Engineered serine protease inhibitor prevents furin-catalyzed activation of the fusion glycoprotein and production of infectious measles virus

We have identified the major cellular endoprotease that activates the fusion (F) glycoprotein of measles virus (MV) and have engineered a serine protease inhibitor (serpin) to target the endoprotease and inhibit the production of infectious MV. The F-protein precursor of MV was not cleaved efficiently into the mature F protein in human colon carcinoma cells lacking functional furin, indicating that furin is the major enzyme responsible for activation of the MV F protein. A human serpin α₁-antitrypsin variant was engineered to specifically inhibit furin. When expressed from a recombinant vaccinia virus in primate cells infected by MV, the engineered serpin (α₁- PDX) specifically inhibited furin-catalyzed cleavage of the F-protein precursor without affecting synthesis of other MV proteins. We generated human glioma cells stably expressing α₁-PDX. MV infection in these cells did not result in syncytia. The infected cells produced all the MV proteins, but the F-protein precursor remained largely uncleaved. This did not prevent virus assembly. However, the released virions contained inactive F-protein precursor rather than mature F protein, and infectious-virus titers were reduced by 3 to 4 orders of magnitude. These results show that a mature F protein is not required for the assembly of MV but is crucial for virus infectivity. The engineered serpin may offer a novel molecular antiviral approach against MV.