A monomeric human apolipoprotein E carboxyl-terminal domain.

ApoE plays a critical role in lipoprotein metabolism and plasma lipid homeostasis through its high-affinity binding to the LDL-receptor family. In solution, apoE is an oligomeric protein and the C-terminal domain causes apoE's aggregation. The aggregation property presents a major difficulty for the structural determination of this protein. A high-level expression system of the apoE C-terminal domain is reported here. Using protein engineering techniques, we identified a monomeric, biologically active apoE C-terminal domain mutant. This mutant replaces five bulky hydrophobic residues in the region of residues 253-289 with either smaller hydrophobic or polar/charged residues (F257A, W264R, V269A, L279Q, and V287E). The solubility of the mutant is significantly increased ( approximately 10-fold). Cross-linking experiments indicate that this mutant is 100% monomeric even at 5 mg/mL. CD and guanidine hydrochloride denaturation results indicate that the mutant maintains an identical alpha-helical secondary structure and stability as compared with those of the wild-type protein. DMPC-binding assays demonstrate an identical vesicle clearance rate shared by both the mutant and the wild-type apoE C-terminal domain. In addition, electron microscopic results show identical recombinant HDL particles prepared with both the mutant and the wild-type proteins. These results indicate that residues F257, W264, V269, L279, and V287 are critical residues for aggregation but may not be important in maintaining the structure, stability, and lipid-binding activity of this apoE domain, suggesting that apoE may use different "epitopes" for its aggregation property, helical structure/stability, and lipid-binding activity. Finally, preliminary NMR data demonstrated that we have collected high-quality NMR spectra, allowing for an NMR structural determination of the apoE C-terminal domain.