Structural differences between apolipoprotein E3 and E4 as measured by ¹⁹ F NMR

The apolipoprotein E family contains three major isoforms (ApoE4, E3, and E2) that are directly involved with lipoprotein metabolism and cholesterol transport. ApoE3 and apoE4 differ in only a single amino acid with an arginine in apoE4 changed to a cysteine at position 112 in apoE3. Yet only apoE4 is recognized as a risk factor for Alzheimer's disease. Here we used ¹⁹F NMR to examine structural differences between apoE4 and apoE3 and the effect of the C‐terminal domain on the N‐terminal domain. After incorporation of 5‐¹⁹F‐tryptophan the 1D ¹⁹F NMR spectra were compared for the N‐terminal domain and for the full length proteins. The NMR spectra of the N‐terminal region (residues 1–191) are reasonably well resolved while those of the full length wild‐type proteins are broad and ill‐defined suggesting considerable conformational heterogeneity. At least four of the seven tryptophan residues in the wild type protein appear to be solvent exposed. NMR spectra of the wild‐type proteins were compared to apoE containing four mutations in the C‐terminal region that gives rise to a monomeric form either of apoE3 under native conditions (Zhang et al., Biochemistry 2007; 46: 10722–10732) or apoE4 in the presence of 1 M urea. For either wild‐type or mutant proteins the differences in tryptophan resonances in the N‐terminal region of the protein suggest structural differences between apoE3 and apoE4. We conclude that these differences occur both as a consequence of the Arg158Cys mutation and as a consequence of the interaction with the C‐terminal domain.