Dissociation of paramyxovirus interferon evasion activities: universal and virus-specific requirements for conserved V protein amino acids in MDA5 interference. J Virol. 2010;84:11152-11163. [PMID: 20719949 DOI: 10.1128/jvi.01375-10]

A tryptophan-rich motif in the human parainfluenza virus type 2 V protein is critical for the blockade of toll-like receptor 7 (TLR7)- and TLR9-dependent signaling

Plasmacytoid dendritic cells (pDCs) do not produce alpha interferon (IFN-α) unless viruses cause a systemic infection or overcome the first-line defense provided by conventional DCs and macrophages. We show here that even paramyxoviruses, whose infections are restricted to the respiratory tract, have a V protein able to prevent Toll-like receptor 7 (TLR7)- and TLR9-dependent IFN-α induction specific to pDCs. Mutational analysis of human parainfluenza virus type 2 demonstrates that the second Trp residue of the Trp-rich motif (Trp-X(3)-Trp-X(9)-Trp) in the C-terminal domain unique to V, a determinant for IRF7 binding, is critical for the blockade of TLR7/9-dependent signaling.

Identification of human parainfluenza virus type 2 (HPIV-2) V protein amino acid residues that reduce binding of V to MDA5 and attenuate HPIV-2 replication in nonhuman primates

Human parainfluenza virus type 2 (HPIV-2), an important pediatric respiratory pathogen, encodes a V protein that inhibits type I interferon (IFN) induction and signaling. Using reverse genetics, we attempted the recovery of a panel of V mutant viruses that individually contained one of six cysteine-to-serine (residues 193, 197, 209, 211, 214, and 218) substitutions, one of two paired charge-to-alanine (R175A/R176A and R205A/K206A) substitutions, or a histidine-to-phenylalanine (H174F) substitution. This mutagenesis was performed using a cDNA-derived HPIV-2 virus that expressed the V and P coding sequences from separate mRNAs. Of the cysteine substitutions, only C193S, C214S, and C218S yielded viable virus, and only the C214S mutant replicated well enough for further analysis. The H174F, R175A/R176A, and R205A/K206A mutants were viable and replicated well. The H174F and R205A/K206A mutants did not differ from the wild-type (WT) V in their ability to physically interact with MDA5, a cytoplasmic sensor of nonself RNA that induces type I IFN. Like WT HPIV-2, these mutants inhibited IFN-β induction and replicated efficiently in African green monkeys (AGMs). In contrast, the C214S and R175A/R176A mutants did not bind MDA5 efficiently, did not inhibit interferon regulatory factor 3 (IRF3) dimerization or IFN-β induction, and were attenuated in AGMs. These findings indicate that V binding to MDA5 is important for HPIV-2 virulence in nonhuman primates and that some V protein residues involved in MDA5 binding are not essential for efficient HPIV-2 growth in vitro. Using a transient expression system, 20 additional mutant V proteins were screened for MDA5 binding, and the region spanning residues 175 to 180 was found to be essential for this activity.

The measles virus V protein binds to p65 (RelA) to suppress NF-kappaB activity

Nuclear factor κB (NF-κB) transcription factors are involved in controlling numerous cellular processes, including inflammation, innate and adaptive immunity, and cell survival. Here we show that the immunosuppressive measles virus (MV; Morbillivirus genus, Paramyxoviridae) has evolved multiple functions to interfere with canonical NF-κB signaling in epithelial cells. The MV P, V, and C proteins, also involved in preventing host cell interferon responses, were found to individually suppress NF-κB-dependent reporter gene expression in response to activation of the tumor necrosis factor (TNF) receptor, RIG-I-like receptors, or Toll-like receptors. NF-κB activity was most efficiently suppressed in the presence of V, while expression of P or C resulted in moderate inhibition. As indicated by reporter gene assays involving overexpression of the IκB kinase (IKK) complex, which phosphorylates the inhibitor of κB to liberate NF-κB, V protein targets a downstream step in the signaling cascade. Coimmunoprecipitation experiments revealed that V specifically binds to the Rel homology domain of the NF-κB subunit p65 but not of p50. Notably, the short C-terminal domain of the V protein, which is also involved in binding STAT2, IRF7, and MDA5, was sufficient for the interaction and for preventing reporter gene activity. As observed by confocal microscopy, the presence of V abolished nuclear translocation of p65 upon TNF-α stimulation. Thus, MV V appears to prevent NF-κB-dependent gene expression by retaining p65 in the cytoplasm. These findings reveal NF-κB as a key target of MV and stress the importance of the V protein as the major viral immune-modulatory factor.