Host factors involved in viral replication are potentially attractive antiviral targets that are complementary to specific inhibitors of viral enzymes, since resistant mutations against the latter are likely to emerge during long-term treatment. It has been reported recently that cyclosporine, which binds to a family of cellular proteins, cyclophilins, inhibits hepatitis C virus (HCV) replication in vitro. Here, the activities of various cyclosporine derivatives were evaluated in the HCV replicon system. There was a strong correlation between the anti-HCV activity and cyclophilin-binding affinity of these compounds. Of these, NIM811 has been selected as a therapeutic candidate for HCV infection, since it binds to cyclophilins with higher affinity than cyclosporine but is devoid of the significant immunosuppressive activity associated with cyclosporine. NIM811 induced a concentration-dependent reduction of HCV RNA in the replicon cells with a 50% inhibitory concentration of 0.66 M at 48 h. Furthermore, a greater than three-log 10 viral RNA reduction was achieved after treating the cells with as little as 1 M of NIM811 for 9 days. In addition, the combination of NIM811 with alpha interferon significantly enhanced anti-HCV activities without causing any increase of cytotoxicity. Taken together, these promising in vitro data warrant clinical investigation of NIM811, an inhibitor of novel mechanism, for the treatment of hepatitis C.
Protein priming of viral RNA synthesis plays an essential role in the replication of picornavirus RNA. Both poliovirus and coxsackievirus encode a small polypeptide, VPg, which serves as a primer for addition of the first nucleotide during synthesis of both positive and negative strands. This study examined the effects on the VPg uridylylation reaction of the RNA template sequence, the origin of VPg (coxsackievirus or poliovirus), the origin of 3D polymerase (coxsackievirus or poliovirus), the presence and origin of interacting protein 3CD, and the introduction of mutations at specific regions in the poliovirus 3D polymerase. Substantial effects associated with VPg origin were traced to differences in VPg-polymerase interactions. The effects of 3CD proteins and mutations at polymerase-polymerase intermolecular Interface I were most consistent with allosteric effects on the catalytic 3D polymerase molecule. In conclusion, the efficiency and specificity of VPg uridylylation by picornavirus polymerases is greatly influenced by allosteric effects of ligand binding that are likely to be relevant during the viral replicative cycle.Picornaviruses have a single copy, positive-strand RNA genome with a small peptide, VPg (3B), covalently attached to the 5Ј-terminal nucleotide and a poly(A) segment at the 3Ј end. RNA replication occurs in the cytoplasm of infected cells, proceeding through negative-strand intermediates which, in turn, serve as templates for production of progeny positive strands. A long-standing question has been how both positiveand negative-strand syntheses are initiated. Characterization of a reaction catalyzed by poliovirus 3D polymerase, in which the tyrosine of the VPg peptide was trans-esterified (uridylylated) in the presence of a poly(A) template to form VPg-pU (pU), provided insight into the mechanism of strand initiation (34): given the ability of poliovirus 3D polymerase to uridylylate VPg, the peptide-nucleotide conjugate could serve as the protein primer for progeny RNA strand synthesis. VPg must bind directly to poliovirus polymerase, because it can serve as an enzymatic substrate; furthermore, VPg-polymerase interactions have been observed in two-hybrid experiments (42). While the binding site on the poliovirus 3D polymerase for the VPg substrate of the uridylylation reaction has not yet been characterized, the binding site for its likely proteolytic precursor, 3AB, has been identified as a distinct site on the back of the palm of the polymerase molecule via extensive alaninescanning mutagenesis (22).Although VPg uridylylation could provide a protein primer for use in either positive-or negative-strand synthesis, this reaction is not sufficient to describe the mechanism of initiation for viral RNA synthesis in infected cells. For example, the use of a poly(A) template for VPg uridylylation does not provide specificity for a particular virus. This specificity could be provided if an RNA sequence or structure within the picornavirus genome were the authentic RNA template for VPg uridylylation...
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