Activation of Sendai virus infectivity by an enzyme in chicken amniotic fluid.

Sendai virus grown in LLC-MK2 cells is known to have low infectivity, no detectable haemolysing ability and predominantly uncleaved F glycoprotein. Treatment of such virus with chicken amniotic fluid resulted in a 10(3)- to 10(4)-fold increase in infectivity, the development of haemolysing ability, and cleavage of the F glycoprotein. The 'Sendai activating enzyme' (SAE) responsible for these effects required the presence of 1 mM-Ca2+ ions for maximum activity. It was inhibited by phenylmethylsulphonyl fluoride and soybean trypsin inhibitor but was unaffected by sulphydryl-blocking agents. The results of gel filtration tests suggested that the molecular weight of SAE was about 55 000. SAE may be the natural proteolytic activator of Sendai virus in a soluble form.