The Envelope Protein Encoded by the A33R Gene Is Required for Formation of Actin-Containing Microvilli and Efficient Cell-to-Cell Spread of Vaccinia Virus

The vaccinia virus (VV) A33R gene encodes a highly conserved 23- to 28-kDa glycoprotein that is specifically incorporated into the viral outer envelope. The protein is expressed early and late after infection, consistent with putative early and late promoter sequences. To determine the role of the protein, two inducible A33R mutants were constructed, one with the late promoter and one with the early and late A33R promoter elements. Decreased A33R expression was associated with small plaques that formed comets in liquid medium. Using both an antibiotic resistance gene and a color marker, an A33R deletion mutant, vA33Δ, was isolated, indicating that the A33R gene is not essential for VV replication. The plaques formed by vA33Δ, however, were tiny, indicating that the A33R protein is necessary for efficient cell-to-cell spread. Rescue of the large-plaque phenotype was achieved by inserting a new copy of the A33R gene into the thymidine kinase locus, confirming the specific genetic basis of the phenotype. Although there was a reduction in intracellular virus formed in cells infected with vA33Δ, the amount of infectious virus in the medium was increased. The virus particles in the medium had the buoyant density of extracellular enveloped viruses (EEV). Additionally, amounts of vA33Δ cell-associated extracellular enveloped viruses (CEV) were found to be normal. Immunogold electron microscopy of cells infected with vA33Δ demonstrated the presence of the expected F13L and B5R proteins in wrapping membranes and EEV; however, fully wrapped vA33Δ intracellular enveloped viruses (IEV) were rare compared to partially wrapped particles. Specialized actin tails that propel IEV particles to the periphery and virus-tipped microvilli (both common in wild-type-infected cells) were absent in cells infected with vA33Δ. This is the first deletion mutant in a VV envelope gene that produces at least normal amounts of fully infectious EEV and CEV and yet has a small-plaque phenotype. These data support a new model for VV spread, emphasizing the importance of virus-tipped actin tails.     

Identification of second-site mutations that enhance release and spread of vaccinia virus

The spread of most strains of vaccinia virus in cell monolayers occurs predominantly via extracellular enveloped virions that adhere to the tips of actin-containing microvilli and to a lesser extent via diffusion of released virions. The mechanism by which virions adhere to the cell surface is unknown, although several viral proteins may be involved. The present investigation was initiated with the following premise: spontaneous mutations that increase virus release will be naturally selected by propagating a virus unable to spread by means of actin tails. Starting with an A36R deletion mutant that forms small, round plaques, five independent virus clones with enhanced spread due to the formation of comet or satellite plaques were isolated. The viral membrane glycoprotein genes of the isolates were sequenced; four had mutations causing C-terminal truncations of the A33R protein, and one had a serine replacing proline 189 of the B5R protein. The comet-forming phenotype was specifically reproduced or reversed by homologous recombination using DNA containing the mutated or natural sequence, respectively. Considerably more extracellular enveloped virus was released into the medium by the second-site mutants than by the parental A36R deletion mutant, explaining their selection in tissue culture as well as their comet-forming phenotype. The data suggest that the B5R protein and the C-terminal region of the A33R protein are involved in adherence of cell-associated enveloped virions to cells. In spite of their selective advantage in cultured cells, the second-site mutants were not detectably more virulent than the A36R deletion mutant when administered to mice by the intranasal route.