APPROACH TO THE INVOLVEMENT OF INFLUENZA-B NEURAMINIDASE IN THE CLEAVAGE OF HA BY HOST-CELL PROTEASE USING LOW PH-INDUCED CELL-FUSION REACTION

ts7, a temperature-sensitive mutant defective in neuraminidase (NA) of influenza B/ Kanagawa/73, lacks NA enzymatic activity at the nonpermi ssive temperature (37.5 C). When MDCK cells were infected with the mut ant at the permissive temperature (32 C) and exposed to pH 5.2 medium, extensive cell fusion occurred. In contrast, at the nonpermissive tem perature cells did not show cell fusion at all unless they were pretre ated with trypsin, suggesting that at 37.5 C the hemagglutinin (HA) of ts7 is expressed at the cell surface in an uncleaved form. It was als o found that the replacement of RNA segment 6 of ts7 with that of wild -type B/Lee resulted in the emergence of low pH-induced fusion activit y as well as NA enzymatic activity at the incubation temperature of 37 .5 C and that the addition of bacterial NA to the cultures infected wi th ts7 at 37.5 C early in infection brought about low pH-induced cell fusion. We suggest that the removal of neuraminic acid from the carboh ydrate moiety of HA by NA is essential for the cleavage of HA by cellu lar protease.