Hemifusion activity of a chimeric influenza virus hemagglutinin with a putative fusion peptide from hepatitis B virus

Entry of enveloped viruses is often mediated by an aminoterminal hydrophobic fusion peptide of a viral surface protein. The S domain of the hepatitis B virus surface protein contains a putative fusion peptide at position 7-18, but no systems are available to study its function directly. We tested the functionality of this peptide and a related peptide from another hepadnavirus in the context of the well-characterized influenza virus hemagglutinin H7 using gene mutation. The chimeric hemagglutinins could be expressed stably in CV 1 cells and were transported to the cell surface. The chimeras were incompletely cleaved by cellular proteases but cleavage could be completed by trypsin treatment of the cells. The chimeras did not differ in receptor binding, i.e. erythrocyte binding. Hemifusion and fusion pore formation were detected with membrane or cytosolic fluorescent dye-labeled erythrocytes as target structures of the hemagglutinin. Five of six different chimeras mediated hemifusion in 20–54% of the hemagglutinin-expressing cells, complete fusion and syncytium formation was not observed. The data suggest that the sequence 7-18 of the hepatitis B S domain may indeed initiate the first step of viral entry, i.e. hemifusion.