HOST CELL-VIRUS CROSS-TALK - PHOSPHORYLATION OF A HEPATITIS-B VIRUS ENVELOPE PROTEIN MEDIATES INTRACELLULAR SIGNALING

Phosphorylation of cytosolic pre-S domains of the duck hepatitis B vir us (DHBV) large envelope protein (L) was identified as a regulatory mo dification involved in intracellular signaling. By using biochemical a nd mass spectrometric analyses of phosphopeptides obtained from metabo lically radiolabeled L protein, a single phosphorylation site was iden tified at serine 118 as part of a PX(S/T)P motif, which is strongly pr eferred by ERR-type mitogen-activated protein kinases (MAP kinases). E RK2 specifically phosphorylated L at serine 118 in vitro, and L phosph orylation was inhibited by a coexpressed MAP kinase-specific phosphata se. Furthermore, L phosphorylation and ERR activation were shown to be induced in parallel by various stimuli. Functional analysis with tran sfected cells showed that DHBV L possesses the ability to activate gen e expression in trans and, by using mutations eliminating (S-->A) or m imicking (S-->D) serine phosphorylation, that this function correlates with L phosphorylation. These mutations had, however, no major effect s on virus production in cell culture and in vivo, indicating that L p hosphorylation and transactivation are not essential for hepadnavirus replication and morphogenesis. Together, these data suggest a role of the L protein in intracellular host-virus cross talk by varying the le vels of pre-S phosphorylation in response to the state of the cell.