Persistent reovirus infections of L cells select mutations in viral attachment protein σ1 that alter oligomer stability

During maintenance of L-cell cultures persistently infected with reovirus, mutations are selected in viruses and cells. Cells cured of persistent infection support growth of viruses isolated from persistently infected cultures (PI viruses) significantly better than that of wild-type (wt) viruses. In a previous study, the capacity of PI virus strain L/C to grow better than wt strain type 1 Lang (TIL) in cured cells was mapped genetically to the SI gene (R. S. Kauffman, R. Ahmed, and B. N. Fields, Virology 131:79-87, 1983), which encodes vital attachment protein σ1. To investigate mechanisms by which mutations in S1 confer growth of PI viruses in cured cells, we determined the SI gene nucleotide sequences of L/C virus and six additional PI viruses isolated from independent persistently infected L-cell cultures. The SI sequences of these viruses contained from one to three mutations, and with the exception of P1 2A1, mutations in each S1 gene resulted in changes in the deduced amino acid sequence of σ1 protein. Using electrophoresis conditions that favor migration of σ1 oligomers, we found that σ1 proteins of L/C, PI IAI, Pl 3-1, and PI 5-1 migrated as monomers, whereas σl proteins of wt reovirus and PI 2AI migrated as oligomers. These findings suggest that mutations in σl protein affecting stability of σ1 oligomers are important for the capacity of PI viruses to infect mutant cells selected during persistent infection. Since no mutation was found in the deduced amino acid sequence of PI 2AI σl protein, we used TIL x PI 2AI reassortant viruses to identify viral genes associated with the capacity of this Pl virus to grow better than wt in cured cells. The capacity of PI 2AI to grow better than T1L in cured cells was mapped to the S4 gene, which encodes outer-capsid protein σ3. This finding suggests that in some cases, mutations in σ3 protein in the absence of σ1 mutations confer growth of P1 viruses in mutant cells. To confirm the importance of the S1 gene in Pl virus growth in cured cells, we used TIL x PI 3-1 reassortant viruses to genetically map the capacity of this P1 virus to grow better than wt in cured cells. In contrast to our results using PI 2A1, we found that growth of PI 3- 1 in cured cells was determined by the σl-encoding S1 gene. Given that the σ1 and σ3 proteins play important roles in reovirus disassembly, findings made in this study suggest that stability of the viral outer capsid is an important determinant of the capacity of reoviruses to adapt to host cells during persistent infection.