Mild proteolysis induces a ready-to-fuse state on Sendai virus envelope.

The Sendai virus fuses with host cell membranes in a pH-independent manner through an unknown mechanism. Here we report that mild trypsin pre-treatments of Sendai virions, for example 15 min at 4 degrees C, give Sendai virions the ability to fuse at a rate up to 10-fold higher than control. By using human erythrocytes as host cell membranes, viral fusion was assessed by hemolysis as well as fluorescence dequenching of octadecyl rhodamine B chloride. The mild protease treatment strikingly shortens the lag time taken by the virus to start the fusion process. Similar data were obtained on reconstituted Sendai virus envelope. Among proteases, tested as fusion enhancer, trypsin is more effective than either endoproteinase Lys-C, chymotrypsin, or endoproteinase Arg-C. After removal of trypsin from treated virions the fusion rate enhancement remains for hours at room temperature. The lack of protease specificity, together with the impossibility to detect any new N-terminal products, suggests that only a small percentage of viral envelope components are cleaved, still a large enough number to set the envelope in a ready-to-fuse state.