THE EXTRACELLULAR DOMAIN OF VACCINIA VIRUS PROTEIN B5R AFFECTS PLAQUEPHENOTYPE, EXTRACELLULAR ENVELOPED VIRUS RELEASE, AND INTRACELLULAR ACTIN TAIL FORMATION

Vaccinia virus produces two morphologically distinct forms of infectio us virus, termed intracellular mature virus (IMV) and extracellular en veloped virus (EEV), EEV is important for virus dissemination within a host and has different surface proteins which bind to cell receptors different from those used by IMV. Sig genes are known to encode EEV-sp ecific proteins, One of these, B5R, encodes a 42-kDa glycoprotein with amino acid similarity to members of the complement control protein su perfamily and contains four copies of a 50- to 70-amino-acid repeat ca lled the short consensus repeat (SCR). Deletion of B5R causes a small- plaque phenotype, a 10-fold reduction in EEV formation, and virus atte nuation in vivo. In this study, we inserted mutated versions of the B5 R gene lacking different combinations of the SCRs into a virus deletio n mutant lacking the B5R gene. The resultant viruses each formed small plaques only slightly larger than those of the deletion mutant; howev er, the virus containing only SCR 1 formed plaques slightly larger tha n those of viruses with SCRs 1 and 2 or SCRs 1, 2, and 3. All of these viruses produced approximately 50-fold more infectious EEV than wild- type virus and formed comet-shaped plaques under liquid overlay. Despi te producing more EEV, the mutant viruses were unable to induce the po lymerization of actin on intracellular virus particles. The implicatio ns of these results for our understanding of EEV formation, release, a nd infectivity are discussed.