Mutations in the DI-DII Linker of Human Parainfluenza Virus Type 3 Fusion Protein Result in Diminished Fusion Activity

Human parainfluenza virus type 3 (HPIV3) can cause severe respiratory tract diseases in infants and young children, but no licensed vaccines or antiviral agents are currently available for treatment. Fusing the viral and target cell membranes is a prerequisite for its entry into host cells and is directly mediated by the fusion (F) protein. Although several domains of F are known to have important effects on regulating the membrane fusion activity, the roles of the DI-DII linker (residues 369–374) of the HPIV3 F protein in the fusogenicity still remains ill-defined. To facilitate our understanding of the role of this domain might play in F-induced cell-cell fusion, nine single mutations were engineered into this domain by site-directed mutagenesis. A vaccinia virus-T7 RNA polymerase transient expression system was employed to express the wild-type or mutated F proteins. These mutants were analyzed for membrane fusion activity, cell surface expression, and interaction between F and HN protein. Each of the mutated F proteins in this domain has a cell surface expression level similar to that of wild-type F. All of them resulted in a significant reduction in fusogenic activity in all steps of membrane fusion. Furthermore, all these fusion-deficient mutants reduced the amount of the HN-F complexes at the cell surface. Together, the results of our work suggest that this region has an important effect on the fusogenic activity of F.