MEMBRANE-FUSION PROMOTED BY INCREASING SURFACE DENSITIES OF THE PARAMYXOVIRUS F-PROTEINS AND HN-PROTEINS - COMPARISON OF FUSION REACTIONS MEDIATED BY SIMIAN-VIRUS-5-F, HUMAN PARAINFLUENZA VIRUS TYPE-3-F, AND INFLUENZA-VIRUS HA

The membrane fusion reaction promoted by the paramyxovirus simian viru s 5 (SV5) and human parainfluenza virus type 3 (HPN-3) fusion (F) prot eins and hemagglutinin-neuraminidase (HN) proteins was characterized w hen the surface densities of F and HN were varied. Using a quantitativ e content mixing assay, it was found that the extent of SV5 F-mediated fusion was dependent on the surface density of the SV5 F protein but independent of the density of SV5 HN protein, indicating that HN serve s only a binding function in the reaction. However, the extent of HPIV -3 F protein promoted fusion reaction was found to be dependent on sur face density of HPIV-3 HN protein, suggesting that the HPIV-3 HN prote in is a direct participant in the fusion reaction. Analysis of the kin etics of lipid mixing demonstrated that both initial rates and final e xtents of fusion increased with rising SV5 F protein surface densities , suggesting that multiple fusion pores can be active during SV5 F pro tein-promoted membrane fusion. Initial rates and extent of lipid mixin g were also found to increase with increasing influenza virus hemagglu tinin protein surface density, suggesting parallels between the mechan ism of fusion promoted by these two viral fusion proteins.