A cellular protein which binds hepatitis B virus but not hepatitis B surface antigen

The envelope of hepatitis B virus (HBV) consists of three related proteins known as the large (L), middle (M) and small (S) hepatitis B surface antige ns (HBsAg), L-HBsAg has a 108-119 amino acid extension at the N terminus co mpared with M-HBsAg and contains the preS1 sequence of the HBV envelope. Pr evious research has identified this region as the likely virus attachment p rotein which is thought to interact with the cellular receptor for the viru s. However, as the receptor has still not been identified unequivocally, we used the preS1 region of L-HBsAg to screen a human liver cDNA library by t he yeast two-hybrid system. Several positive clones were isolated which enc oded cellular proteins that interacted with the HBV preS1 protein. The spec ificity was examined in an independent manner in experiments in which bacul ovirus-derived glutathione S-transferase (GST)-preS1 was incubated with S-3 5-labelled protein expressed by in vitro translation from the positive clon es. The intensity of the interactions using this alternative approach mirro red those observed in the yeast two-hybrid system and two proteins (an unid entified protein and a mitochondrial protein) were selected for further stu dy. The specificity of the binding reaction between the preS1 protein and t hese two proteins was further confirmed in a competition assay; HBV purifie d from serum, but not purified HBsAg, was able to compete with preS1 and th us block GST-preS1 binding to the unidentified protein but not to the mitoc hondrial protein. The unidentified protein was then expressed as a fusion p rotein with GST and this was able to bind HBV virions in a direct manner.