Domains of apoE Required for Binding To apoE Receptor 2 and To Phospholipids: Implications For The Functions Of apoE in the Brain

We have studied the contribution of the carboxy terminal domains of lipid-free apoE isolated from apoE-expressing cell cultures in binding to phospholipids and have determined the affinities of reconstituted POPC−apoE particles for the apoER2. It was found that the initial rate of association of apoE2, apoE3, apoE4, and a mutant form apoE4R158M to multilamellar DMPC vesicles was similar and was reduced and eventually diminished by gradual deletion of the carboxy terminal segments. The truncated apoE forms retained their ability to associate with plasma lipoproteins. Receptor binding studies were performed using the ldlA-7 cells expressing apoER2 and transiently transfected COS-M6 and the appropriate control untransfected cells. Specific binding to apoER2 was obtained by subtracting from the total binding to the receptor-expressing cells the nonspecific binding values of the untransfected cells. POPC−apoE particles generated using apoE3, apoE4, the truncated apoE4-259, apoE4-229, apoE4-202, and apoE-165, and the mutant apoE4R158M all bound tightly to the apoER2 (Kd range of 12 ± 3 to 19 ± 4 μg/mL). POPC−apoE2 bound with reduced affinity (Kd = 31 ± 5.3 μg/mL). The findings establish that the apoER2 binding domain of apoE is in the 1−165 amino terminal region, whereas the carboxy terminal 230−299 region of apoE is required for efficient initial association with phospholipids.