X Protein of Hepatitis B Virus Resembles a Serine Protease Inhibitor

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Abstract
The × protein of hepatitis B virus (HBV) has been shown to be a trans‐activator for viral and cellular genes. Amino acid sequences in × protein were found to be highly homologous to functionally essential sequences in the “Kunitz domain,’characteristic of Kunitz‐type serine protease inhibitors. Mutations at these sequences completely abolished trans‐activation. Consequently, HBV × protein resembles a serine protease inhibitor or its analogue, and may bring about trans‐activation by activating certain transcriptional factors through proteolytic cleavage alteration.