Solution structure and RNA-binding activity of the N-terminal leucine- repeat region of hepatitis delta antigen

Abstract

Hepatitis delta virus (HDV) is a satellite virus of the hepatitis B virus (HBV) which provides the surface antigen for the vital coat. The RNA genome of HDV encodes two proteins: the small delta antigen and the large delta antigen. The two proteins resemble each other except for the presence of an additional 19 amino acids at the C terminus of the latter species. We have found that the N-terminal leucine-repeat region of hepatitis delta antigen (HDAg) binds to the autolytic domain of HDV genomic RNA and attenuates its autolytic activity. A 27-residue polypeptide corresponding to residues 24-50 of HDAg, designated dAg24-50, was synthesized, and its solution structure was found to be an α-helix by circular dichroism and 1H- nuclear magnetic resonance (NMR) techniques. Binding affinity of dAg24-50 with HDV genomic RNA was found to increase with its α-helical content, and it was further confirmed by modifying its N- and C-terminal groups. Furthermore, the absence of RNA binding activity in the mutant peptides, dAgM(24-50am) and dAgM(Ac24-50am), in which Lys38, Lys39, and Lys40 were changed to Glu, indicates a possible involvement of these residues in their binding activity. Structural knowledge of the N-terminal leucine-repeat region of HDAg thus provides a molecular basis for the understanding of its role in the interaction with RNA.