Conditions for pox virus-specific microvilli formation studied during synchronized virus assembly

Late in the infectious cycle of pox viruses numerous specialized microvilli are formed on the surface of the infected cell. They consist of a microfilament bundle containing actin, α-actinin, filamin, and fimbrin and a virus particle at the tip. Formation of pox virus-specific microvilli is inhibited by rifampicin or hydroxyurea treatment. Under conditions of synchronized maturation following removal of rifampicin or hydroxyurea microvilli appear on the cell surface concomitantly with the formation of mature virus particles in the cell. If, however, final particle maturation is prevented by the addition of sodium fluoride, cycloheximide, actinomycin D, or isatin-β-thiosemicarbazone after removal of rifampicin no microvilli can be detected. The inhibition of microvilli formation in rifampicin treated cells is due to the antivaccinia effect of the drug since a rifampicin-resistant vaccinia mutant is able to induce microvilli formation even in the presence of the inhibitor. Two temperature-sensitive pox virus mutants with different defects in virus maturation are able to induce microvilli only at the permissive and not at the restrictive temperature. It is concluded that induction of microvilli can only occur if mature virus particles are formed. Alterations of the surface of the infected cell like the presence of early antigen, hemagglutinin, and cell fusing activity are not required for microvilli induction.