Products of the "X" gene in hepatitis B and related viruses

The X region in hepatitis B virus DNA potentially encodes a polypeptide 154 amino acids in length. Two synthetic peptides spanning residues 100 to 115 (peptide 99) and 115 to 131 (peptide 100) in a hydrophilic domain within the carboxy terminal third of the proposed gene product were made and used to raise peptide antisera in rabbits. Such antisera specifically bound to X reactive determinants in liver-derived core antigen particles from humans (HBcAg), ducks (DHBcAg), ground squirrels (GSHcAg) and woodchucks (WHcAg) at each step of core antigen purification. This reactivity was blocked by addition of excess synthetic peptide, and neither sera were reactive with other purified antigens such as HBsAg. Individual polypeptides associated with these core particles were also reactive by Western blotting. These findings suggest that X reactive determinants are present in the core particles of hepatitis B virus and related viruses, and that one or more core-associated polypeptides may have both X and core antigenic determinants. The possible significance of these observations upon the genetic organization and expression of the X gene is discussed.