Parainfluenza virus 5 (PIV5) belongs to the rubulavirus genus in the paramyxovirus family, which includes many well-known human and animal pathogens as well as emerging viruses such as Hendra virus and Nipah virus. The PIV5 V protein contains a cysteine-rich C-terminal domain which is conserved among all paramyxoviruses. The V protein can block both interferon (IFN) signaling by causing degradation of STAT1 and IFN production by blocking IRF-3 nuclear import. Previously, it was reported that recombinant SV5 lacking the C terminus of the V protein (rPIV5VΔC) induces a severe cytopathic effect (CPE) in tissue culture whereas wild-type (wt) PIV5 infection does not. In chapter 2, the nature of the CPE was investigated. Our data suggested that: (1) The CPE induced by rPIV5VΔC is apoptosis; (2) wt V protein inhibited the apoptosis induced by rPIV5VΔC; (3) IFN was not required for the apoptosis; (4) Caspase-2 and 3 were activated but not required for rPIV5VÄC induced apoptosis; (5) caspsae-12 is activated; (6) Endoplasmic reticulum (ER) stress makers such as GRP78, GRP94 are upregulated in rPIV5VΔC infected cells, suggesting that rPIV5VΔC might induce apoptosis by triggering ER stress.

Recently, it is reported that PIV5 V protein plays a important role in regulating PIV5 RNA synthesis in the mini-genome, possibly through a host protein. In chapter 3, the mechanism of the regulation of viral RNA synthesis by PIV5 V protein is investigated. The data we have suggested that Akt1, a serine/theronine kinase, also known as protein kinase B (PKB), interacted with PIV5 V protein. Both inhibitors and siRNA of Akt1 reduced PIV5 replication, indicating Akt plays a critical role in PIV5 replication. Furthermore, inhibitors of Akt also reduced the replication of Mumps virus (MuV), Measles virus (MeV), Respiratory Synthcytial Virus (RSV) and Vesicular Stomatitis Virus (VSV), indicating that Akt plays an important role in replication of many non-segmented negative single-stranded RNA viruses (NNSVs). The P protein of NNSVs is phosphorylated at multiple sites and is a part of viral RNA polymerase complex. Inhibition of Akt activity reduced the level of P phosphorylation, which is known to play a critical role in viral RNA synthesis. Akt plays important roles in many cellular processes such as cell survival, metabolism, growth, proliferation and mobility and has been targeted for cancer therapy. The finding that NNSV replication requires Akt will lead to better understanding how these viruses replicate, as well as novel strategies to treat infectious diseases caused by NNSVs.