THE NUCLEOLIN BINDING-ACTIVITY OF HEPATITIS-DELTA ANTIGEN IS ASSOCIATED WITH NUCLEOLUS TARGETING

Hepatitis delta antigens (HDAgs) are important for the replication and assembly of hepatitis delta virus (HDV), To understand the associatio n between HDAgs and cellular proteins and the mechanism of viral multi plication, we have studied the interaction between HDAgs and nucleolin , a major nucleolar phosphoprotein, The interaction between HDAgs and nucleolin was first demonstrated by immunofluorescence staining studie s, HDAgs and endogenous nucleolin were colocalized in the nucleoli of cultured cells transfected with plasmids encoding the small and large HDAg, Coimmunoprecipitation results indicated that the NH2-terminal do main of HDAg was essential for its binding to nucleolin, In vitro liga nd binding assays revealed two nucleolin binding sites, NBS1 and NBS2, Each spanned amino acid residues 35-50 and 51-65, respectively, with a conserved core sequence H(K/R)XK. HDV replication was modulated by e xogenous human nucleolin, In addition, a small HDAg mutant S-d65/75, w hich possesses both NBS1 and NBS2, was capable of transactivating HDV replication, whereas the small HDAg mutant S-d50/75, which retained NB S1 but not NBS2, was unable to support the replication of HDV, Thus, t he nucleolin binding activity of HDAg is critical for its nucleolar ta rgeting and is involved in the modulation of HDV replication.