Three sites of the hepatitis B virus X protein cooperatively interact with cellular proteins

The X protein of hepatitis B virus is known to be a trans-activator of viral and cellular genes and to be a serine protease inhibitor as well. X protein has no DNA-binding activity, but is postulated to exert its trans-activation function by interacting with cellular proteins. To investigate interaction sites of X protein with cellular proteins, we carried out an immunoprecipitation inhibition assay using several different anti-X antibodies in the presence or absence of cellular proteins. Results elucidated three separate sites (aa 65-72, aa 105-115, and aa 131-142; U22, X1, and Z44 sites, respectively) of the X protein that cooperatively interacted with cellular proteins. Analyses with a series of mutant X proteins also supported the interactions at the U22, X1, and Z44 sites. Based on the CAT activity assay, the essential regions for the trans-activation function of X protein overlapped with these three interaction sites. Furthermore, these interaction sites also coincide with the structures necessary for the serine protease inhibitor activity. Thus, the trans-activation function and serine protease inhibitor activity of X protein may be exerted by interaction with cellular proteins through at least these three sites.