Characterization of RNA-binding domains of hepatitis delta antigen.

Hepatitis delta antigen (HDAg), the only protein encoded by the hepatitis delta virus (HDV), binds specifically genomic and antigenomic strands of the HDV RNA. In a previous study, three recombinant HDAg subdomains were synthesized, covering residues 11 to 78, 79 to 163 and 164 to 212, and only the middle domain was shown to be responsible for the binding to HDV RNA. To investigate HDAg sequences involved in HDV RNA binding, we synthesized five peptides, 15 to 29 residues in length, and tested their ability to bind HDV RNA using a simple non-radioactive ELISA with digoxigenin-labelled HDV genomic or antigenomic RNA probes. The specificity of interactions was demonstrated by comparison with control peptides and non-HDV RNA probes, and with an inhibition assay using recombinant HDAg. The HDAg-binding domain found within the middle region (79 to 163) of HDAg was more finely mapped: it is located between residues 79 and 107. In addition, another domain (residues 2 to 27) of HDAg was also found to bind specifically to HDV RNA. These two peptides share sequence similarities at residues 2 to 10 and 97 to 107 with other RNA-binding domains.