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.

Sendai virus (SeV) leader (le) and trailer (tr) RNAs are short transcripts generated during abortive antigenome and genome synthesis, respectively. Recombinant SeV (rSeV) that express tr-like RNAs from the leader region are non-cytopathic and, moreover, prevent wild-type SeV from inducing apoptosis in mixed infections. These rSeV thus appear to have gained a function. Here we report that tr RNA binds to a cellular protein with many links to apoptosis (TIAR) via the AU-rich sequence 5¢ UUUUAAAUUUU. Duplication of this AU-rich sequence alone within the le RNA confers TIAR binding on this le* RNA and a non-cytopathic phenotype to these rSeV in cell culture. Transgenic overexpression of TIAR during SeV infection promotes apoptosis and reverses the anti-apoptotic effects of le* RNA expression. Moreover, TIAR overexpression and SeV infection act synergistically to induce apoptosis. These short viral RNAs may act by sequestering TIAR, a multivalent RNA recognition motif (RRM) family RNA-binding protein involved in SeV-induced apoptosis. In this view, tr RNA is not simply a by-product of abortive genome synthesis, but is also an antigenome transcript that modulates the cellular antiviral response.

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