Redistribution of the delta antigens in cells replicating the genome of hepatitis delta virus

When the small form of the delta antigen (delta Ag-S) was expressed from a cDNA expression plasmid and subsequently detected by immunofluorescence, it was found localized to the nucleoli. However, if the cDNA was cotransfected with a cDNA expressing a mutated hepatitis delta virus (HDV) genome that could only replicate by using the delta Ag-S provided by the first plasmid, then most of the delta Ag-S was redistributed to the nucleoplasm, largely to specific discrete nucleoplasmic sites or speckles; this pattern was stable for at least 50 days after transfection. These speckles coincided with those detected with an antibody to SC35, an essential non small nuclear ribonucleoprotein splicing factor. Others have shown that SC35 speckles correspond to active sites of DNA-directed transcription by RNA polymerase II and also of RNA processing. We also found, in contrast to the cotransfections with the mutant HDV and the delta Ag-S provided in trans, that cells transfected with, wild-type HDV showed a variable pattern of staining. The SC35-like speckle pattern of accumulation of delta antigen (delta Ag) was maintained for only 6 days, after which the pattern began to change. By 18 days posttransfection, a variety of different delta Ag staining patterns were observed. This pattern of change occurs at a time when the large form of the delta antigen (delta Ag-L) appears and HDV RNA synthesis begins to shut down. Our studies therefore support the interpretation that HDV RNA and delta Ag-S accumulate at SC35 speckle sites in the nucleoplasm. We speculate that these may be the sites at which HDV RNA is transcribed by RNA polymerase II and/or sites of HDV RNA processing. Furthermore, when delta Ag-L, as well as other mutant delta Ag accumulate, the speckle association is disrupted, thereby stopping HDV RNA replication.