RESUMO
Both forms of the hepatitis delta antigen (HDAg) encoded by hepatitis delta virus are active only as oligomers. Previous studies showed that quadrin, a synthetic 50-residue peptide containing residues 12-60 from the N-terminus of HDAg, interferes with HDAg oligomerization, forms an alpha-helical coiled coil in solution, and forms a novel square octamer in the crystal consisting of four antiparallel coiled-coil dimers joined at the corners by hydrophobic binding of oligomerization sites located at each end of the dimers. We designed and synthesized deltoid (CH3CO-[Cys23]HDAg-(12-27)-seryl-tRNA synthetae-(59-65)-[Cys42]HDAg-(34-60)-Tyr-NH2), a chimeric protein that structurally resembles one end of the quadrin dimer and contains a single oligomerization site. The 51-residue chain of deltoid contains a seven-residue alpha-hairpin loop in place of the remainder of the quadrin dimer plus Cys12 and Cys31 for forming an intrachain disulfide bridge. Reduced, unbridged deltoid (Tm=61 degrees C, DeltaG(H2O)=-1.7 kcal mol(-1)) was less stable to denaturation by heat or guanidine HCl than oxidized, intrachain disulfide-bridged deltoid (Tm>80 degrees C, DeltaG(H2O)=-2.6 kcal mol(-1)). Each form is an alpha-helical dimer that reversibly dissociates into two monomers (Kd=80 microM).
