Proteolytic activation of vaccinia virus for the penetration phase of infection

Abstract

Treatment of vaccinia virus strain IHD-J with trypsin enhanced the titer by specific activation of the infecting efficiency of the virus, not by dispersion of aggregated virus. When the penetration step was performed in the presence of an inhibitor of serine protease (PMSF), the titer of untreated virus was reduced, whereas trypsin-treated virus infected effectively even in the presence of the drug. There is a PMSF-sensitive step in the viral uncoating process, the function of which is replaceable by treatment of the virus with trypsin. Using PMSF sensitivity and trypsin-catalyzed activation, vaccinia virus populations were classified into three fractions differing in infectivity: a fraction that is infectious in the presence of PMSF (activated form), a fraction that is not infectious in the presence of PMSF but is infectious under usual tissue culture conditions (infectious form), and a fraction that is not infectious under usual conditions and requires treatment with trypsin to become activated (protected form). The ratios in a purified virus sample were 1:3:16, respectively. The ratios of the three forms assayed at various times after infection showed that the activated form and/or infectious form was dominant until 20 hr and the protected form was the large majority after 24 hr of infection. The progeny virus in the medium was of the activated form.