Cell fusion induced by herpes simplex virus is promoted and suppressed by different viral glycoproteins

Some of the factors that regulate membrane fusion resulting in polykaryocyte formationhave been investigated, using the model system of human cells infected with mutants of herpes simplex virus (HSV). One of the mutant viruses used in this study (MP) failed to produce the viral glycoprotein designated C2--a nonlethal defect that has previously been correlated with the polykaryocyte-inducing phenotype of this and other mutant strains (wild-type strains of HSV usually induce the aggregation of infected cells rather than their fusion). The other mutant virus (tsB5), a temperature-sensitive conditional-lethal mutant, failed to produce glycoprotein B2 at non-permissive temperature, whereas the synthesis of all other viral products appeared to be normal. We produced and isolated seven recombinants of MP and tsB5 that expressed both of the parental alterations in glycoprotein synthesis. All of the re-combinant viruses induced the fusion of infected cells at 34 degrees (correlated with the absence of C2 expression) but were unable to cause cell fusion at 39 degrees (correlated with the absence of C2 and of B2 expression), even after infection at multiplicities high enough to ensure that all cells in the cultures synthesized viral macromolecules. These results and studies on the dominance or recessiveness of the fusion-inducing phenotype in mixed infections provide evidence that glycoprotein B2 plays a critical role in the promotion of cell fusion and that glycoprotein C2 can act to suppress fusion.