| We have studied the structure/function of the paramyxovirus F protein, the relationship of the F and HN glycoprotein, and the mechanism of paramyxovirus-induced cell fusion. Using a synthetic oligonucleotide as a probe, we cloned the PI2 F gene and determined its nucleotide sequence and deduced amino acid sequence. PI2 F was found to be most closely related with SV5 F. By expressing PI2 F and HN in eukaryotic cells using vaccinia-T7 system, we demonstrated that F alone was insufficient to mediate fusion; cell fusion caused by paramyxoviruses required both F and HN to participate. The function of HN could not be replaced by nonspecific attachment molecules such as the lectin wheat germ agglutinin (WGA). Together with experiments using exogenous neuraminidase, we demonstrated that both the interaction with F and the attachment to sialic acid receptors by HN were essential to cell fusion. Based on our observations that only coexpression of homologous F/HN induced cell fusion, we proposed a new concept that a functional type-specific interaction occurs between F and HN in the cell fusion process. Neither F nor HN can be replaced even by proteins of a related virus type within the same viral genus.;To investigate the role and relationship of F and HN in the fusion process, we performed cross-linking/coprecipitation, surface labeling/coprecipitation and capping experiments. Evidence was obtained that homologous F and HN were physically associated with each other on the cell surface. Thus, we proposed another novel concept that a functional "fusion complex" may be formed by homologous F and HN. We hypothesized that such a complex formation may result in a conformational change of F, which converts the fusion ectodomain into a more hydrophobic entity. Alternatively, the oligomeric F and HN may form a patch structure which leads to the further contact of F with the target cell membrane.;Finally, by using fluorescence dequenching assay, we obtained quantitative data which supported our conclusions that only homologous F and HN cause cell fusion and F is insufficient to mediate fusion even in the presence of the lectin WGA.;The results of these studies contribute to a better understanding of requirements for paramyxovirus-induced cell fusion and the role and relationship of F and HN in the fusion process. |
