Effects of proteolytic enzymes on the infectivity, haemagglutinating activity and protein composition of bluetongue virus type 20

The effects on virus infectivity, haemagglutinating (HA) activity and polypeptide composition of bluetongue virus type 20 (BTV 20) were determined after digestion with the proteolytic enzymes, chymotrypsin, thermolysin and trypsin. Virus infectivity increased eight to 50-fold after exposure periods which reflected the activity of the proteases. Identical maximum increases in HA activity (i.e. 4096, 1024 and 128 HAU per 0.05 ml with sheep, bovine and human erythrocytes, respectively) occurred with each of the three proteases. Peak increases in virus infectivities and HA activities occurred after similar exposure periods. Outer capsid protein VP2 was the most sensitive virus protein to proteolytic digestion, being cleaved into a number of smaller polypeptides that remained attached to the virus particle. Digestion with chymotrypsin and thermolysin yielded four common cleavage products, designated P93, P76, P54 and P25 according to their estimated molecular weight, which suggested that they shared at least three cleavage sites. VP2 cleavage products resulting from digestion with trypsin differed somewhat from those of chymotrypsin and thermolysin, although the generation of polypeptides P93, P54 and P25.5 suggested the existence of common cleavage sites for the three proteases. Possible mechanisms whereby proteolytic cleavage of VP2 may enhance the infectivity and HA activity of BTV 20 are discussed.