Putative Delta Antigen Kinase

HDAg presents a very complicated phosphorylation pattern. No single ser-ine/threonine mutation can abolish its phosphorylation completely. The conserved serine 2, threonine/serine 95, serine 123 and serine 177 are most likely residues for phosphorylation in HDAg. If the phosphorylation is critical for HDV replication, phosphorylation at the remaining nonconserved serine/threonine residues may not have significant effect for HDV

Kinase inhibition assay suggested that the CK II phosphorylates serine 2 and serine 123 (Yeh et al. 1996). However, there is still no direct evidence to prove serine 2 and 123 are phosphorylated in vivo by CK II.

Ion trap tandem mass spectrometry confirmed that serine 177 is phosphorylated in an S-HDAg-expressing stable cell line (Chen et al. 2002). An in-gel kinase assay indicated that double-stranded RNA activated kinase (PKR) could phophorylate S-HDAg. Furthermore, immunoprecipitation-purified endogenous PKR could phosphorylate recombinant SHDAg at serine 177,180, and threonine 182 (Chen et al. 2002). Among the residues phosphorylated in vitro by PKR, serine 177 is the only one that has also been identified to be phosphorylated in vivo. Apart from the phosphorylated residues on S-HDAg, another critical question for HDAg phosphorylation is the nature and number of kinase(s). Previous studies have shown that HDV replication was not reduced in cells expressing dominant negative PKR mutants. Therefore, PKR was considered to be a regulatory kinase but not the essential kinase for HDAg phosphorylation. In HDV cDNA-transfected cells, HDV replication was not suppressed by interferon treatment even though the level of PKR was increased. This result implies that part of the function of S-HDAg may resemble the trans-acting protein (Tat) of HIV-1 and vaccinia virus K3L protein that behave as a decoy substrate to inhibit PKR activity.

To more completely understand the role of phosphorylated S-HDAg in the HDV life cycle, it will be essential to further identify the HDAg modification sites and the responsible kinase(s).

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