The structure of human apolipoprotein E2, E3 and E4 in solution 1. Tertiary and quaternary structure

Three recombinant apoE isoforms fused with an amino-terminal extension of 43 amino acids were produced in a heterologous expression system in E. coli. Their state of association in aqueous phase was analyzed by size-exclusion liquid chromatography, sedimentation velocity and sedimentation equilibrium experiments. By liquid chromatography, all three isoforms consisted of three major species with Stokes radii of 4.0, 5.0 and 6.6 nm. Sedimentation velocity confirmed the presence of monomers, dimers and tetramers as major species of each isoform. The association schemes established by sedimentation equilibrium experiments corresponded to monomer-dimer-tetramer-octamer for apoE2, monomer-dimer-tetramer for apoE3 and monomer-dimer-tetramer-octamer for apoE4. Each of the three isoforms exhibits a distinct self-association pattern. The apolipoprotein multi-domain structure was mapped by limited proteolysis with trypsin, chymotrypsin, elastase, subtilisin and Staphylococcus aureus V8 protease. All five enzymes produced stable intermediates during the degradation of the three apoE isoforms, as described for plasma apoE3. The recombinant apoE isoforms, thus, consist of N- and C-terminal domains. The presence of the fusion peptide did not appear to alter the apolipoprotein tertiary organization. However, a 30 kDa amino-terminal fragment appeared during the degradation of the recombinant apoE isoforms resulting from cleavage in the 273-278 region. This region, not accessible in plasma apoE3, results from a different conformation of the C-terminal domain in the recombinant isoforms. A specific pattern for the apoE4 C-terminal domain was observed during the proteolysis. The region 230-260 in apoE4, in contrast to that of apoE3 and apoE2, was not accessible to proteases, probably due to the existence of a longer helix in this region of apoE4 stabilized by an interdomain interaction.