PROPROTEIN-PROCESSING ENDOPROTEASES PC6 AND FURIN BOTH ACTIVATE HEMAGGLUTININ OF VIRULENT AVIAN INFLUENZA-VIRUSES

Among the proprotein-processing subtilisin-related endoproteases, furi n has been a leading candidate for the enzyme that activates the hemag glutinin (HA) of virulent avian influenza viruses. In the present stud y, we examined the cleavage activity of tno other recently isolated ub iquitous subtilisin-related pretenses, PACE4 and PC6, using wild-type HA of A/turkey/Ireland/1378/83 (H5N8) and a series of its mutant HAs. Vaccinia virus-expressed wild-type IEA was not cleaved in human colon adenocarcinoma LoVo cells, which lack active furin. This processing de fect was corrected by the expression of furin and PC6 but not of PACE4 and a control wild-type vaccinia virus. PC6 showed a sequence specifi city similar to that with the endogenous proteases in cultured cells. When LoVo cells were infected with a virulent avian virus, A/turkey/On tario/7732/66 (H5N9), only noninfectious virions were produced because of the lack of HA. cleavage. However, when tile cells were coinfected with vaccinia virus that expressed either furin or PC6, the avian vir us underwent multiple cycles of replication, indicating that both furi n and PC6 specifically cleave the virulent virus HA at the authentic s ite. These data suggest that PC6, as well as furin, can activate virul ent avian influenza viruses in vivo, implying the presence of multiple HA cleavage enzymes in animals.