MILD PROTEOLYSIS INDUCES A READY-TO-FUSE STATE ON SENDAI VIRUS ENVELOPE

The Sendai virus fuses with host cell membranes in a pH-independent ma nner through an unknown mechanism, Here we report that mild trypsin pr e-treatments of Sendai virions, for example 15 min at 4 degrees C, giv e Sendai virions the ability to fuse at a rate up to 10-fold higher th an control, By using human erythrocytes as host cell membranes, viral fusion was assessed by hemolysis as well as fluorescence dequenching o f octadecyl rhodamine B chloride, The mild protease treatment striking ly shortens the lag time taken by the virus to start the fusion proces s, 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 enhan cement remains for hours at room temperature, The lack of protease spe cificity, together with the impossibility to detect any new N-terminal products, suggests that only a small percentage of viral envelope com ponents are cleaved, still a large enough number to set the envelope i n a ready-to-fuse state. (C) 1998 Federation of European Biochemical S ocieties.