Identification of RNA-binding regions on the P and V proteins of human parainfluenza virus type 2

W protein expression by Newcastle disease virus

Differential editing of transcripts from the Newcastle disease virus (NDV) phosphoprotein gene results in mRNAs capable of encoding the phosphoprotein (P), the V protein, and the W protein which share a common N-terminus but specify different C-termini. Whereas the expression and viral incorporation of the P - and V proteins by NDV has been documented, evidence for the existence of a W protein was lacking. To analyze expression of the NDV W protein, two peptides encompassing predicted antigenic sites of the unique C-terminal W protein amino acid sequence of NDV Clone 30 were used for the generation of W-specific rabbit antisera. One of them detected plasmid-expressed W protein and identified W protein after infection by indirect immunofluorescence and Western blot analyses. W protein was absent in cells infected by a newly generated recombinant NDV lacking W protein expression. Furthermore, Western blot and mass spectrometric analyses indicated the incorporation of W protein into viral particles. Confocal microscopic analyses of infected cells revealed nuclear accumulation of W protein that could be attributed to a bipartite nuclear localization sequence (NLS) within its unique C-terminal part. Redistribution of the W protein to the cytoplasm within transfected cells confirmed functionality of the NLS after mutation of its two basic clusters. This finding was additionally corroborated in cells infected with a recombinant virus expressing the mutated W protein.

Nucleocytoplasmic shuttling of the human parainfluenza virus type 2 phosphoprotein

Human parainfluenza virus type 2 phosphoprotein (P) is an essential component of viral polymerase. The P gene encodes both P and accessory V proteins by a specific gene editing mechanism. Therefore, the N-terminal 164 amino acids of P protein are common to V protein. Interestingly, while P protein is located in the cytoplasm, V protein is found mainly in the nucleus. Using deletion mutants, we show the presence of a nuclear localization signal (NLS) in the P/V common domain, and a nuclear export signal (NES) in the C-terminal P specific region. The NLS region makes a complex with importin α5 or 7. In the presence of leptomycin B, P protein is retained in the nucleus, indicating that it contains a CRM1-dependent NES. We identified the NLS (⁶⁵PVKPRRKK⁷²) and the NES (²²⁵IIELLKGLDL²³⁴) using β-galactosidase fusion proteins. Moreover, nucleocytoplasmic shuttling of P protein appears to be important for efficient viral polymerase activity.

Recombinant human parainfluenza virus type 2 with mutations in V that permit cellular interferon signaling are not attenuated in non-human primates

The HPIV2 V protein inhibits type I interferon (IFN) induction and signaling. To manipulate the V protein, whose coding sequence overlaps that of the polymerase-associated phosphoprotein (P), without altering the P protein, we generated an HPIV2 virus in which P and V are expressed from separate genes (rHPIV2-P+V). rHPIV2-P+V replicated like HPIV2-WT in vitro and in non-human primates. HPIV2-P+V was modified by introducing two separate mutations into the V protein to create rHPIV2-L101E/L102E and rHPIV2-Delta122-127. In contrast to HPIV2-WT, both mutant viruses were unable to degrade STAT2, leaving virus-infected cells susceptible to IFN. Neither mutant, nor HPIV2-WT, induced significant amounts of IFN-beta in infected cells. Surprisingly, neither rHPIV2-L101E/L102E nor rHPIV2-Delta122-127 was attenuated in two species of non-human primates. This indicates that loss of HPIV2's ability to inhibit IFN signaling is insufficient to attenuate virus replication in vivo as long as IFN induction is still inhibited.

Human parainfluenza virus type 2 V protein inhibits interferon production and signaling and is required for replication in non-human primates

In wild-type human parainfluenza virus type 2 (WT HPIV2), one gene (the P/V gene) encodes both the polymerase-associated phosphoprotein (P) and the accessory V protein. We generated a HPIV2 virus (rHPIV2-V^ko) in which the P/V gene encodes only the P protein to examine the role of V in replication in vivo and as a potential live attenuated virus vaccine. Preventing expression of V protein severely impaired virus recovery from cDNA and growth in vitro, particularly in IFN-competent cells. rHPIV2-V^ko, unlike WT HPIV2, strongly induced IFN-β and permitted IFN signaling, leading to establishment of a robust antiviral state. rHPIV2-V^ko infection induced extensive syncytia and cytopathicity that was due to both apoptosis and necrosis. Replication of rHPIV2-V^ko was highly restricted in the respiratory tract of African green monkeys and in differentiated primary human airway epithelial (HAE) cultures, suggesting that V protein is essential for efficient replication of HPIV2 in organized epithelial cells and that rHPIV2-V^ko is over-attenuated for use as a live attenuated vaccine.

Human parainfluenza virus type 2 V protein inhibits genome replication by binding to the L protein: possible role in promoting viral fitness

The human parainfluenza virus type 2 (hPIV2) V protein plays important roles in inhibiting the host interferon response and promoting virus growth, but its role in hPIV2 replication and transcription is not clear. A green fluorescent protein (GFP)-expressing a negative-sense minigenomic construct of hPIV2 has been established by standard technology, with helper plasmids expressing the nucleocapsid protein (NP), phosphoprotein (P), and large RNA polymerase (L) protein, to examine the role of V protein. We found that the simultaneous expression of wild-type V protein in the minigenome system inhibited GFP expression, at least in part, by inhibiting minigenome replication. In contrast, expression of C terminally truncated or mutant hPIV2 V proteins had no effect. Moreover, the V protein of simian virus 41, the rubulavirus most closely related virus to hPIV2, also inhibited GFP expression, whereas that of PIV5, a more distantly related rubulavirus, did not. Using these other rubulavirus V proteins, as well as various mutant hPIV2 V proteins, we found that the ability of V protein to inhibit GFP expression correlated with its ability to bind to L protein via its C-terminal V protein-specific region, but there was no correlation with NP binding. A possible role for this inhibition of genome replication in promoting viral fitness is discussed.

The conserved carboxyl terminus of human parainfluenza virus type 2 V protein plays an important role in virus growth

Our previous results have shown that some residues of V protein-specific domain in human parainfluenza virus type 2 (hPIV2) are essential not only for STAT protein degradation but also for promoting virus growth. Here, we demonstrated that the virus growth of these recombinant hPIV2s (rPIV2) expressing mutated V proteins were improved in HeLa cell transiently expressing the wild-type V protein, but not in the cells constitutively expressing it. Consequently, we identified the region of the V protein that is essential for its oligomerization and for complex formation with NP protein. We also identified a host protein, AlP1/Alix, involved in apoptosis and efficient budding of several enveloped viruses as an interacting partner of the V and NP proteins. Depletion of AIP1/Alix by small interfering RNA suppressed virus growth. These data suggest that the conserved carboxyl terminus of the V protein plays an important role in virus growth.