Amino acid substitutions in leucine zipper motif in the F-specific domain of human parainfluenza virus 3 HN protein play important roles in the protein function.

Objectives: To explore the effects of leucine zipper motif in F-specificity domain of human parainfluenza virus 3 (hPIV3) HN protein on the membrane fusion promotion activity.

Methods: Site-directed mutagenesis was performed to generate mutants in leucine zipper motif of hPIV3 HN protein. Combined mutants were obtained from individual mutants. Each mutant was co-expressed with the wild-type (wt) hPIV3 F gene in eukaryotes, using the vaccinia-T7 RNA polymerase expression system. Cell fusion functions were assayed with Giemsa staining and reporter gene method. The expression of HN protein on cell surface was analyzed with fluorescence-activated cell sorter (FACS). Hemadsorption assay was performed to determine the receptor-binding activity of HN mutants.

Results: The conserved amino acids in the F-specificity domain of the hPIV3 HN protein were mutated and 9 single mutants were obtained. Based on the single mutants, 2 combined mutants were obtained. Compared to the wt HN protein, the membrane fusion promotion activity of each mutant HN protein was, to some extent, decreased. Among these single mutants, I125A showed the lowest fusion promotion activity, with 34.8% of fusion promotion activity compared to wt HN. In contrast, the fusion promotion activity of I128A was the highest, showing 90.9% of wt HN. In combined mutations, no cell fusion was observed, suggesting fusion promotion activity was negligible. All mutants had a limited effect on receptor-binding activity, and I125A showed the lowest activity, with 85.86% of wt HN. FACS analysis indicated that all mutants were expressed on the cell surface.

Conclusions: Leucine zipper motif in the F-specificity domain had an important effect on the fusion promotion ability of hPIV3 HN protein. A mutation in the motif will diminish or abolish the fusion promotion activity of hPIV3 HN protein. A complete leucine zipper motif was prerequisite to the fusion promotion of HN protein.