HCV NS5A and IRF9 compete for CypA binding.
OBJECTIVE
Cyclophilin A (CypA) is vital for HCV replication. Cyp inhibitors successfully decrease viral loads in HCV-infected patients. However, their mechanisms of action remain unknown. Since interferon (IFN) can also suppress HCV replication, we asked whether a link between CypA and the IFN response exists.


METHODS
We used cellular and recombinant pulldown approaches to investigate the possibility of a specific association of CypA with host ligands.


RESULTS
We found for the first time that CypA binds to a major component of the IFN response - the IFN regulatory factor 9 (IRF9). IRF9 is the DNA-binding component of the transcriptional IFN-stimulated gene factor 3 (ISGF3). CypA binds directly to IRF9 via its peptidyl-prolyl isomerase (PPIase) pocket. Cyp inhibitors such as cyclosporine A (CsA) or non-immunosuppressive derivates such as alisporivir and SCY-635, prevent IRF9-CypA complex formation. CypA binds to the C-terminal IRF-association-domain (IAD), but not to the DNA-binding or linker domains of IRF9. Remarkably, CypA associates with the multimeric ISGF3 complex. We also obtained evidence that CypA neutralization enhances IFN-induced transcription. Interestingly, the hepatitis C virus (HCV) non-structural 5A (NS5A) protein, which is known to modulate the IFN response, competes with IRF9 for CypA binding and can prevent the formation of IRF9-CypA complexes.


CONCLUSIONS
This study demonstrates for the first time that CypA binds specifically to a component of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, IRF9. This study also reveals a novel opportunity of HCV to modulate the IFN response via NS5A.
Department of Immunology & Microbial Science, The Scripps Research Institute, La Jolla, CA 92037, USA.
Michael
M
Bobardt
Sam
S
Hopkins
James
J
Baugh
Udayan
U
Chatterji
Felicia
F
Hernandez
John
J
Hiscott
Ann
A
Sluder
Kai
K
Lin
Philippe A
PA
Gallay
D001665:Binding Sites / Q000502:physiology; D001667:Binding, Competitive / Q000502:physiology; D021984:Cyclophilin A / Q000378:metabolism*; D056945:Hep G2 Cells; D016174:Hepacivirus / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D019698:Hepatitis C, Chronic / Q000235:genetics / Q000378:metabolism / Q000821:virology*; D006801:Humans; D050834:Interferon-Stimulated Gene Factor 3, gamma Subunit / Q000235:genetics / Q000378:metabolism*; D007372:Interferons / Q000378:metabolism; D053612:Janus Kinases / Q000378:metabolism; D008024:Ligands; D050794:STAT1 Transcription Factor / Q000378:metabolism; D050795:STAT2 Transcription Factor / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D015533:Transcriptional Activation / Q000502:physiology; D019562:Viral Load / Q000502:physiology; D017361:Viral Nonstructural Proteins / Q000235:genetics / Q000378:metabolism*; D014779:Virus Replication / Q000502:physiology; D012324:RNA-Dependent RNA Polymerase
D021984:Cyclophilin A; D050834:Interferon-Stimulated Gene Factor 3, gamma Subunit; D007372:Interferons; D053612:Janus Kinases; D008024:Ligands; D050794:STAT1 Transcription Factor; D050795:STAT2 Transcription Factor; D017361:Viral Nonstructural Proteins; C494227:IRF9 protein, human; C494079:STAT1 protein, human; C494084:STAT2 protein, human; C087036:NS-5 protein, hepatitis C virus; D012324:RNA-Dependent RNA Polymerase