Large hepatitis delta antigen is phosphorylated at multiple sites and phosphorylation is associated with protein...

Hepatitis delta antigen (HDAg) consists of two species, small HDAg (SHDAg) and large HDAg (LHDAg), which are identical in sequence with the exception that the large form contains an additional 19 amino acids at the C-terminus. Both HDAgs are nuclear phosphoproteins. However, LHDAg is hyperphosphorylated, i.e. it is at least 10 times more phosphorylated than SHDAg. To determine the phosphorylation site(s) of the LHDAg, we mutated all the conserved serine residues and expressed these mutant proteins using a recombinant baculovirus expression system. By labeling insect cells in vivo with (32)P-orthophosphate and immunoprecipitation, we showed that LHDAg is phosphorylated at multiple serine residues. Although LHDAg contains two additional serines at its 19-amino acid extension, mutations of these two amino acids did not affect the overall phosphorylation level. Most importantly, the phosphorylation level of middle domain-deleted LHDAg (M75del) was significantly higher than that of wild-type LHDAg. We conclude that phosphorylation of the LHDAg occurs at multiple sites and that hyperphosphorylation is associated with alteration of protein conformation.