Activation of an influenza virus A/turkey/Oregon/71 HA insertion variant by the subtilisin-like endoprotease furin.

The hemagglutinin (HA) gene of the influenza A turkey/Oregon/71 variant Tc1 adapted to primary chicken embryo cells contains an insertion of 54 nucleotides that encodes a peptide adjacent to the HA cleavage site, which is responsible for increased cleavability by ubiquitous cellular proteases. After coexpression with human furin from cDNA by vaccinia virus vectors and by an endogenous protease, the HA of Tc1, which possesses the amino acid sequence R-T-A-R at the cleavage site, is proteolytically processed. Site-directed mutagenesis of the cleavage site indicated that the arginine in position -4 is critical for HA activation by furin. Deletion of the insert revealed that the amino acid sequence -1 to -4 predisposes the protein for furin recognition.