Hybrid association between human apolipoproteins A-I and A-II in aqueous solution and in phospholipid recombinants

The formation of hybrid association products between apolipoprotein A-I and apolipoprotein A-II from human high-density lipoprotein was investigated in solutions of these apolipoprotein and in recombinant particles with dimyristoylphosphatidylcholine (DMPC). It was found that these two proteins interact in solution to form hybrid association products, but not to a marked degree. When these two proteins were incubated together with DMPC, it was likewise found that there was little tendency to reside on the same particle, as judged from the absence of hybrid oligomers by chemical cross-linking. By a modified immunoelectrophoretic method it was found that only about 15% of the A-II and 10% of the A-I were precipitated by the heterologous antiserum; from this it is concluded that 80-90% of these proteins do not form hybrid recombinants with the other protein. These results suggest that in the delipidated state, as well as in discoidal recombinants, there do not exist strong protein-protein interactions between A-I and A-II. This implies that even in the high-density lipoprotein, where both proteins coexist in the same particle, the A-II does not stabilize the molecular structure through interactions with A-I, and its role in this molecule remains obscure.