INTERACTION OF APOLIPOPROTEIN-AII WITH THE PUTATIVE HIGH-DENSITY-LIPOPROTEIN RECEPTOR

There is strong evidence to indicate that binding of HDL by cells is d ue to recognition of apoproteins residing on the surface of the lipopr otein by the putative HDL receptor(s). Although both of the major HDL apoproteins, AI and AII, are recognized by the putative receptor, the nature of the binding interaction and the domains of the apoproteins i nvolved are largely unknown. Previous data from this laboratory led to the proposal of a model to explain how HDL particles containing AII i nteracted with the HDL receptor in a different manner as compared to H DL particles which contain apoAI but not apoAII [Vadiveloo, P.K., & Fi dge, N.H. (1992) Biochem. J. 284, 145-151]. The model predicted that e ach chain of the apoAII homodimer contained a binding domain capable o f interacting with the HDL receptor. This model was tested in the curr ent study by preparing apoAII monomers, complexing them with phospholi pid, and determining the ability of these complexes to bind to putativ e HDL receptors in rat liver plasma membranes (RLPM) and bovine aortic endothelial cell membranes (BAECM) by ligand blotting. The data showe d that these complexes were bound by HB1 and HB2 from RLPM, and to the 110-kDa HDL binding protein from BAECM, providing critical evidence t o support the model. Further investigation into the binding interactio n revealed that apoAII complexed with phospholipid (apoAII-PC) bound m ore than delipidated apoAII, which bound more than delipidated apoAII monomers. Thus, optimum binding required the presence of lipid. Since the amount of binding correlated with the amount of alpha-helical cont ent of the molecules (i.e., apoAII-PC > delipidated apoAII > delipidat ed apoAII monomers), these findings are consistent with the hypothesis that alpha-helical regions are involved in recognition of apoAII by t he putative HDL receptor. ApoAII has a single methionine residue at po sition 26. Investigations into whether the oxidation state of the meth ionine residue affected receptor interaction were undertaken. The resu lts of these experiments showed that apoAII with methionine residues i n either the reduced or the oxidized state was able to bind to the put ative HDL receptors.