HEPATITIS-B VIRUS SURFACE-ANTIGEN BINDS TO APOLIPOPROTEIN-H

We have previously demonstrated that a plasma membrane-enriched fracti on isolated from human liver is capable of binding recombinant hepatit is B surface antigen (rHBsAg) (P. Pontisso, M. A. Petit, M. Bankowski, and M. E. Peeples, J. Virol. 63:1981-1988, 1989). In this study we ha ve separated the plasma membrane proteins by sodium dodecyl sulfate-po lyacrylamide gel electrophoresis and used a ligand-blotting technique to identify a 46-kDa rHBsAg-binding protein. This protein could be rem oved from the membranes with a weakly acidic buffer, implying that it is peripherally bound. Examination of human serum revealed that the 46 -kDa binding protein is a serum protein. Isolation of plasma lipoprote ins revealed that the binding protein is in part associated with chylo microns and high-density lipoproteins, both of which are targeted to t he hepatocyte during the normal course of lipid metabolism. The bindin g protein was identified as apolipoprotein H (apo H), also known as be ta 2-glycoprotein I, on the basis of copurification of the rHBsAg-bind ing activity with the apo H protein and the ability of cDNA-expressed apo H to bind rHBsAg. Serum-derived HBsAg also binds to apo H, indicat ing that binding is not unique to rHBsAg. Binding is saturable, requir es only the small S protein of rHBsAg, and is inhibited by excess rHBs Ag, antibodies to HBsAg, and antibodies to apo H. The binding activity of apo H is destroyed upon reduction, indicating that 1 or more of it s 22 disulfide bonds are required for interaction with rHBsAg. The pos sibility that an interaction between hepatitis B virus particles and l ipoprotein particles may facilitate entry of the virus into hepatocyte s is discussed.