Two apolipoprotein E5 variants illustrate the importance of the position of additional positive charge on receptor-binding activity.
Apolipoprotein (apo) E polymorphism has a significant effect on plasma cholesterol and low density lipoprotein cholesterol concentrations. The association of two apoE5 isoforms with elevated plasma low density lipoprotein cholesterol levels in two unrelated subjects led us to investigate the primary structures and receptor-binding properties of their apoE. Cysteamine modification and isoelectric focusing demonstrated that the apoE5 isoform from subject 1 did not contain cysteine but that the apoE5 isoform from subject 2 contained one residue of cysteine. The structural mutation in the apoE5 isoform of subject 1 was determined by peptide sequencing. Like apoE4, this variant had arginine at position 112 but differed from apoE4 by the substitution of arginine for proline at position 84. When purified and subjected to a competitive binding assay, this apoE5(84 Pro----Arg, 112 Cys----Arg) variant had the same receptor-binding activity as normal apoE3. Because subject 2 was of Japanese descent and her apoE5 contained one cysteine residue, we suspected that it would contain the lysine-forglutamic acid mutation at position 3 that has been described previously in Japanese subjects. This was confirmed by directly sequencing the first 10 amino acid residues of her apoE. When subjected to the competitive binding assay, the total apoE from subject 2, which consisted of approximately equal amounts of normal apoE3 and apoE5(3 Glu----Lys), had a binding activity of 188%, confirming the previously reported enhanced binding of this variant. These results demonstrate that the enhancement of receptor-binding activity of more basic isoforms of apoE depends on the position at which additional positively charged amino acids are incorporated.