The Hepatitis B Virus Receptor: Book'em, Dano?

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Abstract
In a previous study, we have identified endonexin II (E–II) on human liver plasma membranes as a specific, Ca–dependent, small hepatitis B surface antigen (HBsAg)–binding protein. In this article, we describe the spontaneous development of anti–HBs antibodies in rabbits immunized with native or recombinant human liver E–II and in chickens immunized with the F(ab′)2 fragment of rabbit anti–human liver E–II immunoglobulin G. Anti–HBs activity was not observed in rabbits immunized with rat liver E–II. Cross–reactivity of anti–E–II antibodies to HBsAg epitopes was excluded, since anti–HBs and anti–E–II activities can be separated by E–II affinity chromatography. The existence of an anti–idiotypic antibody is further demonstrated by competitive binding of human liver E–II and this antibody (Ab2) to small HBsAg, suggesting that Ab2 mimics a specific E–II epitope that interacts with small HBsAg. In addition, it was demonstrated that anti–HBs antibodies developed in rabbits after immunization with intact human liver E–II or in chickens after immunization with F(ab′)2 fragments of rabbit anti–human liver E–II immunoglobulin G recognize the same epitopes on small HBsAg. These findings strongly indicate that human liver E–II is a very specific small HBsAg–binding protein and support the assumption that human liver E–II is the hepatitis B virus receptor protein. Binding of viral envelope proteins to specific receptors on human hepatocytes is considered to be an important step in HBV infection. In this study, we demonstrate that a 34–kDa human liver plasma membrane protein specifically binds to small HBsAg in a Ca2+–dependent manner. By partial amino acid sequence analysis of preparatively isolated 34–kDa protein comigrating with HBsAg–binding protein obtained from binding assay on IEF/SDS–PAGE, we have identified this HBsAg–binding protein as Endonexin II (E–II). Native human liver E–II inhibits binding of HBsAg to intact human hepatocytes and shows specific binding to small HBsAg. This binding can be inhibited by human liver plasma membrane proteins, recombinant E–II, or anti–E–II antibodies. Despite 90% sequence homology, rat liver E–II does not bind to small HBsAg and does not inhibit significantly (less than 20%) binding of HBsAg to intact hepatocytes. Cross–linking of small HBsAg and radiolabeled human liver E–II resulted in a specific additional protein complex on PAGE with an apparent molecular weight of 90 kDa, corresponding to a complex of E–II and small HBsAg with a ratio of 2 to 1 or 1 to 2. These findings indicate that E–II, found in human liver, is a specific HBsAg–binding protein and might play an important role in the initiation of HBV infection.