Interaction of frog virus 3 with the cytomatrix. III. Role of microfilaments in virus release.

The role of microfilaments in the release of frog virus 3 (FV3) from the plasma membrane was studied. Scanning electron microscopic study of FV3-infected baby hamster kidney (BHK) cells showed that late in infection (15 hr), numerous microvillus-like projections containing virions and microfilaments occur on the cell surface. Two microfilament-disrupting drugs, cytochalasin B and cytochalasin D, inhibited both the formation of microvillus-like projections and virus release. In the drug-treated cells, the virions accumulated in large numbers beneath the plasma membrane (transmission electron microscopy), suggesting that both drugs affected the release of the virus at the level of plasma membrane rather than the traverse of the virus to the plasma membrane. Two-dimensional gel analysis of actin from FV3-infected and uninfected cells revealed the following. There was no difference in the synthesis of actin in infected versus uninfected cells. However, the actin of infected cells is more acidic than its counterpart in uninfected cells. Temporally, the change in actin preceded the formation of microvilli-like projections involved in virus release. The change in actin is virus induced and is linked to virus maturation since a ts mutant of FV3 (ts9467), which is deficient in virus production at the restrictive temperature (30 degrees), did not modify actin. The mutant, at the permissive temperature (25 degrees), produced virions and altered the actin. Together, the above results attribute an active role for microfilaments in virus release.