Role for the vaccinia virus A36R outer envelope protein in the formation of virus-tipped actin-containing microvilli and cell-to-cell virus spread

A small-plaque-forming vaccinia virus mutant with a deletion in the A36R gene encoding an outer envelope protein (Parkinson and Smith, Virology, 204, 376-390, 1994) was shown to assemble wrapped forms of intra-and extracellular virus particles and to mediate acid-induced polykaryon formation. The intracellular virions, however, did not acquire actin tails and those on the cell surface were not associated with specialized microvilli. This phenotype is similar to that of the A34R (E. J. Wolffe, E. Katz, A. Weisberg, and a. Moss, J. Virol. 71, 3904-3915, 1997) and A33R (R. Roper, E.J. Wolffe, A Weisberg, and a. Moss, J. Virol., in press) deletion mutants. Taken together, these data support a model in which the envelope proteins encoded by the A33R, A34R, and A36R genes are all required for nucleation of actin tails, which facilitate dissemination rather than egress of virus particles.