miRNAs are a species of small-regulatory RNA that post-transcriptionally
regulate gene expression via the RNA induced silencing complex (RISC). They
are encoded ubiquitously among animals and plants, and have recently been
shown to be encoded by the majority of herpesviruses. It seems likely that
herpesvirus encoded miRNAs have evolved as a tool for the manipulation of
host-cellular and viral-gene expression during infection.
Human cytomegalovirus (HCMV) is a clinically important herpesvirus that
represents a significant cause of morbidity and mortality in the immune-compromised.
HCMV encodes as many as 25 miRNAs during infection, but the
function of the majority of these is not known.
Identifying the targets of HCMV miRNAs will not only establish a basis for
understanding the role of miRNAs within the context of HCMV infection, but
also provide a means for discovering novel host-virus interactions. Using RISC
immunoprecipitation and siRNA screening, host-cellular targets of viral miRNAs
that play important roles in the biology of HCMV were identified. ATP6VOC, a
key component of the vacuolar-ATPase, was shown to be a target of miR-US25-1
and subsequent siRNA knockdown of ATP6VOC resulted in the almost complete
inhibition of infectious virion production. Despite this, ATP6VOC knock-down
did not inhibit viral entry, DNA synthesis, or gene expression, highlighting a
possible role for ATP6VOC in the assembly and egress of HCMV.
A critical step in HCMV assembly and egress is the formation of the juxta-nuclear
virion assembly compartment (VAC). The HCMV VAC is derived from host-cellular
endocytic and secretory vacuoles, and is crucial for the efficient nuclear
egress of nucleocapsids, cyotplasmic tegumentation, final envelopment, and the
egress of mature virions. Using siRNA knock-down, immunofluorescence-microscopy,
and western-blot analysis, a crucial role for ATP6VOC and
v-ATPase function in the formation of the VAC was demonstrated. siRNA knock-down
of ATP6VOC resulted in a failure in the reorganisation of trans-golgi and
early-endosomal compartments during infection, resulting in a failure in VAC
formation.
These findings demonstrate a crucial role for ATP6VOC during infection, and in
so doing identify a novel host factor that is required for HCMV assembly
