FUNCTIONAL AND STRUCTURAL SIMILARITY BETWEEN THE X-PROTEIN OF HEPATITIS-B VIRUS AND NUCLEOSIDE DIPHOSPHATE KINASES

One of the four genes encoded by hepatitis B virus (HBV) is the regula tory 17 kDa protein called HBx (or pX). HBx is a transcription transac tivator of many cellular and viral regulatory elements. We report here that recombinant HBx supports transcription in vitro and has phosphot ransfer enzymatic activity. In the presence of EDTA, a phosphoryl-HBx is formed that releases the phosphate residue upon the addition of Mg2 +. This two-step NTP hydrolysis reaction is characteristic of a group of enzymes termed nucleoside diphosphate kinases (NDPKs). Remarkably, structural similarity between HBx and NDPKs is also evident. Our findi ngs suggest that HBx has evolved from this group of enzymes but acquir ed additional activities that satisfy the viral needs.