The hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins of two paramyxoviruses, human parainfluenza virus type 2 (PIV2) and simian virus 41 (SV41), were expressed in HeLa cells by transfecting with recombinant plasmid harboring each glycoprotein gene. Expressed F proteins could not induce cell fusion by themselves, but evoked prominent cell fusion when coexpressed with homologous HN proteins. It was also proved that PIV2 HN protein could weakly promote SV41 F-mediated cell fusion. By analyzing the fusion-promoting function of chimeric HN proteins of PIV2 and SV41, it was revealed that the N-terminal region (about 16% of total amino acids) of either PIV2 HN or SV41 HN protein could define the type-specific fusion-promoting function for homologous F protein. Analyses of additional chimeras indicated that the N-terminal region in PIV2 HN protein (designated region I, consisting of 94 amino acids) could be reduced to a 58-amino-acid region (region I') which was located at the membrane-proximal end of the ectodomain. Furthermore, PIV2 HN protein proved to promote cell fusion mediated by PIV4A F protein. Unexpectedly, analyses of another set of chimeras revealed that the promoting function of PIV2 HN protein for PIV4A F-mediated cell fusion was not merely carried by its region I but also by another region ranging from residue 148 to 209 (region II). Finally, it was indicated that regions I' (in the presumed stalk domain) and II (in the globular head) in PIV2 HN protein might play important roles in promoting cell fusion mediated by the F proteins.

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Some paramyxovirus V proteins induce STAT protein degradation, and the amino acids essential for this process in the human parainfluenza virus type 2 (hPIV2) V protein have been studied. Various recombinant hPIV2s and cell lines constitutively expressing various mutant V proteins were generated. We found that V proteins with replacement of Cys residues of the Cys cluster were still able to bind STATs but were unable to induce their degradation. The hPIV2 V protein binds STATs via a W-(X) 3 -W-(X) 9 -W Trp motif located just upstream of the Cys cluster. Replacements of two or more Trp residues in this motif resulted in a failure to form a V/STAT2 complex. We have also identified two Phe residues of the hPIV2 V protein that are essential for STAT degradation, namely, Phe207, lying within the Cys cluster, and Phe143, in the P/V common region of the protein. Interestingly, infection of BHK cells with hPIV2 led to the specific degradation of STAT1 and not STAT2. Other evidence for the cell species specificity of hPIV2-induced STAT degradation is presented. Finally, a V-minus hPIV2, which can express only the P protein from its P gene, was generated and partially characterized. In contrast to V-minus viruses of other paramyxovirus genera, this V-minus rubulavirus was highly debilitated, and its growth even in Vero cells was very limited. The structural rubulavirus V proteins, as expected, are thus clearly important in promoting virus growth, independent of their anti-interferon (IFN) activity. Interestingly, many of the residues that are essential for anti-IFN activity, e.g., the Cys of this cluster and Phe207 within this cluster, as well as the Trp of this motif, are also essential for promoting virus growth.The interferon (IFN) system is the first line of host defense against virus infection. Viruses of the Paramyxovirinae, similarly to other viruses, have evolved proteins that specifically inhibit IFN-induced innate antiviral responses, at least in part through direct inhibition of cellular STAT proteins that are responsible for IFN signal transduction. The V proteins encoded by the rubulaviruses simian virus 5 (SV5), SV41, and mumps virus (MuV) and by Newcastle disease virus (NDV, an avulavirus) block IFN signaling by targeting STAT1 for degradation (1,5,6,14,20,25,33,45,46,49,51), whereas the V protein of human parainfluenza virus type 2 (hPIV2, a rubulavirus) targets STAT2 for degradation (25,32). Moreover, the V proteins of measles virus (morbillivirus) and Nipah virus and Hendra virus (henipaviruses) have been shown to inhibit IFN signaling by preventing STAT1 and STAT2 nuclear accumulation (30,37,38,41). Sendai virus (SeV) and hPIV3 also block IFN signaling, and this anti-IFN ability has been shown to be a property of these respirovirus C proteins (7,8,10,11,17,19,42). In cells that are highly IFN competent, the Sendai virus C protein also induces the intracellular loss of STAT1 (9). The rubulavirus V protein-dependent degradation of STAT proteins involves degradation complexes that contain the V protein, STA...

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