Cleavability of hemagglutinin from an extremely virulent strain of avian influenza virus containing a unique cleavage site sequence.

An avian influenza virus, A/turkey/England/50-92/91 (H5N1), showed extremely high virulence in chickens, although its hemagglutinin (HA) cleavage site sequence (R-K-R-K-T-R), having a nonbasic (Thr) residue at the second position (P-2) from the carboxyl terminus of HA1, does not conform to the previously established consensus sequence motif, X-X-R/K-X-R/K-R (X = nonbasic residue), for highly virulent phenotype of the H5 virus. When we evaluated the HA cleavability of this strain in chicken embryo fibroblast culture, we observed that, unlike other HAs with a Thr residue at P-2, this HA was efficiently cleaved. These findings suggest that a nonbasic residue at the P-2 does not affect its recognition and catalyzation by cleavage enzymes that are otherwise influenced by steric structure around the cleavage site.