HDV Produces Two Forms of HDAg from the Same Gene

Hepatitis delta virus (HDV) is often compared to viroids because of the characteristic unbranched rod secondary structure formed by its RNA and the relatively small size of its genome. However, unlike viroids, HDV does contain one gene that encodes the sole viral protein, HDAg. Early analyses showed two electrophoretic forms of HDAg in liver and viral particles isolated from serum (Bergmann andGerin 1986; Bonino et al. 1981,1984,1986). (These forms were sometimes referred to by their apparent molecular weights, p-24 and p-27; they are denoted here as S-HDAg and L-HDAg for short and long, respectively.) Following the cloning of HDV cDNAs (Makino et al. 1987; Wang et al. 1986), a series of studies illuminated the functional roles of S-HDAg and L-HDAg in HDV replication: S-HDAg is required for replication of HDV RNA, and L-HDAg is required for the formation of HDV particles (Chang et al. 1991; Glenn et al. 1992; Hwang et al. 1992). Early studies found that L-HDAg also inhibits HDV RNA replication (Chao et al. 1990; Kuo et al. 1989), but more recent analyses suggestthatthismight notalwaysbethe case, particularlyfor antigenome RNA synthesis (Macnaughton and Lai 2002; Modahl and Lai 2000).

Cloning and sequencing of the genome in 1986 indicated heterogeneity at several positions in the 1679 nucleotide (nt) genome (Wang et al. 1986). This variability affected the predicted length of HDAg: some clones contained a UAG (amber) stop as the 196th codon and encoded a 195 amino acid protein, other clones had UGG at this location and encoded a protein 214 amino acids in length (Wanget al. 1986; Xiaet al. 1990). Expression of protein from clones that contained either the UAG or UGG sequence showed that the former encoded S-HDAg and the latter L-HDAg (Weiner et al. 1988; Xia et al. 1990). Subsequently, a series of studies in cultured cells and in a chimpanzee infected by injection of an HDV cDNA clone led to the remarkable discovery that the heterogeneity at this position arose during the course of HDV replication. Although transfected cDNAs encoded only S-HDAg, both S-HDAg and L-HDAg were detected (Luo et al. 1990; Sureau et al. 1989). No L-HDAg was detected when cells were transfected with an expression construct for S-HDAg that did not produce replicating HDV RNA. Thus, the appearance of L-HDAg was linked to HDV replication. Because of the different functions of S-HDAg and L-HDAg, the synthesis of L-HDAg late in the replication cycle is an example of a classic switch from viral RNA replication to genome packaging.

Analysis of HDV RNA isolated from the serum of the transfected chimpanzee and from transfected cultured cells showed that heterogeneity appeared at the position corresponding to the adenosine in the UAG stop codon for S-HDAg (Luo et al. 1990). Subsequent studies in transfected cells showed that the appearance of L-HDAg and sequence heterogeneity at this site are temporally correlated; moreover, mutations that abolished the appearance of heterogeneity also prevented L-HDAg production (Casey et al. 1992). These studies indicated that some genomes encoding S-HDAg are converted, or edited, to encode L-HDAg during the course of HDV replication. The site at which editing occurs has been termed amber/W in accord with the codon change that accompanies the sequence modification. Because of the essential functions of S-HDAg and L-HDAg editing plays a central role in the HDV replication cycle.

0 0

Post a comment