Differences in the binding capacity of human apolipoprotein E3 and E4 to size-fractionated lipid emulsions

We describe sensitive new approaches for detecting and quantitating protein lipid interactions using analytical ultracentrifugation and continuous size-distribution analysis [Schuck (2000) Biophys. J. 78, 1606-1619]. The new methods were developed to investigate the binding of human apolipoprotein E (apoE) isoforms to size-fractionated lipid emulsions, and demonstrate that apoE3 binds preferentially to small lipid emulsions, whereas apoE4 exhibits a preference for large lipid particles. Although the apparent binding affilnity for large emulsions is similar (K-d approximate to 0.5 muM), the maximum binding capacity for apoE4 is significantly higher than for apoE3 (3.0 and 1.8 amino acids per phospholipid, respectively). This indicates that apoE4 has a smaller binding footprint at saturation. We propose that apoE isoforms differentiate between lipid surfaces on the basis of size, and that these differences in lipid binding are due to a greater propensity of apoE4 to adopt a more compact closed conformation. Implications for the role of apoE4 in blood lipid transport and disease are discussed.