Transcriptional activation by the product of open reading frame 50 of Kaposi's sarcoma-associated herpesvirus is required for lytic viral reactivation in B cells
Dm. Lukac et al., Transcriptional activation by the product of open reading frame 50 of Kaposi's sarcoma-associated herpesvirus is required for lytic viral reactivation in B cells, J VIROLOGY, 73(11), 1999, pp. 9348-9361
Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is a lymphotropic virus
strongly linked to the development of KS, an endothelial cell neoplasm fre
quent in persons with AIDS. Reactivation from latency in B cells is thought
to be an important antecedent to viral spread to endothelial cells during
KS pathogenesis. Earlier experiments have posited a role for the transcript
ional activator encoded by KSHV open reading frame 50 (ORF50) in such react
ivation, since ectopic overexpression of this protein induces reactivation
in latently infected B cells. Here we have explored several aspects of the
expression, structure, and function of this protein bearing on this role. T
he ORF50 gene is expressed very early in lytic reactivation, before several
other genes implicated as candidate regulatory genes in related viruses, a
nd its expression can upregulate their promoters in transient assays. The p
rotein is extensively phosphorylated in vivo and bears numerous sites for p
hosphorylation by protein kinase C, activators of which are potent stimulat
ors of lytic induction. The C terminus of the ORF50 protein contains a doma
in that can strongly activate transcription when targeted to DNA; deletion
of this domain generates an allele that expresses a truncated protein which
retains the ability to form multimers with full-length ORF50 and functions
as a dominant-negative protein. Expression of this allele in latently infe
cted cells ablates spontaneous reactivation from latency and strikingly sup
presses viral replication induced by multiple stimuli, including phorbol es
ter, ionomycin, and sodium butyrate. These results indicate that the ORF50
gene product plays an essential role in KSHV lytic replication and are cons
istent with its action as a putative molecular switch controlling the induc
tion of virus from latency.