Unique properties of the large antigen of hepatitis delta virus

The large form of the hepatitis delta virus (HDV) protein (L) can be isopre nylated near its C terminus, and this modification is considered essential for particle assembly. Using gel electrophoresis, we separated L into two s pecies of similar mobilities. The slower species could be labelled by the i ncorporation of [C-14]mevalonolactone and is interpreted to be isoprenylate d L (L-i). In serum particles, infected liver, transfected cells, and assem bled particles, 25 to 85% of L was isoprenylated. Isoprenylation was also d emonstrated by C-14 incorporation in vitro with a rabbit reticulocyte coupl ed transcription-translation system. However, the species obtained migrated even slower than that detected by labeling in vivo. Next, in studies of HD V particle assembly in the presence of the surface proteins of human hepati tis B virus, we observed the following. (i) Relative to L, L-i was preferen tially assembled into virus-like particles. (ii) L-i could coassemble the u nmodified L and the small delta protein, S. (iii) In contrast, a form of L with a deletion in the dimerization domain was both isoprenylated and assem bled, but it could not support the coassembly of S. Finally, to test the ex pectation that the isoprenylation of L would increase its hydrophobicity, w e applied a phase separation strategy based on micelle formation with the n onionic detergent Triton X-114. We showed the following. (i) The unique C-t erminal 19 amino acids present on L relative to S caused a significant incr ease in the hydrophobicity. (ii) This increase was independent of isoprenyl ation. (iii) In contrast, other, artificial modifications at either the N o r C terminus of S did not increase the hydrophobicity. (iv) The increased h ydrophobicity was not sufficient for particle assembly; nevertheless, we sp eculate that it might facilitate virion assembly.