For the pestivirus bovine viral
diarrhea virus (BVDV) cytopathogenic (cp) and noncytopathogenic
(noncp) strains can be distinguished in cell culture. Only the
noncp strains have the capability to establish in cattle lifelong
persistent infections. Enhanced viral RNA synthesis and permanent
expression of viral non-structural protein 3 (NS3) are correlated
with the cytopathogenicity of BVDV. NS3 is an essential component
of the viral replicase which can not be functionally replaced
by uncleaved NS2-3. In general, the release of NS3 by cleavage
of NS2-3 is catalyzed by an autoprotease residing in NS2. Surprisingly,
in noncp BVDV infected cells efficient cleavage of NS2-3 is restricted
to the first hours after infection; thereby viral RNA replication
is downregulated at later time points. According to our data the
cellular J-domain protein Jiv (J-domain protein interacting with
viral protein) represents an essential cofactor of the viral NS2
protease. In noncp BVDV infected cells the intracellular amount
of Jiv limits the number of released NS3 molecules and thereby
the number of active replication complexes. When this limitation
is abrogated, e.g. by an increase of the intracellular Jiv level,
also noncp BVDV strains become cytopathogenic. cp BVDV strains
generate NS3 independent of cellular Jiv by a variety of different
mechanisms depending in each case on the specific genomic alterations
present in the respective genome.
In conclusion, our data demonstrate that the regulation of the
viral NS2 autoprotease by a cellular cofactor is crucial for the
capability of BVDV to establish lifelong persistent infection.
(i) Jiv and its fragment Jiv90 can induce NS2-3 cleavage in the
context of NS2-3, as well as in trans. Analysis of Jiv / NS2 interaction
revealed two Jiv binding sites in NS2. The consequences of Jiv
binding to NS2 shall be clarified on structural level.
(ii) The NS2 autoprotease can be divided into a protease domain
and a substrate domain. When both domains are coexpressed together
with Jiv90 an active NS2 protease assembles. This process shall
be studied with purified components. Provided that further cellular
factors are essential these are to be identified and characterized.
(iii) Role of the viral NS proteins in cytopathogenicity: Surprisingly,
expression of NS2-3-4A of cp BVDV (NS3 release) as well as of
noncp BVDV (no NS3 release) in eukaryotic cells induces cell death
by apoptosis. It shall be determined which NS proteins determine
the viral biotype.