The binding of specific plasma lipoproteins to the high affinity cell surface receptors of fibroblasts is determined by the presence of specific apoproteins, the apo-B of low density lipoproteins and the apo-E of certain high density lipoproteins. Despite the observations which indicated that the recognition site responsible for receptor interaction resides exclusively with the B or E apoproteins, total delipidation of the apo-E-containing lipoproteins abolished their ability to bind to the receptors. To test the postulate that the phospholipid portion of the lipoprotein was essential to establish and maintain the proper conformation of the recognition site, the E apoprotein was added to dimyristoylphosphatidylcholine(DMPC) vesicles and the protein l phospholipid complexes were isolated and tested for binding activity. It was established that the binding activity could be restored to the canine or human E apoproteins following apo-E l DMPC complex formation. The apo-E* DMPC binding activity was equal to or greater than that observed with the native apo-E-containing canine lipoproteins, an activity which has previously been shown to be lo-to IOO-fold greater than that observed for low density lipoproteins. Furthermore, the E apoprotein in the DMPC complexes was taken up and degraded by the fibroblasts in binding, internalization, and degradation studies performed at 35 C. Qnly apo-EaDMPC possessed binding activity and not DMPC complexes with AI, C-II, or C-III human apoproteins. Thus, phospholipids appear to function not only to bind the apoprotein but also to confer to the E apoprotein the requisite physical state or conformation required for its binding to the cell surface receptor.

Specific plasma lipoproteins bind to high affinity receptors on the plasma membranes of several different cell types (for review see Refs. 1 and 2). The two classes of lipoproteins that bind to the cell surface receptors are the low density lipoproteins (LDL)’and a special class of lipoproteins referred to as