aligning the region known to be crucial for receptor binding, in the vicinity of residues 140-160.