Discrete carboxyl-terminal segments of apolipoprotein E mediate lipoprotein association and protein oligomerization

The carboxyl terminus of apolipoprotein (apo) E is required for lipoprotein association and for tetramer formation. To correlate these roles with specific regions within the carboxyl terminus, a series of apoE3 variants with carboxyl-terminal truncations at residues 266, 244, 223, and 191 were expressed in Escherichia coli. As determined by gel permeation and sedimentation equilibrium centrifugation, the four truncated variants were monomeric in solution. Compared to native apoE3 (299 residues), all had reduced affinity for lipoproteins, as assessed by incubation of 125I-labeled proteins with plasma followed by fractionation of lipoprotein classes by gel filtration. The 266-residue variant associated with very low density lipoproteins and high density lipoproteins, but was partly non-lipoprotein-bound (25% of total). Shorter variants, with 244 or fewer residues, did not associate with very low density lipoproteins and only associated slightly (approximately 20%) with high density lipoproteins, with the major portion non-lipoprotein-bound (65-73%). After these proteins were injected into rabbits, the clearance rate was proportional to the plasma level of non-lipoprotein-bound protein. These results indicate lipoprotein association modulates the clearance of apoE, residues within the segment 267-299 are critical for apoE tetramerization and facilitate lipoprotein association, and residues within the segment 245-266 also contribute to lipoprotein association.