Purification and characterization of two high-density-lipoprotein-binding proteins from rat and human liver.
The liver plays a major role in the metabolism of plasma high-density lipoprotein (HDL). Several groups have postulated, but others refuted, the existence of a classical membrane receptor which recognizes HDL. In the present study, we identified and purified two liver HDL-binding proteins of 120 kDa (HB1) and 100 kDa (HB2), with apparent specificity for HDL3 devoid of E apolipoprotein. The plasma membrane was the richest source of the HDL-binding protein. Both proteins bound A-I and A-II apolipoproteins and retained HDL-binding activity after final purification. HB1 activity, but not that of HB2, was lost after treatment with beta-mercaptoethanol, but reduction did not change the apparent molecular mass of either protein. Antibodies against HB1 or HB2 did not cross-react, and preliminary structural investigations provide evidence to suggest that HB1 and HB2 are not structurally related. We thus provide evidence for at least two liver plasma-membrane proteins which bind HDL apolipoproteins, suggesting that protein-protein interaction participates to some degree in the mechanism of HDL recognition by cells.