Targeted delivery of human neurofibromin and c-Raf-1 mutants to the cytoplasmic membrane by use of the influenza virus hemagglutinin

Fusion activity of transmembrane and cytoplasmic domain chimeras of the influenza virus glycoprotein hemagglutinin

The role of the sequence of transmembrane and cytoplasmic/intraviral domains of influenza virus hemagglutinin (HA, subtype H7) for HA-mediated membrane fusion was explored. To analyze the influence of the two domains on the fusogenic properties of HA, we designed HA-chimeras in which the cytoplasmic tail and/or transmembrane domain of HA was replaced with the corresponding domains of the fusogenic glycoprotein F of Sendai virus. These chimeras, as well as constructs of HA in which the cytoplasmic tail was replaced by peptides of human neurofibromin type 1 (NF1) or c-Raf-1, NF78 (residues 1441 to 1518), and Raf81 (residues 51 to 131), respectively, were expressed in CV-1 cells by using the vaccinia virus-T7 polymerase transient-expression system. Wild-type and chimeric HA were cleaved properly into two subunits and expressed as trimers. Membrane fusion between CV-1 cells and bound human erythrocytes (RBCs) mediated by parental or chimeric HA proteins was studied by a lipid-mixing assay with the lipid-like fluorophore octadecyl rhodamine B chloride (R18). No profound differences in either extent or kinetics could be observed. After the pH was lowered, the above proteins also induced a flow of the aqueous fluorophore calcein from preloaded RBCs into the cytoplasm of the protein-expressing CV-1 cells, indicating that membrane fusion involves both leaflets of the lipid bilayers and leads to formation of an aqueous fusion pore. We conclude that neither HA-specific sequences in the transmembrane and cytoplasmic domains nor their length is crucial for HA-induced membrane fusion activity.